Chapter 17 Group Replication

Table of Contents

17.1 Group Replication Background
17.1.1 Replication Technologies
17.1.2 Group Replication Use Cases
17.1.3 Group Replication Details
17.2 Getting Started
17.2.1 Deploying Group Replication in Single-Primary Mode
17.2.2 Deploying Group Replication Locally
17.3 Monitoring Group Replication
17.3.1 Group Replication Server States
17.3.2 The replication_group_members Table
17.3.3 The replication_group_member_stats Table
17.4 Group Replication Operations
17.4.1 Deploying in Multi-Primary or Single-Primary Mode
17.4.2 Tuning Recovery
17.4.3 Network Partitioning
17.4.4 Using MySQL Enterprise Backup with Group Replication
17.5 Group Replication Security
17.5.1 Group Replication IP Address Allowlisting
17.5.2 Group Replication Secure Socket Layer (SSL) Support
17.5.3 Group Replication and Virtual Private Networks (VPNs)
17.6 Group Replication System Variables
17.7 Requirements and Limitations
17.7.1 Group Replication Requirements
17.7.2 Group Replication Limitations
17.8 Frequently Asked Questions
17.9 Group Replication Technical Details
17.9.1 Group Replication Plugin Architecture
17.9.2 The Group
17.9.3 Data Manipulation Statements
17.9.4 Data Definition Statements
17.9.5 Distributed Recovery
17.9.6 Observability
17.9.7 Group Replication Performance

This chapter explains MySQL Group Replication and how to install, configure and monitor groups. MySQL Group Replication is a MySQL Server plugin that enables you to create elastic, highly-available, fault-tolerant replication topologies.

Groups can operate in a single-primary mode with automatic primary election, where only one server accepts updates at a time. Alternatively, for more advanced users, groups can be deployed in multi-primary mode, where all servers can accept updates, even if they are issued concurrently.

There is a built-in group membership service that keeps the view of the group consistent and available for all servers at any given point in time. Servers can leave and join the group and the view is updated accordingly. Sometimes servers can leave the group unexpectedly, in which case the failure detection mechanism detects this and notifies the group that the view has changed. This is all automatic.

The chapter is structured as follows:

17.1 Group Replication Background

This section provides background information on MySQL Group Replication.

The most common way to create a fault-tolerant system is to resort to making components redundant, in other words the component can be removed and the system should continue to operate as expected. This creates a set of challenges that raise complexity of such systems to a whole different level. Specifically, replicated databases have to deal with the fact that they require maintenance and administration of several servers instead of just one. Moreover, as servers are cooperating together to create the group several other classic distributed systems problems have to be dealt with, such as network partitioning or split brain scenarios.

Therefore, the ultimate challenge is to fuse the logic of the database and data replication with the logic of having several servers coordinated in a consistent and simple way. In other words, to have multiple servers agreeing on the state of the system and the data on each and every change that the system goes through. This can be summarized as having servers reaching agreement on each database state transition, so that they all progress as one single database or alternatively that they eventually converge to the same state. Meaning that they need to operate as a (distributed) state machine.

MySQL Group Replication provides distributed state machine replication with strong coordination between servers. Servers coordinate themselves automatically when they are part of the same group. The group can operate in a single-primary mode with automatic primary election, where only one server accepts updates at a time. Alternatively, for more advanced users the group can be deployed in multi-primary mode, where all servers can accept updates, even if they are issued concurrently. This power comes at the expense of applications having to work around the limitations imposed by such deployments.

There is a built-in group membership service that keeps the view of the group consistent and available for all servers at any given point in time. Servers can leave and join the group and the view is updated accordingly. Sometimes servers can leave the group unexpectedly, in which case the failure detection mechanism detects this and notifies the group that the view has changed. This is all automatic.

For a transaction to commit, the majority of the group have to agree on the order of a given transaction in the global sequence of transactions. Deciding to commit or abort a transaction is done by each server individually, but all servers make the same decision. If there is a network partition, resulting in a split where members are unable to reach agreement, then the system does not progress until this issue is resolved. Hence there is also a built-in, automatic, split-brain protection mechanism.

All of this is powered by the provided Group Communication System (GCS) protocols. These provide a failure detection mechanism, a group membership service, and safe and completely ordered message delivery. All these properties are key to creating a system which ensures that data is consistently replicated across the group of servers. At the very core of this technology lies an implementation of the Paxos algorithm. It acts as the group communication engine.

17.1.1 Replication Technologies

Before getting into the details of MySQL Group Replication, this section introduces some background concepts and an overview of how things work. This provides some context to help understand what is required for Group Replication and what the differences are between classic asynchronous MySQL Replication and Group Replication.

17.1.1.1 Primary-Secondary Replication

Traditional MySQL Replication provides a simple Primary-Secondary approach to replication. There is a primary (source) and there are one or more secondaries (replicas). The primary executes transactions, commits them and then they are later (thus asynchronously) sent to the secondaries to be either re-executed (in statement-based replication) or applied (in row-based replication). It is a shared-nothing system, where all servers have a full copy of the data by default.

Figure 17.1 MySQL Asynchronous Replication

A transaction received by the source is executed, written to the binary log, then committed, and a response is sent to the client application. The record from the binary log is sent to the relay logs on Replica 1 and Replica 2 before the commit takes place on the source. On each of the replicas, the transaction is applied, written to the replica's binary log, and committed. The commit on the source and the commits on the replicas are all independent and asynchronous.

There is also semisynchronous replication, which adds one synchronization step to the protocol. This means that the Primary waits, at commit time, for the secondary to acknowledge that it has received the transaction. Only then does the Primary resume the commit operation.

Figure 17.2 MySQL Semisynchronous Replication

A transaction received by the source is executed and written to the binary log. The record from the binary log is sent to the relay logs on Replica 1 and Replica 2. The source then waits for an acknowledgement from the replicas. When both of the replicas have returned the acknowledgement, the source commits the transaction, and a response is sent to the client application. After each replica has returned its acknowlegement, it applies the transaction, writes it to the binary log, and commits it. The commit on the source depends on the acknowledgement from the replicas, but the commits on the replicas are independent from each other and from the commit on the source.

In the two pictures above, you can see a diagram of the classic asynchronous MySQL Replication protocol (and its semisynchronous variant as well). Diagonal arrows represent messages exchanged between servers or messages exchanged between servers and the client application.

17.1.1.2 Group Replication

Group Replication is a technique that can be used to implement fault-tolerant systems. The replication group is a set of servers that each have their own entire copy of the data (a shared-nothing replication scheme), and interact with each other through message passing. The communication layer provides a set of guarantees such as atomic message and total order message delivery. These are very powerful properties that translate into very useful abstractions that one can resort to build more advanced database replication solutions.

MySQL Group Replication builds on top of such properties and abstractions and implements a multi-source update everywhere replication protocol. A replication group is formed by multiple servers and each server in the group may execute transactions independently at any time. However, all read-write transactions commit only after they have been approved by the group. In other words, for any read-write transaction the group needs to decide whether it commits or not, so the commit operation is not a unilateral decision from the originating server. Read-only transactions need no coordination within the group and commit immediately.

When a read-write transaction is ready to commit at the originating server, the server atomically broadcasts the write values (the rows that were changed) and the corresponding write set (the unique identifiers of the rows that were updated). Because the transaction is sent through an atomic broadcast, either all servers in the group receive the transaction or none do. If they receive it, then they all receive it in the same order with respect to other transactions that were sent before. All servers therefore receive the same set of transactions in the same order, and a global total order is established for the transactions.

However, there may be conflicts between transactions that execute concurrently on different servers. Such conflicts are detected by inspecting and comparing the write sets of two different and concurrent transactions, in a process called certification. During certification, conflict detection is carried out at row level: if two concurrent transactions, that executed on different servers, update the same row, then there is a conflict. The conflict resolution procedure states that the transaction that was ordered first commits on all servers, and the transaction ordered second aborts, and is therefore rolled back on the originating server and dropped by the other servers in the group. For example, if t1 and t2 execute concurrently at different sites, both changing the same row, and t2 is ordered before t1, then t2 wins the conflict and t1 is rolled back. This is in fact a distributed first commit wins rule. Note that if two transactions are bound to conflict more often than not, then it is a good practice to start them on the same server, where they have a chance to synchronize on the local lock manager instead of being rolled back as a result of certification.

For applying and externalizing the certified transactions, Group Replication permits servers to deviate from the agreed order of the transactions if this does not break consistency and validity. Group Replication is an eventual consistency system, meaning that as soon as the incoming traffic slows down or stops, all group members have the same data content. While traffic is flowing, transactions can be externalized in a slightly different order, or externalized on some members before the others. For example, in multi-primary mode, a local transaction might be externalized immediately following certification, although a remote transaction that is earlier in the global order has not yet been applied. This is permitted when the certification process has established that there is no conflict between the transactions. In single-primary mode, on the primary server, there is a small chance that concurrent, non-conflicting local transactions might be committed and externalized in a different order from the global order agreed by Group Replication. On the secondaries, which do not accept writes from clients, transactions are always committed and externalized in the agreed order.

The following figure depicts the MySQL Group Replication protocol and by comparing it to MySQL Replication (or even MySQL semisynchronous replication) you can see some differences. Note that some underlying consensus and Paxos related messages are missing from this picture for the sake of clarity.

Figure 17.3 MySQL Group Replication Protocol

A transaction received by Source 1 is executed. Source 1 then sends a message to the replication group, consisting of itself, Source 2, and Source 3. When all three members have reached consensus, they certify the transaction. Source 1 then writes the transaction to its binary log, commits it, and sends a response to the client application. Sources 2 and 3 write the transaction to their relay logs, then apply it, write it to the binary log, and commit it.

17.1.2 Group Replication Use Cases

Group Replication enables you to create fault-tolerant systems with redundancy by replicating the system state to a set of servers. Even if some of the servers subsequently fail, as long it is not all or a majority, the system is still available. Depending on the number of servers which fail the group might have degraded performance or scalability, but it is still available. Server failures are isolated and independent. They are tracked by a group membership service which relies on a distributed failure detector that is able to signal when any servers leave the group, either voluntarily or due to an unexpected halt. There is a distributed recovery procedure to ensure that when servers join the group they are brought up to date automatically. There is no need for server fail-over, and the multi-source update everywhere nature ensures that even updates are not blocked in the event of a single server failure. To summarize, MySQL Group Replication guarantees that the database service is continuously available.

It is important to understand that although the database service is available, in the event of an unexpected server exit, those clients connected to it must be redirected, or failed over, to a different server. This is not something Group Replication attempts to resolve. A connector, load balancer, router, or some form of middleware are more suitable to deal with this issue. For example see MySQL Router 8.0.

To summarize, MySQL Group Replication provides a highly available, highly elastic, dependable MySQL service.

17.1.2.1 Examples of Use Case Scenarios

The following examples are typical use cases for Group Replication.

  • Elastic Replication - Environments that require a very fluid replication infrastructure, where the number of servers has to grow or shrink dynamically and with as few side-effects as possible. For instance, database services for the cloud.

  • Highly Available Shards - Sharding is a popular approach to achieve write scale-out. Use MySQL Group Replication to implement highly available shards, where each shard maps to a replication group.

  • Alternative to Source-Replica replication - In certain situations, using a single source server makes it a single point of contention. Writing to an entire group may prove more scalable under certain circumstances.

  • Autonomic Systems - Additionally, you can deploy MySQL Group Replication purely for the automation that is built into the replication protocol (described already in this and previous chapters).

17.1.3 Group Replication Details

This section presents details about some of the services that Group Replication builds on.

17.1.3.1 Group Membership

In MySQL Group Replication, a set of servers forms a replication group. A group has a name, which takes the form of a UUID. The group is dynamic and servers can leave (either voluntarily or involuntarily) and join it at any time. The group adjusts itself whenever servers join or leave.

If a server joins the group, it automatically brings itself up to date by fetching the missing state from an existing server. If a server leaves the group, for instance it was taken down for maintenance, the remaining servers notice that it has left and reconfigure the group automatically.

Group Replication has a group membership service that defines which servers are online and participating in the group. The list of online servers is referred to as a view. Every server in the group has a consistent view of which servers are the members participating actively in the group at a given moment in time.

Group members must agree not only on transaction commits, but also on which is the current view. If existing members agree that a new server should become part of the group, the group is reconfigured to integrate that server in it, which triggers a view change. If a server leaves the group, either voluntarily or not, the group dynamically rearranges its configuration and a view change is triggered.

In the case where a member leaves the group voluntarily, it first initiates a dynamic group reconfiguration, during which all members have to agree on a new view without the leaving server. However, if a member leaves the group involuntarily, for example because it has stopped unexpectedly or the network connection is down, it cannot initiate the reconfiguration. In this situation, Group Replication's failure detection mechanism recognizes after a short period of time that the member has left, and a reconfiguration of the group without the failed member is proposed. As with a member that leaves voluntarily, the reconfiguration requires agreement from the majority of servers in the group. However, if the group is not able to reach agreement, for example because it partitioned in such a way that there is no majority of servers online, the system is not able to dynamically change the configuration, and blocks to prevent a split-brain situation. This situation requires intervention from an administrator.

It is possible for a member to go offline for a short time, then attempt to rejoin the group again before the failure detection mechanism has detected its failure, and before the group has been reconfigured to remove the member. In this situation, the rejoining member forgets its previous state, but if other members send it messages that are intended for its pre-crash state, this can cause issues including possible data inconsistency. If a member in this situation participates in XCom's consensus protocol, it could potentially cause XCom to deliver different values for the same consensus round, by making a different decision before and after failure.

To counter this possibility, from MySQL 5.7.22, servers are given a unique identifier when they join a group. This enables Group Replication to be aware of the situation where a new incarnation of the same server (with the same address but a new identifier) is trying to join the group while its old incarnation is still listed as a member. The new incarnation is blocked from joining the group until the old incarnation can be removed by a reconfiguration. If Group Replication is stopped and restarted on the server, the member becomes a new incarnation and cannot rejoin until the suspicion times out.

17.1.3.2 Failure Detection

Group Replication includes a failure detection mechanism that is able to find and report which servers are silent and as such assumed to be dead. At a high level, the failure detector is a distributed service that provides information about which servers may be dead (suspicions). Suspicions are triggered when servers go mute. When server A does not receive messages from server B during a given period, a timeout occurs and a suspicion is raised. Later if the group agrees that the suspicions are probably true, then the group decides that a given server has indeed failed. This means that the remaining members in the group take a coordinated decision to exclude a given member.

Suspicions are triggered when servers go mute. When server A does not receive messages from server B during a given period, a timeout occurs and a suspicion is raised.

If a server gets isolated from the rest of the group, then it suspects that all others have failed. Being unable to secure agreement with the group (as it cannot secure a quorum), its suspicion does not have consequences. When a server is isolated from the group in this way, it is unable to execute any local transactions.

17.1.3.3 Fault-tolerance

MySQL Group Replication builds on an implementation of the Paxos distributed algorithm to provide distributed coordination between servers. As such, it requires a majority of servers to be active to reach quorum and thus make a decision. This has direct impact on the number of failures the system can tolerate without compromising itself and its overall functionality. The number of servers (n) needed to tolerate f failures is then n = 2 x f + 1.

In practice this means that to tolerate one failure the group must have three servers in it. As such if one server fails, there are still two servers to form a majority (two out of three) and allow the system to continue to make decisions automatically and progress. However, if a second server fails involuntarily, then the group (with one server left) blocks, because there is no majority to reach a decision.

The following is a small table illustrating the formula above.

Group Size

Majority

Instant Failures Tolerated

1

1

0

2

2

0

3

2

1

4

3

1

5

3

2

6

4

2

7

4

3

The next Chapter covers technical aspects of Group Replication.

17.2 Getting Started

MySQL Group Replication is provided as a plugin to MySQL server, and each server in a group requires configuration and installation of the plugin. This section provides a detailed tutorial with the steps required to create a replication group with at least three members.

Tip

An alternative way to deploy multiple instances of MySQL is by using InnoDB Cluster, which uses Group Replication and wraps it in a programmatic environment that enables you to easily work with groups of MySQL server instances in the MySQL Shell 8.0 (part of MySQL 8.0). In addition, InnoDB Cluster interfaces seamlessly with MySQL Router and simplifies deploying MySQL with high availability. See Using MySQL AdminAPI.

17.2.1 Deploying Group Replication in Single-Primary Mode

Each of the MySQL server instances in a group can run on an independent physical host machine, which is the recommended way to deploy Group Replication. This section explains how to create a replication group with three MySQL Server instances, each running on a different host machine. See Section 17.2.2, “Deploying Group Replication Locally” for information about deploying multiple MySQL server instances running Group Replication on the same host machine, for example for testing purposes.

Figure 17.4 Group Architecture

Three server instances, S1, S2, and S3, are deployed as an interconnected group, and clients communicate with each of the server instances.

This tutorial explains how to get and deploy MySQL Server with the Group Replication plugin, how to configure each server instance before creating a group, and how to use Performance Schema monitoring to verify that everything is working correctly.

17.2.1.1 Deploying Instances for Group Replication

The first step is to deploy at least three instances of MySQL Server, this procedure demonstrates using multiple hosts for the instances, named s1, s2 and s3. It is assumed that MySQL Server was installed on each of the hosts, see Chapter 2, Installing and Upgrading MySQL. Group Replication is a built-in MySQL plugin provided with MySQL Server 5.7.17 and later. For more background information on MySQL plugins, see Section 5.5, “MySQL Server Plugins”.

In this example, three instances are used for the group, which is the minimum number of instances to create a group. Adding more instances increases the fault tolerance of the group. For example if the group consists of three members, in event of failure of one instance the group can continue. But in the event of another failure the group can no longer continue processing write transactions. By adding more instances, the number of servers which can fail while the group continues to process transactions also increases. The maximum number of instances which can be used in a group is nine. For more information see Section 17.1.3.2, “Failure Detection”.

17.2.1.2 Configuring an Instance for Group Replication

This section explains the configuration settings required for MySQL Server instances that you want to use for Group Replication. For background information, see Section 17.7, “Requirements and Limitations”.

Storage Engines

For Group Replication, data must be stored in the InnoDB transactional storage engine (for details of why, see Section 17.7.1, “Group Replication Requirements”). The use of other storage engines, including the temporary MEMORY storage engine, might cause errors in Group Replication. Set the disabled_storage_engines system variable as follows to prevent their use:

disabled_storage_engines="MyISAM,BLACKHOLE,FEDERATED,ARCHIVE,MEMORY"

Note that with the MyISAM storage engine disabled, when you are upgrading a MySQL instance to a release where mysql_upgrade is still used (before MySQL 8.0.16), mysql_upgrade might fail with an error. To handle this, you can re-enable that storage engine while you run mysql_upgrade, then disable it again when you restart the server. For more information, see Section 4.4.7, “mysql_upgrade — Check and Upgrade MySQL Tables”.

Replication Framework

The following settings configure replication according to the MySQL Group Replication requirements.

server_id=1
gtid_mode=ON
enforce_gtid_consistency=ON
master_info_repository=TABLE
relay_log_info_repository=TABLE
binlog_checksum=NONE
log_slave_updates=ON
log_bin=binlog
binlog_format=ROW

These settings configure the server to use the unique identifier number 1, to enable global transaction identifiers and to store replication metadata in system tables instead of files. Additionally, it instructs the server to turn on binary logging, use row-based format and disable binary log event checksums. For more details see Section 17.7.1, “Group Replication Requirements”.

Group Replication Settings

At this point the option file ensures that the server is configured and is instructed to instantiate the replication infrastructure under a given configuration. The following section configures the Group Replication settings for the server.

plugin_load_add='group_replication.so'
transaction_write_set_extraction=XXHASH64
group_replication_group_name="aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa"
group_replication_start_on_boot=off
group_replication_local_address= "s1:33061"
group_replication_group_seeds= "s1:33061,s2:33061,s3:33061"
group_replication_bootstrap_group=off
  • plugin-load-add adds the Group Replication plugin to the list of plugins which the server loads at startup. This is preferable in a production deployment to installing the plugin manually.

  • Configuring group_replication_group_name tells the plugin that the group that it is joining, or creating, is named "aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa".

    The value of group_replication_group_name must be a valid UUID. This UUID is used internally when setting GTIDs for Group Replication events in the binary log. You can use SELECT UUID() to generate a UUID.

  • Configuring the group_replication_start_on_boot variable to off instructs the plugin to not start operations automatically when the server starts. This is important when setting up Group Replication as it ensures you can configure the server before manually starting the plugin. Once the member is configured you can set group_replication_start_on_boot to on so that Group Replication starts automatically upon server boot.

  • Configuring group_replication_local_address sets the network address and port which the member uses for internal communication with other members in the group. Group Replication uses this address for internal member-to-member connections involving remote instances of the group communication engine (XCom, a Paxos variant).

    Important

    This address must be different to the hostname and port used for SQL and it must not be used for client applications. It must be only be used for internal communication between the members of the group while running Group Replication.

    The network address configured by group_replication_local_address must be resolvable by all group members. For example, if each server instance is on a different machine with a fixed network address, you could use the IP address of the machine, such as 10.0.0.1. If you use a host name, you must use a fully qualified name, and ensure it is resolvable through DNS, correctly configured /etc/hosts files, or other name resolution processes. From MySQL 8.0.14, IPv6 addresses (or host names that resolve to them) can be used as well as IPv4 addresses. A group can contain a mix of members using IPv6 and members using IPv4. For more information on Group Replication support for IPv6 networks and on mixed IPv4 and IPv6 groups, see Support For IPv6 And For Mixed IPv6 And IPv4 Groups.

    The recommended port for group_replication_local_address is 33061. group_replication_local_address is used by Group Replication as the unique identifier for a group member within the replication group. You can use the same port for all members of a replication group as long as the host names or IP addresses are all different, as demonstrated in this tutorial. Alternatively you can use the same host name or IP address for all members as long as the ports are all different, for example as shown in Section 17.2.2, “Deploying Group Replication Locally”.

  • Configuring group_replication_group_seeds sets the hostname and port of the group members which are used by the new member to establish its connection to the group. These members are called the seed members. Once the connection is established, the group membership information is listed at performance_schema.replication_group_members. Usually the group_replication_group_seeds list contains the hostname:port of each of the group member's group_replication_local_address, but this is not obligatory and a subset of the group members can be chosen as seeds.

    Important

    The hostname:port listed in group_replication_group_seeds is the seed member's internal network address, configured by group_replication_local_address and not the SQL hostname:port used for client connections, and shown for example in performance_schema.replication_group_members table.

    The server that starts the group does not make use of this option, since it is the initial server and as such, it is in charge of bootstrapping the group. In other words, any existing data which is on the server bootstrapping the group is what is used as the data for the next joining member. The second server joining asks the one and only member in the group to join, any missing data on the second server is replicated from the donor data on the bootstrapping member, and then the group expands. The third server joining can ask any of these two to join, data is synchronized to the new member, and then the group expands again. Subsequent servers repeat this procedure when joining.

    Warning

    When joining multiple servers at the same time, make sure that they point to seed members that are already in the group. Do not use members that are also joining the group as seeds, because they might not yet be in the group when contacted.

    It is good practice to start the bootstrap member first, and let it create the group. Then make it the seed member for the rest of the members that are joining. This ensures that there is a group formed when joining the rest of the members.

    Creating a group and joining multiple members at the same time is not supported. It might work, but chances are that the operations race and then the act of joining the group ends up in an error or a time out.

  • Configuring group_replication_bootstrap_group instructs the plugin whether to bootstrap the group or not. In this case, even though s1 is the first member of the group we set this variable to off in the option file. Instead we configure group_replication_bootstrap_group when the instance is running, to ensure that only one member actually bootstraps the group.

    Important

    The group_replication_bootstrap_group variable must only be enabled on one server instance belonging to a group at any time, usually the first time you bootstrap the group (or in case the entire group is brought down and back up again). If you bootstrap the group multiple times, for example when multiple server instances have this option set, then they could create an artificial split brain scenario, in which two distinct groups with the same name exist. Always set group_replication_bootstrap_group=off after the first server instance comes online.

Configuration for all servers in the group is quite similar. You need to change the specifics about each server (for example server_id, datadir, group_replication_local_address). This is illustrated later in this tutorial.

17.2.1.3 User Credentials

Group Replication uses the asynchronous replication protocol to achieve Section 17.9.5, “Distributed Recovery”, synchronizing group members before joining them to the group. The distributed recovery process relies on a replication channel named group_replication_recovery which is used to transfer transactions from donor members to members that join the group. Therefore you need to set up a replication user with the correct permissions so that Group Replication can establish direct member-to-member recovery replication channels.

Start the MySQL server instance and then connect a client to it. Create a MySQL user with the REPLICATION SLAVE privilege. This process can be captured in the binary log and then you can rely on distributed recovery to replicate the statements used to create the user. Alternatively, you can disable binary logging using SET SQL_LOG_BIN=0; and then create the user manually on each member, for example if you want to avoid the changes being propagated to other server instances. If you do decide to disable binary logging, ensure you renable it once you have configured the user.

In the following example the user rpl_user with the password password is shown. When configuring your servers use a suitable user name and password.

mysql> CREATE USER rpl_user@'%' IDENTIFIED BY 'password';
mysql> GRANT REPLICATION SLAVE ON *.* TO rpl_user@'%';
mysql> FLUSH PRIVILEGES;

If binary logging was disabled, enable it again once the user has been created using SET SQL_LOG_BIN=1;.

Once the user has been configured, use the CHANGE MASTER TO statement to configure the server to use the given credentials for the group_replication_recovery replication channel the next time it needs to recover its state from another member. Issue the following, replacing rpl_user and password with the values used when creating the user.

mysql> CHANGE MASTER TO MASTER_USER='rpl_user', MASTER_PASSWORD='password' \\
		      FOR CHANNEL 'group_replication_recovery';
      

Distributed recovery is the first step taken by a server that joins the group and does not have the same set of transactions as the group members. If these credentials are not set correctly for the group_replication_recovery replication channel and the rpl_user as shown, the server cannot connect to the donor members and run the distributed recovery process to gain synchrony with the other group members, and hence ultimately cannot join the group. See Section 17.9.5, “Distributed Recovery”.

Similarly, if the server cannot correctly identify the other members via the server's hostname the recovery process can fail. It is recommended that operating systems running MySQL have a properly configured unique hostname, either using DNS or local settings. This hostname can be verified in the Member_host column of the performance_schema.replication_group_members table. If multiple group members externalize a default hostname set by the operating system, there is a chance of the member not resolving to the correct member address and not being able to join the group. In such a situation use report_host to configure a unique hostname to be externalized by each of the servers.

17.2.1.4 Launching Group Replication

Once server s1 has been configured and started, install the Group Replication plugin. If you used plugin_load_add='group_replication.so' in the option file then the Group Replication plugin is installed and you can proceed to the next step. In the event that you decide to install the plugin manually, connect to the server and issue the following:

INSTALL PLUGIN group_replication SONAME 'group_replication.so';
Important

The mysql.session user must exist before you can load Group Replication. mysql.session was added in MySQL version 5.7.19. If your data dictionary was initialized using an earlier version you must perform the MySQL upgrade procedure (see Section 2.11, “Upgrading MySQL”). If the upgrade is not run, Group Replication fails to start with the error message There was an error when trying to access the server with user: mysql.session@localhost. Make sure the user is present in the server and that mysql_upgrade was ran after a server update..

To check that the plugin was installed successfully, issue SHOW PLUGINS; and check the output. It should show something like this:

mysql> SHOW PLUGINS;
+----------------------------+----------+--------------------+----------------------+-------------+
| Name                       | Status   | Type               | Library              | License     |
+----------------------------+----------+--------------------+----------------------+-------------+
| binlog                     | ACTIVE   | STORAGE ENGINE     | NULL                 | PROPRIETARY |

(...)

| group_replication          | ACTIVE   | GROUP REPLICATION  | group_replication.so | PROPRIETARY |
+----------------------------+----------+--------------------+----------------------+-------------+

17.2.1.5 Bootstrapping the Group

The process of starting a group for the first time is called bootstrapping. You use the group_replication_bootstrap_group system variable to bootstrap a group. The bootstrap should only be done by a single server, the one that starts the group and only once. This is why the value of the group_replication_bootstrap_group option was not stored in the instance's option file. If it is saved in the option file, upon restart the server automatically bootstraps a second group with the same name. This would result in two distinct groups with the same name. The same reasoning applies to stopping and restarting the plugin with this option set to ON. Therefore to safely bootstrap the group, connect to s1 and issue:

mysql> SET GLOBAL group_replication_bootstrap_group=ON;
mysql> START GROUP_REPLICATION;
mysql> SET GLOBAL group_replication_bootstrap_group=OFF;

Once the START GROUP_REPLICATION statement returns, the group has been started. You can check that the group is now created and that there is one member in it:

mysql> SELECT * FROM performance_schema.replication_group_members;
+---------------------------+--------------------------------------+-------------+-------------+---------------+
| CHANNEL_NAME              | MEMBER_ID                            | MEMBER_HOST | MEMBER_PORT | MEMBER_STATE  |
+---------------------------+--------------------------------------+-------------+-------------+---------------+
| group_replication_applier | ce9be252-2b71-11e6-b8f4-00212844f856 |   s1        |       3306  | ONLINE        |
+---------------------------+--------------------------------------+-------------+-------------+---------------+

The information in this table confirms that there is a member in the group with the unique identifier ce9be252-2b71-11e6-b8f4-00212844f856, that it is ONLINE and is at s1 listening for client connections on port 3306.

For the purpose of demonstrating that the server is indeed in a group and that it is able to handle load, create a table and add some content to it.

mysql> CREATE DATABASE test;
mysql> USE test;
mysql> CREATE TABLE t1 (c1 INT PRIMARY KEY, c2 TEXT NOT NULL);
mysql> INSERT INTO t1 VALUES (1, 'Luis');

Check the content of table t1 and the binary log.

mysql> SELECT * FROM t1;
+----+------+
| c1 | c2   |
+----+------+
|  1 | Luis |
+----+------+

mysql> SHOW BINLOG EVENTS;
+---------------+-----+----------------+-----------+-------------+--------------------------------------------------------------------+
| Log_name      | Pos | Event_type     | Server_id | End_log_pos | Info                                                               |
+---------------+-----+----------------+-----------+-------------+--------------------------------------------------------------------+
| binlog.000001 |   4 | Format_desc    |         1 |         123 | Server ver: 5.7.34-log, Binlog ver: 4                              |
| binlog.000001 | 123 | Previous_gtids |         1 |         150 |                                                                    |
| binlog.000001 | 150 | Gtid           |         1 |         211 | SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:1'  |
| binlog.000001 | 211 | Query          |         1 |         270 | BEGIN                                                              |
| binlog.000001 | 270 | View_change    |         1 |         369 | view_id=14724817264259180:1                                        |
| binlog.000001 | 369 | Query          |         1 |         434 | COMMIT                                                             |
| binlog.000001 | 434 | Gtid           |         1 |         495 | SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:2'  |
| binlog.000001 | 495 | Query          |         1 |         585 | CREATE DATABASE test                                               |
| binlog.000001 | 585 | Gtid           |         1 |         646 | SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:3'  |
| binlog.000001 | 646 | Query          |         1 |         770 | use `test`; CREATE TABLE t1 (c1 INT PRIMARY KEY, c2 TEXT NOT NULL) |
| binlog.000001 | 770 | Gtid           |         1 |         831 | SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:4'  |
| binlog.000001 | 831 | Query          |         1 |         899 | BEGIN                                                              |
| binlog.000001 | 899 | Table_map      |         1 |         942 | table_id: 108 (test.t1)                                            |
| binlog.000001 | 942 | Write_rows     |         1 |         984 | table_id: 108 flags: STMT_END_F                                    |
| binlog.000001 | 984 | Xid            |         1 |        1011 | COMMIT /* xid=38 */                                                |
+---------------+-----+----------------+-----------+-------------+--------------------------------------------------------------------+

As seen above, the database and the table objects were created and their corresponding DDL statements were written to the binary log. Also, the data was inserted into the table and written to the binary log. The importance of the binary log entries is illustrated in the following section when the group grows and distributed recovery is executed as new members try to catch up and become online.

17.2.1.6 Adding Instances to the Group

At this point, the group has one member in it, server s1, which has some data in it. It is now time to expand the group by adding the other two servers configured previously.

17.2.1.6.1 Adding a Second Instance

In order to add a second instance, server s2, first create the configuration file for it. The configuration is similar to the one used for server s1, except for things such as the server_id. These different lines are highlighted in the listing below.

[mysqld]

#
# Disable other storage engines
#
disabled_storage_engines="MyISAM,BLACKHOLE,FEDERATED,ARCHIVE,MEMORY"

#
# Replication configuration parameters
#
server_id=2
gtid_mode=ON
enforce_gtid_consistency=ON
master_info_repository=TABLE
relay_log_info_repository=TABLE
binlog_checksum=NONE
log_slave_updates=ON
log_bin=binlog
binlog_format=ROW

#
# Group Replication configuration
#
transaction_write_set_extraction=XXHASH64
group_replication_group_name="aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa"
group_replication_start_on_boot=off
group_replication_local_address= "s2:33061"
group_replication_group_seeds= "s1:33061,s2:33061,s3:33061"
group_replication_bootstrap_group= off

Similar to the procedure for server s1, with the option file in place you launch the server. Then configure the recovery credentials as follows. The commands are the same as used when setting up server s1 as the user is shared within the group. This member needs to have the same replication user configured in Section 17.2.1.3, “User Credentials”. If you are relying on distributed recovery to configure the user on all members, when s2 connects to the seed s1 the replication user is relicated to s1. If you did not have binary logging enabled when you configured the user credentials on s1, you must create the replication user on s2. In this case, connect to s2 and issue:

SET SQL_LOG_BIN=0;
CREATE USER rpl_user@'%' IDENTIFIED BY 'password';
GRANT REPLICATION SLAVE ON *.* TO rpl_user@'%';
SET SQL_LOG_BIN=1;
CHANGE MASTER TO MASTER_USER='rpl_user', MASTER_PASSWORD='password' \\
	FOR CHANNEL 'group_replication_recovery';

If necessary, install the Group Replication plugin, see Section 17.2.1.4, “Launching Group Replication”.

Start Group Replication and s2 starts the process of joining the group.

mysql> START GROUP_REPLICATION;

Unlike the previous steps that were the same as those executed on s1, here there is a difference in that you do not need to boostrap the group because the group already exiists. In other words on s2 group_replication_bootstrap_group is set to off, and you do not issue SET GLOBAL group_replication_bootstrap_group=ON; before starting Group Replication, because the group has already been created and bootstrapped by server s1. At this point server s2 only needs to be added to the already existing group.

Tip

When Group Replication starts successfully and the server joins the group it checks the super_read_only variable. By setting super_read_only to ON in the member's configuration file, you can ensure that servers which fail when starting Group Replication for any reason do not accept transactions. If the server should join the group as read-write instance, for example as the primary in a single-primary group or as a member of a multi-primary group, when the super_read_only variable is set to ON then it is set to OFF upon joining the group.

Checking the performance_schema.replication_group_members table again shows that there are now two ONLINE servers in the group.

mysql> SELECT * FROM performance_schema.replication_group_members;
+---------------------------+--------------------------------------+-------------+-------------+---------------+
| CHANNEL_NAME              | MEMBER_ID                            | MEMBER_HOST | MEMBER_PORT | MEMBER_STATE  |
+---------------------------+--------------------------------------+-------------+-------------+---------------+
| group_replication_applier | 395409e1-6dfa-11e6-970b-00212844f856 |   s1        |        3306 | ONLINE        |
| group_replication_applier | ac39f1e6-6dfa-11e6-a69d-00212844f856 |   s2        |        3306 | ONLINE        |
+---------------------------+--------------------------------------+-------------+-------------+---------------+

When s2 attempted to join the group, Section 17.9.5, “Distributed Recovery” ensured that s2 applied the same transactions which s1 had applied. Once this process completed, s2 could join the group as a member, and at this point it is marked as ONLINE. In other words it must have already caught up with server s1 automatically. Once s2 is ONLINE, it then begins to process transactions with the group. Verify that s2 has indeed synchronized with server s1 as follows.

mysql> SHOW DATABASES LIKE 'test';
+-----------------+
| Database (test) |
+-----------------+
| test            |
+-----------------+

mysql> SELECT * FROM test.t1;
+----+------+
| c1 | c2   |
+----+------+
|  1 | Luis |
+----+------+

mysql> SHOW BINLOG EVENTS;
+---------------+------+----------------+-----------+-------------+--------------------------------------------------------------------+
| Log_name      | Pos  | Event_type     | Server_id | End_log_pos | Info                                                               |
+---------------+------+----------------+-----------+-------------+--------------------------------------------------------------------+
| binlog.000001 |    4 | Format_desc    |         2 |         123 | Server ver: 5.7.34-log, Binlog ver: 4                              |
| binlog.000001 |  123 | Previous_gtids |         2 |         150 |                                                                    |
| binlog.000001 |  150 | Gtid           |         1 |         211 | SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:1'  |
| binlog.000001 |  211 | Query          |         1 |         270 | BEGIN                                                              |
| binlog.000001 |  270 | View_change    |         1 |         369 | view_id=14724832985483517:1                                        |
| binlog.000001 |  369 | Query          |         1 |         434 | COMMIT                                                             |
| binlog.000001 |  434 | Gtid           |         1 |         495 | SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:2'  |
| binlog.000001 |  495 | Query          |         1 |         585 | CREATE DATABASE test                                               |
| binlog.000001 |  585 | Gtid           |         1 |         646 | SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:3'  |
| binlog.000001 |  646 | Query          |         1 |         770 | use `test`; CREATE TABLE t1 (c1 INT PRIMARY KEY, c2 TEXT NOT NULL) |
| binlog.000001 |  770 | Gtid           |         1 |         831 | SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:4'  |
| binlog.000001 |  831 | Query          |         1 |         890 | BEGIN                                                              |
| binlog.000001 |  890 | Table_map      |         1 |         933 | table_id: 108 (test.t1)                                            |
| binlog.000001 |  933 | Write_rows     |         1 |         975 | table_id: 108 flags: STMT_END_F                                    |
| binlog.000001 |  975 | Xid            |         1 |        1002 | COMMIT /* xid=30 */                                                |
| binlog.000001 | 1002 | Gtid           |         1 |        1063 | SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:5'  |
| binlog.000001 | 1063 | Query          |         1 |        1122 | BEGIN                                                              |
| binlog.000001 | 1122 | View_change    |         1 |        1261 | view_id=14724832985483517:2                                        |
| binlog.000001 | 1261 | Query          |         1 |        1326 | COMMIT                                                             |
+---------------+------+----------------+-----------+-------------+--------------------------------------------------------------------+

As seen above, the second server has been added to the group and it has replicated the changes from server s1 automatically using distributed recovery. In other words, the transactions applied on s1 up to the point in time that s2 joined the group have been replicated to s2.

17.2.1.6.2 Adding Additional Instances

Adding additional instances to the group is essentially the same sequence of steps as adding the second server, except that the configuration has to be changed as it had to be for server s2. To summarise the required commands:

1. Create the configuration file

[mysqld]

#
# Disable other storage engines
#
disabled_storage_engines="MyISAM,BLACKHOLE,FEDERATED,ARCHIVE,MEMORY"

#
# Replication configuration parameters
#
server_id=3
gtid_mode=ON
enforce_gtid_consistency=ON
master_info_repository=TABLE
relay_log_info_repository=TABLE
binlog_checksum=NONE
log_slave_updates=ON
log_bin=binlog
binlog_format=ROW

#
# Group Replication configuration
#
group_replication_group_name="aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa"
group_replication_start_on_boot=off
group_replication_local_address= "s3:33061"
group_replication_group_seeds= "s1:33061,s2:33061,s3:33061"
group_replication_bootstrap_group= off

2. Start the server and connect to it. Configure the recovery credentials for the group_replication_recovery channel.

SET SQL_LOG_BIN=0;
CREATE USER rpl_user@'%' IDENTIFIED BY 'password';
GRANT REPLICATION SLAVE ON *.* TO rpl_user@'%';
FLUSH PRIVILEGES;
SET SQL_LOG_BIN=1;
CHANGE MASTER TO MASTER_USER='rpl_user', MASTER_PASSWORD='password'  \\
FOR CHANNEL 'group_replication_recovery';

4. Install the Group Replication plugin and start it.

INSTALL PLUGIN group_replication SONAME 'group_replication.so';
START GROUP_REPLICATION;

At this point server s3 is booted and running, has joined the group and caught up with the other servers in the group. Consulting the performance_schema.replication_group_members table again confirms this is the case.

mysql> SELECT * FROM performance_schema.replication_group_members;
+---------------------------+--------------------------------------+-------------+-------------+---------------+
| CHANNEL_NAME              | MEMBER_ID                            | MEMBER_HOST | MEMBER_PORT | MEMBER_STATE  |
+---------------------------+--------------------------------------+-------------+-------------+---------------+
| group_replication_applier | 395409e1-6dfa-11e6-970b-00212844f856 |   s1        |       3306  | ONLINE        |
| group_replication_applier | 7eb217ff-6df3-11e6-966c-00212844f856 |   s3        |       3306  | ONLINE        |
| group_replication_applier | ac39f1e6-6dfa-11e6-a69d-00212844f856 |   s2        |       3306  | ONLINE        |
+---------------------------+--------------------------------------+-------------+-------------+---------------+

Issuing this same query on server s2 or server s1 yields the same result. Also, you can verify that server s3 has caught up:

mysql> SHOW DATABASES LIKE 'test';
+-----------------+
| Database (test) |
+-----------------+
| test            |
+-----------------+

mysql> SELECT * FROM test.t1;
+----+------+
| c1 | c2   |
+----+------+
|  1 | Luis |
+----+------+

mysql> SHOW BINLOG EVENTS;
+---------------+------+----------------+-----------+-------------+--------------------------------------------------------------------+
| Log_name      | Pos  | Event_type     | Server_id | End_log_pos | Info                                                               |
+---------------+------+----------------+-----------+-------------+--------------------------------------------------------------------+
| binlog.000001 |    4 | Format_desc    |         3 |         123 | Server ver: 5.7.34-log, Binlog ver: 4                              |
| binlog.000001 |  123 | Previous_gtids |         3 |         150 |                                                                    |
| binlog.000001 |  150 | Gtid           |         1 |         211 | SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:1'  |
| binlog.000001 |  211 | Query          |         1 |         270 | BEGIN                                                              |
| binlog.000001 |  270 | View_change    |         1 |         369 | view_id=14724832985483517:1                                        |
| binlog.000001 |  369 | Query          |         1 |         434 | COMMIT                                                             |
| binlog.000001 |  434 | Gtid           |         1 |         495 | SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:2'  |
| binlog.000001 |  495 | Query          |         1 |         585 | CREATE DATABASE test                                               |
| binlog.000001 |  585 | Gtid           |         1 |         646 | SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:3'  |
| binlog.000001 |  646 | Query          |         1 |         770 | use `test`; CREATE TABLE t1 (c1 INT PRIMARY KEY, c2 TEXT NOT NULL) |
| binlog.000001 |  770 | Gtid           |         1 |         831 | SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:4'  |
| binlog.000001 |  831 | Query          |         1 |         890 | BEGIN                                                              |
| binlog.000001 |  890 | Table_map      |         1 |         933 | table_id: 108 (test.t1)                                            |
| binlog.000001 |  933 | Write_rows     |         1 |         975 | table_id: 108 flags: STMT_END_F                                    |
| binlog.000001 |  975 | Xid            |         1 |        1002 | COMMIT /* xid=29 */                                                |
| binlog.000001 | 1002 | Gtid           |         1 |        1063 | SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:5'  |
| binlog.000001 | 1063 | Query          |         1 |        1122 | BEGIN                                                              |
| binlog.000001 | 1122 | View_change    |         1 |        1261 | view_id=14724832985483517:2                                        |
| binlog.000001 | 1261 | Query          |         1 |        1326 | COMMIT                                                             |
| binlog.000001 | 1326 | Gtid           |         1 |        1387 | SET @@SESSION.GTID_NEXT= 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa:6'  |
| binlog.000001 | 1387 | Query          |         1 |        1446 | BEGIN                                                              |
| binlog.000001 | 1446 | View_change    |         1 |        1585 | view_id=14724832985483517:3                                        |
| binlog.000001 | 1585 | Query          |         1 |        1650 | COMMIT                                                             |
+---------------+------+----------------+-----------+-------------+--------------------------------------------------------------------+

17.2.2 Deploying Group Replication Locally

The most common way to deploy Group Replication is using multiple server instances, to provide high availability. It is also possible to deploy Group Replication locally, for example for testing purposes. This section explains how you can deploy Group Replication locally.

Important

Group Replication is usually deployed on multiple hosts because this ensures that high-availability is provided. The instructions in this section are not suitable for production deployments because all MySQL server instances are running on the same single host. In the event of failure of this host, the whole group fails. Therefore this information should be used for testing purposes and it should not be used in a production environments.

This section explains how to create a replication group with three MySQL Server instances on one physical machine. This means that three data directories are needed, one per server instance, and that you need to configure each instance independently. This - procedure assumes that MySQL Server was downloaded and unpacked - into the directory named mysql-5.7. Each MySQL server instance requires a specific data directory. Create a directory named data, then in that directory create a subdirectory for each server instance, for example s1, s2 and s3, and initialize each one.

mysql-5.7/bin/mysqld --initialize-insecure --basedir=$PWD/mysql-5.7 --datadir=$PWD/data/s1
mysql-5.7/bin/mysqld --initialize-insecure --basedir=$PWD/mysql-5.7 --datadir=$PWD/data/s2
mysql-5.7/bin/mysqld --initialize-insecure --basedir=$PWD/mysql-5.7 --datadir=$PWD/data/s3

Inside data/s1, data/s2, data/s3 is an initialized data directory, containing the mysql system database and related tables and much more. To learn more about the initialization procedure, see Section 2.10.1, “Initializing the Data Directory”.

Warning

Do not use -initialize-insecure in production environments, it is only used here to simplify the tutorial. For more information on security settings, see Section 17.5, “Group Replication Security”.

Configuration of Local Group Replication Members

When you are following Section 17.2.1.2, “Configuring an Instance for Group Replication”, you need to add configuration for the data directories added in the previous section. For example:

[mysqld]

# server configuration
datadir=<full_path_to_data>/data/s1
basedir=<full_path_to_bin>/mysql-8.0/

port=24801
socket=<full_path_to_sock_dir>/s1.sock

These settings configure MySQL server to use the data directory created earlier and which port the server should open and start listening for incoming connections.

Note

The non-default port of 24801 is used because in this tutorial the three server instances use the same hostname. In a setup with three different machines this would not be required.

Group Replication requires a network connection between the members, which means that each member must be able to resolve the network address of all of the other members. For example in this tutorial all three instances run on one machine, so to ensure that the members can contact each other you could add a line to the option file such as report_host=127.0.0.1.

Then each member needs to be able to connect to the other members on their group_replication_local_address. For example in the option file of member s1 add:

group_replication_local_address= "127.0.0.1:24901"
group_replication_group_seeds= "127.0.0.1:24901,127.0.0.1:24902,127.0.0.1:24903"

This configures s1 to use port 24901 for internal group communication with seed members. For each server instance you want to add to the group, make these changes in the option file of the member. For each member you must ensure a unique address is specified, so use a unique port per instance for group_replication_local_address. Usually you want all members to be able to serve as seeds for members that are joining the group and have not got the transactions processed by the group. In this case, add all of the ports to group_replication_group_seeds as shown above.

The remaining steps of Section 17.2.1, “Deploying Group Replication in Single-Primary Mode” apply equally to a group which you have deployed locally in this way.

17.3 Monitoring Group Replication

Use the Perfomance Schema tables to monitor Group Replication, assuming that the Performance Schema is enabled. Group Replication adds the following tables:

These Perfomance Schema replication tables also show information about Group Replication:

  • performance_schema.replication_connection_status shows information regarding Group Replication, for example the transactions that have been received from the group and queued in the applier queue (the relay log).

  • performance_schema.replication_applier_status shows the state of the Group Replication related channels and threads If there are many different worker threads applying transactions, then the worker tables can also be used to monitor what each worker thread is doing.

The replication channels created by the Group Replication plugin are named:

  • group_replication_recovery - This channel is used for the replication changes that are related to the distributed recovery phase.

  • group_replication_applier - This channel is used for the incoming changes from the group. This is the channel used to apply transactions coming directly from the group.

The following sections describe how to interpret the information available.

17.3.1 Group Replication Server States

There are various states that a server instance can be in. If servers are communicating properly, all report the same states for all servers. However, if there is a network partition, or a server leaves the group, then different information could be reported, depending on which server is queried. If the server has left the group then it cannot report updated information about the other servers' states. If there is a partition, such that quorum is lost, servers are not able to coordinate between themselves. As a consequence, they cannot guess what the status of different servers is. Therefore, instead of guessing their state they report that some servers are unreachable.

Table 17.1 Server State

Field

Description

Group Synchronized

ONLINE

The member is ready to serve as a fully functional group member, meaning that the client can connect and start executing transactions.

Yes

RECOVERING

The member is in the process of becoming an active member of the group and is currently going through the recovery process, receiving state information from a donor.

No

OFFLINE

The plugin is loaded but the member does not belong to any group.

No

ERROR

The state of the member. Whenever there is an error on the recovery phase or while applying changes, the server enters this state.

No

UNREACHABLE

Whenever the local failure detector suspects that a given server is not reachable, because for example it was disconnected involuntarily, it shows that server's state as UNREACHABLE.

No


Important

Once an instance enters ERROR state, the super_read_only option is set to ON. To leave the ERROR state you must manually configure the instance with super_read_only=OFF.

Note that Group Replication is not synchronous, but eventually synchronous. More precisely, transactions are delivered to all group members in the same order, but their execution is not synchronized, meaning that after a transaction is accepted to be committed, each member commits at its own pace.

17.3.2 The replication_group_members Table

The performance_schema.replication_group_members table is used for monitoring the status of the different server instances that are members of the group. The information in the table is updated whenever there is a view change, for example when the configuration of the group is dynamically changed when a new member joins. At that point, servers exchange some of their metadata to synchronize themselves and continue to cooperate together. The information is shared between all the server instances that are members of the replication group, so information on all the group members can be queried from any member. This table can be used to get a high level view of the state of a replication group, for example by issuing:

SELECT * FROM performance_schema.replication_group_members;
+---------------------------+--------------------------------------+--------------+-------------+--------------+
| CHANNEL_NAME              | MEMBER_ID	                           | MEMBER_HOST  | MEMBER_PORT | MEMBER_STATE |
+---------------------------+--------------------------------------+--------------+-------------+--------------+
| group_replication_applier | 041f26d8-f3f3-11e8-adff-080027337932 | example1     |      3306   | ONLINE       |
| group_replication_applier | f60a3e10-f3f2-11e8-8258-080027337932 | example2     |      3306   | ONLINE       |
| group_replication_applier | fc890014-f3f2-11e8-a9fd-080027337932 | example3     |      3306   | ONLINE       |
+---------------------------+--------------------------------------+--------------+-------------+--------------+

Based on this result we can see that the group consists of three members, each member's host and port number which clients use to connect to the member, and the server_uuid of the member. The MEMBER_STATE column shows one of the Section 17.3.1, “Group Replication Server States”, in this case it shows that all three members in this group are ONLINE, and the MEMBER_ROLE column shows that there are two secondaries, and a single primary. Therefore this group must be running in single-primary mode. The MEMBER_VERSION column can be useful when you are upgrading a group and are combining members running different MySQL versions. See Section 17.3.1, “Group Replication Server States” for more information.

For more information about the Member_host value and its impact on the distributed recovery process, see Section 17.2.1.3, “User Credentials”.

17.3.3 The replication_group_member_stats Table

Each member in a replication group certifies and applies transactions received by the group. Statistics regarding the certifier and applier procedures are useful to understand how the applier queue is growing, how many conflicts have been found, how many transactions were checked, which transactions are committed everywhere, and so on.

The performance_schema.replication_group_member_stats table provides group-level information related to the certification process, and also statistics for the transactions received and originated by each individual member of the replication group. The information is shared between all the server instances that are members of the replication group, so information on all the group members can be queried from any member. Note that refreshing of statistics for remote members is controlled by the message period specified in the group_replication_flow_control_period option, so these can differ slightly from the locally collected statistics for the member where the query is made. To use this table to monitor a Group Replication member, issue:

mysql> SELECT * FROM performance_schema.replication_group_member_stats\G

These fields are important for monitoring the performance of the members connected in the group. For example, suppose that one of the group’s members always reports a large number of transactions in its queue compared to other members. This means that the member is delayed and is not able to keep up to date with the other members of the group. Based on this information, you could decide to either remove the member from the group, or delay the processing of transactions on the other members of the group in order to reduce the number of queued transactions. This information can also help you to decide how to adjust the flow control of the Group Replication plugin, see Section 17.9.7.3, “Flow Control”.

17.4 Group Replication Operations

This section describes the different modes of deploying Group Replication, explains common operations for managing groups and provides information about how to tune your groups. .

17.4.1 Deploying in Multi-Primary or Single-Primary Mode

Group Replication operates in the following different modes:

  • single-primary mode

  • multi-primary mode

The default mode is single-primary. It is not possible to have members of the group deployed in different modes, for example one configured in multi-primary mode while another one is in single-primary mode. To switch between modes, the group and not the server, needs to be restarted with a different operating configuration. Regardless of the deployed mode, Group Replication does not handle client-side fail-over, that must be handled by the application itself, a connector or a middleware framework such as a proxy or MySQL Router 8.0.

When deployed in multi-primary mode, statements are checked to ensure they are compatible with the mode. The following checks are made when Group Replication is deployed in multi-primary mode:

  • If a transaction is executed under the SERIALIZABLE isolation level, then its commit fails when synchronizing itself with the group.

  • If a transaction executes against a table that has foreign keys with cascading constraints, then the transaction fails to commit when synchronizing itself with the group.

These checks can be deactivated by setting the option group_replication_enforce_update_everywhere_checks to FALSE. When deploying in single-primary mode, this option must be set to FALSE.

17.4.1.1 Single-Primary Mode

In this mode the group has a single-primary server that is set to read-write mode. All the other members in the group are set to read-only mode (with super-read-only=ON ). This happens automatically. The primary is typically the first server to bootstrap the group, all other servers that join automatically learn about the primary server and are set to read only.

Figure 17.5 New Primary Election

Five server instances, S1, S2, S3, S4, and S5, are deployed as an interconnected group. Server S1 is the primary. Write clients are communicating with server S1, and a read client is communicating with server S4. Server S1 then fails, breaking communication with the write clients. Server S2 then takes over as the new primary, and the write clients now communicate with server S2.

When in single-primary mode, some of the checks deployed in multi-primary mode are disabled, because the system enforces that only a single server writes to the group. For example, changes to tables that have cascading foreign keys are allowed, whereas in multi-primary mode they are not. Upon primary member failure, an automatic primary election mechanism chooses the new primary member. The election process is performed by looking at the new view, and ordering the potential new primaries based on the value of group_replication_member_weight. Assuming the group is operating with all members running the same MySQL version, then the member with the highest value for group_replication_member_weight is elected as the new primary. In the event that multiple servers have the same group_replication_member_weight, the servers are then prioritized based on their server_uuid in lexicographical order and by picking the first one. Once a new primary is elected, it is automatically set to read-write and the other secondaries remain as secondaries, and as such, read-only.

When a new primary is elected, it is only writable once it has processed all of the transactions that came from the old primary. This avoids possible concurrency issues between old transactions from the old primary and the new ones being executed on this member. It is a good practice to wait for the new primary to apply its replication related relay-log before re-routing client applications to it.

If the group is operating with members that are running different versions of MySQL then the election process can be impacted. For example, if any member does not support group_replication_member_weight, then the primary is chosen based on server_uuid order from the members of the lower major version. Alternatively, if all members running different MySQL versions do support group_replication_member_weight, the primary is chosen based on group_replication_member_weight from the members of the lower major version.

17.4.1.2 Multi-Primary Mode

In multi-primary mode, there is no notion of a single primary. There is no need to engage an election procedure because there is no server playing any special role.

Figure 17.6 Client Failover

Five server instances, S1, S2, S3, S4, and S5, are deployed as an interconnected group. All of the servers are primaries. Write clients are communicating with servers S1 and S2, and a read client is communicating with server S4. Server S1 then fails, breaking communication with its write client. This client reconnects to server S3.

All servers are set to read-write mode when joining the group.

17.4.1.3 Finding the Primary

The following example shows how to find out which server is currently the primary when deployed in single-primary mode.

mysql> SHOW STATUS LIKE 'group_replication_primary_member'

17.4.2 Tuning Recovery

Whenever a new member joins a replication group, it connects to a suitable donor and fetches the data that it has missed up until the point it is declared online. This critical component in Group Replication is fault tolerant and configurable. The following section explains how recovery works and how to tune the settings

Donor Selection

A random donor is selected from the existing online members in the group. This way there is a good chance that the same server is not selected more than once when multiple members enter the group.

If the connection to the selected donor fails, a new connection is automatically attempted to a new candidate donor. Once the connection retry limit is reached the recovery procedure terminates with an error.

Note

A donor is picked randomly from the list of online members in the current view.

Enhanced Automatic Donor Switchover

The other main point of concern in recovery as a whole is to make sure that it copes with failures. Hence, Group Replication provides robust error detection mechanisms. In earlier versions of Group Replication, when reaching out to a donor, recovery could only detect connection errors due to authentication issues or some other problem. The reaction to such problematic scenarios was to switch over to a new donor thus a new connection attempt was made to a different member.

This behavior was extended to also cover other failure scenarios:

  • Purged data scenarios - If the selected donor contains some purged data that is needed for the recovery process then an error occurs. Recovery detects this error and a new donor is selected.

  • Duplicated data - If a server joining the group already contains some data that conflicts with the data coming from the selected donor during recovery then an error occurs. This could be caused by some errant transactions present in the server joining the group.

    One could argue that recovery should fail instead of switching over to another donor, but in heterogeneous groups there is chance that other members share the conflicting transactions and others do not. For that reason, upon error, recovery selects another donor from the group.

  • Other errors - If any of the recovery threads fail (receiver or applier threads fail) then an error occurs and recovery switches over to a new donor.

Note

In case of some persistent failures or even transient failures recovery automatically retries connecting to the same or a new donor.

Donor Connection Retries

The recovery data transfer relies on the binary log and existing MySQL replication framework, therefore it is possible that some transient errors could cause errors in the receiver or applier threads. In such cases, the donor switch over process has retry functionality, similar to that found in regular replication.

Number of Attempts

The number of attempts a server joining the group makes when trying to connect to a donor from the pool of donors is 10. This is configured through the group_replication_recovery_retry_count plugin variable . The following command sets the maximum number of attempts to connect to a donor to 10.

mysql> SET GLOBAL group_replication_recovery_retry_count= 10;

Note that this accounts for the global number of attempts that the server joining the group makes connecting to each one of the suitable donors.

Sleep Routines

The group_replication_recovery_reconnect_interval plugin variable defines how much time the recovery process should sleep between donor connection attempts. This variable has its default set to 60 seconds and you can change this value dynamically. The following command sets the recovery donor connection retry interval to 120 seconds.

mysql> SET GLOBAL group_replication_recovery_reconnect_interval= 120;

Note, however, that recovery does not sleep after every donor connection attempt. As the server joining the group is connecting to different servers and not to the same one over and over again, it can assume that the problem that affects server A does not affect server B. As such, recovery suspends only when it has gone through all the possible donors. Once the server joining the group has tried to connect to all the suitable donors in the group and none remains, the recovery process sleeps for the number of seconds configured by the group_replication_recovery_reconnect_interval variable.

17.4.3 Network Partitioning

The group needs to achieve consensus whenever a change that needs to be replicated happens. This is the case for regular transactions but is also required for group membership changes and some internal messaging that keeps the group consistent. Consensus requires a majority of group members to agree on a given decision. When a majority of group members is lost, the group is unable to progress and blocks because it cannot secure majority or quorum.

Quorum may be lost when there are multiple involuntary failures, causing a majority of servers to be removed abruptly from the group. For example in a group of 5 servers, if 3 of them become silent at once, the majority is compromised and thus no quorum can be achieved. In fact, the remaining two are not able to tell if the other 3 servers have crashed or whether a network partition has isolated these 2 alone and therefore the group cannot be reconfigured automatically.

On the other hand, if servers exit the group voluntarily, they instruct the group that it should reconfigure itself. In practice, this means that a server that is leaving tells others that it is going away. This means that other members can reconfigure the group properly, the consistency of the membership is maintained and the majority is recalculated. For example, in the above scenario of 5 servers where 3 leave at once, if the 3 leaving servers warn the group that they are leaving, one by one, then the membership is able to adjust itself from 5 to 2, and at the same time, securing quorum while that happens.

Note

Loss of quorum is by itself a side-effect of bad planning. Plan the group size for the number of expected failures (regardless whether they are consecutive, happen all at once or are sporadic).

The following sections explain what to do if the system partitions in such a way that no quorum is automatically achieved by the servers in the group.

Tip

A primary that has been excluded from a group after a majority loss followed by a reconfiguration can contain extra transactions that are not included in the new group. If this happens, the attempt to add back the excluded member from the group results in an error with the message This member has more executed transactions than those present in the group.

Detecting Partitions

The replication_group_members performance schema table presents the status of each server in the current view from the perspective of this server. The majority of the time the system does not run into partitioning, and therefore the table shows information that is consistent across all servers in the group. In other words, the status of each server on this table is agreed by all in the current view. However, if there is network partitioning, and quorum is lost, then the table shows the status UNREACHABLE for those servers that it cannot contact. This information is exported by the local failure detector built into Group Replication.

Figure 17.7 Losing Quorum

Five server instances, S1, S2, S3, S4, and S5, are deployed as an interconnected group, which is a stable group. When three of the servers, S3, S4, and S5, fail, the majority is lost and the group can no longer proceed without intervention.

To understand this type of network partition the following section describes a scenario where there are initially 5 servers working together correctly, and the changes that then happen to the group once only 2 servers are online. The scenario is depicted in the figure.

As such, lets assume that there is a group with these 5 servers in it:

  • Server s1 with member identifier 199b2df7-4aaf-11e6-bb16-28b2bd168d07

  • Server s2 with member identifier 199bb88e-4aaf-11e6-babe-28b2bd168d07

  • Server s3 with member identifier 1999b9fb-4aaf-11e6-bb54-28b2bd168d07

  • Server s4 with member identifier 19ab72fc-4aaf-11e6-bb51-28b2bd168d07

  • Server s5 with member identifier 19b33846-4aaf-11e6-ba81-28b2bd168d07

Initially the group is running fine and the servers are happily communicating with each other. You can verify this by logging into s1 and looking at its replication_group_members performance schema table. For example:

mysql> SELECT MEMBER_ID,MEMBER_STATE, MEMBER_ROLE FROM performance_schema.replication_group_members;
+--------------------------------------+--------------+-------------+
| MEMBER_ID                            | MEMBER_STATE |-MEMBER_ROLE |
+--------------------------------------+--------------+-------------+
| 1999b9fb-4aaf-11e6-bb54-28b2bd168d07 | ONLINE       | SECONDARY   |
| 199b2df7-4aaf-11e6-bb16-28b2bd168d07 | ONLINE       | PRIMARY     |
| 199bb88e-4aaf-11e6-babe-28b2bd168d07 | ONLINE       | SECONDARY   |
| 19ab72fc-4aaf-11e6-bb51-28b2bd168d07 | ONLINE       | SECONDARY   |
| 19b33846-4aaf-11e6-ba81-28b2bd168d07 | ONLINE       | SECONDARY   |
+--------------------------------------+--------------+-------------+

However, moments later there is a catastrophic failure and servers s3, s4 and s5 stop unexpectedly. A few seconds after this, looking again at the replication_group_members table on s1 shows that it is still online, but several others members are not. In fact, as seen below they are marked as UNREACHABLE. Moreover, the system could not reconfigure itself to change the membership, because the majority has been lost.

mysql> SELECT MEMBER_ID,MEMBER_STATE FROM performance_schema.replication_group_members;
+--------------------------------------+--------------+
| MEMBER_ID                            | MEMBER_STATE |
+--------------------------------------+--------------+
| 1999b9fb-4aaf-11e6-bb54-28b2bd168d07 | UNREACHABLE  |
| 199b2df7-4aaf-11e6-bb16-28b2bd168d07 | ONLINE       |
| 199bb88e-4aaf-11e6-babe-28b2bd168d07 | ONLINE       |
| 19ab72fc-4aaf-11e6-bb51-28b2bd168d07 | UNREACHABLE  |
| 19b33846-4aaf-11e6-ba81-28b2bd168d07 | UNREACHABLE  |
+--------------------------------------+--------------+

The table shows that s1 is now in a group that has no means of progressing without external intervention, because a majority of the servers are unreachable. In this particular case, the group membership list needs to be reset to allow the system to proceed, which is explained in this section. Alternatively, you could also choose to stop Group Replication on s1 and s2 (or stop completely s1 and s2), figure out what happened with s3, s4 and s5 and then restart Group Replication (or the servers).

Unblocking a Partition

Group replication enables you to reset the group membership list by forcing a specific configuration. For instance in the case above, where s1 and s2 are the only servers online, you could chose to force a membership configuration consisting of only s1 and s2. This requires checking some information about s1 and s2 and then using the group_replication_force_members variable.

Figure 17.8 Forcing a New Membership

Three of the servers in a group, S3, S4, and S5, have failed, so the majority is lost and the group can no longer proceed without intervention. With the intervention described in the following text, S1 and S2 are able to form a stable group by themselves.

Suppose that you are back in the situation where s1 and s2 are the only servers left in the group. Servers s3, s4 and s5 have left the group unexpectedly. To make servers s1 and s2 continue, you want to force a membership configuration that contains only s1 and s2.

Warning

This procedure uses group_replication_force_members and should be considered a last resort remedy. It must be used with extreme care and only for overriding loss of quorum. If misused, it could create an artificial split-brain scenario or block the entire system altogether.

Recall that the system is blocked and the current configuration is the following (as perceived by the local failure detector on s1):

mysql> SELECT MEMBER_ID,MEMBER_STATE FROM performance_schema.replication_group_members;
+--------------------------------------+--------------+
| MEMBER_ID                            | MEMBER_STATE |
+--------------------------------------+--------------+
| 1999b9fb-4aaf-11e6-bb54-28b2bd168d07 | UNREACHABLE  |
| 199b2df7-4aaf-11e6-bb16-28b2bd168d07 | ONLINE       |
| 199bb88e-4aaf-11e6-babe-28b2bd168d07 | ONLINE       |
| 19ab72fc-4aaf-11e6-bb51-28b2bd168d07 | UNREACHABLE  |
| 19b33846-4aaf-11e6-ba81-28b2bd168d07 | UNREACHABLE  |
+--------------------------------------+--------------+

The first thing to do is to check what is the local address (group communication identifier) for s1 and s2. Log in to s1 and s2 and get that information as follows.

mysql> SELECT @@group_replication_local_address;

Once you know the group communication addresses of s1 (127.0.0.1:10000) and s2 (127.0.0.1:10001), you can use that on one of the two servers to inject a new membership configuration, thus overriding the existing one that has lost quorum. To do that on s1:

mysql> SET GLOBAL group_replication_force_members="127.0.0.1:10000,127.0.0.1:10001";

This unblocks the group by forcing a different configuration. Check replication_group_members on both s1 and s2 to verify the group membership after this change. First on s1.

mysql> SELECT MEMBER_ID,MEMBER_STATE FROM performance_schema.replication_group_members;
+--------------------------------------+--------------+
| MEMBER_ID                            | MEMBER_STATE |
+--------------------------------------+--------------+
| b5ffe505-4ab6-11e6-b04b-28b2bd168d07 | ONLINE       |
| b60907e7-4ab6-11e6-afb7-28b2bd168d07 | ONLINE       |
+--------------------------------------+--------------+

And then on s2.

mysql> SELECT * FROM performance_schema.replication_group_members;
+--------------------------------------+--------------+
| MEMBER_ID                            | MEMBER_STATE |
+--------------------------------------+--------------+
| b5ffe505-4ab6-11e6-b04b-28b2bd168d07 | ONLINE       |
| b60907e7-4ab6-11e6-afb7-28b2bd168d07 | ONLINE       |
+--------------------------------------+--------------+

When forcing a new membership configuration, make sure that any servers are going to be forced out of the group are indeed stopped. In the scenario depicted above, if s3, s4 and s5 are not really unreachable but instead are online, they may have formed their own functional partition (they are 3 out of 5, hence they have the majority). In that case, forcing a group membership list with s1 and s2 could create an artificial split-brain situation. Therefore it is important before forcing a new membership configuration to ensure that the servers to be excluded are indeed shutdown and if they are not, shut them down before proceeding.

After you have used the group_replication_force_members system variable to successfully force a new group membership and unblock the group, ensure that you clear the system variable. group_replication_force_members must be empty in order to issue a START GROUP_REPLICATION statement.

17.4.4 Using MySQL Enterprise Backup with Group Replication

MySQL Enterprise Backup is a commercially-licensed backup utility for MySQL Server, available with MySQL Enterprise Edition. This section explains how to back up and subsequently restore a Group Replication member using MySQL Enterprise Backup. The same technique can be used to quickly add a new member to a group.

Backing up a Group Replication Member Using MySQL Enterprise Backup

Backing up a Group Replication member is similar to backing up a stand-alone MySQL instance. The following instructions assume that you are already familiar with how to use MySQL Enterprise Backup to perform a backup; if that is not the case, please review the MySQL Enterprise Backup 4.1 User's Guide, especially Backing Up a Database Server. Also note the requirements described in Grant MySQL Privileges to Backup Administrator and Using MySQL Enterprise Backup with Group Replication.

Consider the following group with three members, s1, s2, and s3, running on hosts with the same names:

mysql> SELECT member_host, member_port, member_state FROM performance_schema.replication_group_members;
+-------------+-------------+--------------+
| member_host | member_port | member_state |
+-------------+-------------+--------------+
| s1          |        3306 | ONLINE       |
| s2          |        3306 | ONLINE       |
| s3          |        3306 | ONLINE       |
+-------------+-------------+--------------+

Using MySQL Enterprise Backup, create a backup of s2 by issuing on its host, for example, the following command:

s2> mysqlbackup --defaults-file=/etc/my.cnf --backup-image=/backups/my.mbi_`date +%d%m_%H%M` \
		      --backup-dir=/backups/backup_`date +%d%m_%H%M` --user=root -p \
--host=127.0.0.1 backup-to-image
Note
  • When backing up a secondary member, as MySQL Enterprise Backup cannot write backup status and metadata to a read-only server instance, it might issue warnings similar to the following one during the backup operation:

    181113 21:31:08 MAIN WARNING: This backup operation cannot write to backup
    progress. The MySQL server is running with the --super-read-only option.

    You can avoid the warning by using the --no-history-logging option with your backup command.

Restoring a Failed Member

Assume one of the members (s3 in the following example) is irreconcilably corrupted. The most recent backup of group member s2 can be used to restore s3. Here are the steps for performing the restore:

  1. Copy the backup of s2 onto the host for s3. The exact way to copy the backup depends on the operating system and tools available to you. In this example, we assume the hosts are both Linux servers and use SCP to copy the files between them:

    s2/backups> scp my.mbi_2206_1429 s3:/backups
  2. Restore the backup. Connect to the target host (the host for s3 in this case), and restore the backup using MySQL Enterprise Backup. Here are the steps:

    1. Stop the corrupted server, if it is still running. For example, on Linux distributions that use systemd:

      s3> systemctl stop mysqld
    2. Preserve the configuration file auto.cnf, located in the corrupted server's data directory, by copying it to a safe location outside of the data directory. This is for preserving the server's UUID, which is needed later.

    3. Delete all contents in the data directory of s3. For example:

      s3> rm -rf /var/lib/mysql/*

      If the system variables innodb_data_home_dir, innodb_log_group_home_dir, and innodb_undo_directory point to any directories other than the data directory, they should also be made empty; otherwise, the restore operation fails.

    4. Restore backup of s2 onto the host for s3:

      s3> mysqlbackup --defaults-file=/etc/my.cnf \
        --datadir=/var/lib/mysql \
        --backup-image=/backups/my.mbi_2206_1429  \
      --backup-dir=/tmp/restore_`date +%d%m_%H%M` copy-back-and-apply-log
      Note

      The command above assumes that the binary logs and relay logs on s2 and s3 have the same base name and are at the same location on the two servers. If these conditions are not met, for MySQL Enterprise Backup 4.1.2 and later, you should use the --log-bin and --relay-log options to restore the binary log and relay log to their original file paths on s3. For example, if you know that on s3 the binary log's base name is s3-bin and the relay-log's base name is s3-relay-bin, your restore command should look like:

      mysqlbackup --defaults-file=/etc/my.cnf \
        --datadir=/var/lib/mysql \
        --backup-image=/backups/my.mbi_2206_1429  \
        --log-bin=s3-bin --relay-log=s3-relay-bin \
        --backup-dir=/tmp/restore_`date +%d%m_%H%M` copy-back-and-apply-log

      Being able to restore the binary log and relay log to the right file paths makes the restore process easier; if that is impossible for some reason, see Rebuild the Failed Member to Rejoin as a New Member.

  3. Restore the auto.cnf file for s3. To rejoin the replication group, the restored member must have the same server_uuid it used to join the group before. Supply the old server UUID by copying the auto.cnf file preserved in step 2 above into the data directory of the restored member.

    Note

    If you cannot supply the failed member's original server_uuid to the restored member by restoring its old auto.cnf file, you must let the restored member join the group as a new member; see instructions in Rebuild the Failed Member to Rejoin as a New Member below on how to do that.

  4. Start the restored server. For example, on Linux distributions that use systemd:

    systemctl start mysqld
    Note

    If the server you are restoring is a primary member, perform the steps described in Restoring a Primary Member before starting the restored server.

  5. Restart Group Replication. Connect to the restarted s3 using, for example, a mysql client, and issue the following command:

    mysql> START GROUP_REPLICATION;

    Before the restored instance can become an online member of the group, it needs to apply any transactions that have happened to the group after the backup was taken; this is achieved using Group Replication's distributed recovery mechanism, and the process starts after the START GROUP_REPLICATION statement has been issued. To check the member status of the restored instance, issue:

    mysql> SELECT member_host, member_port, member_state FROM performance_schema.replication_group_members;
    +-------------+-------------+--------------+
    | member_host | member_port | member_state |
    +-------------+-------------+--------------+
    | s1          |        3306 | ONLINE       |
    | s2          |        3306 | ONLINE       |
    | s3          |        3306 | RECOVERING   |
    +-------------+-------------+--------------+
    

    This shows that s3 is applying transactions to catch up with the group. Once it has caught up with the rest of the group, its member_state changes to ONLINE:

    mysql> SELECT member_host, member_port, member_state FROM performance_schema.replication_group_members;
    +-------------+-------------+--------------+
    | member_host | member_port | member_state |
    +-------------+-------------+--------------+
    | s1          |        3306 | ONLINE       |
    | s2          |        3306 | ONLINE       |
    | s3          |        3306 | ONLINE       |
    +-------------+-------------+--------------+
    
    Note

    If the server you are restoring is a primary member, once it has gained synchrony with the group and become ONLINE, perform the steps described at the end of Restoring a Primary Member to revert the configuration changes you had made to the server before you started it.

The member has now been fully restored from the backup and functions as a regular member of the group.

Rebuild the Failed Member to Rejoin as a New Member

Sometimes, the steps outlined above in Restoring a Failed Member cannot be carried out because, for example, the binary log or relay log is corrupted, or it is just missing from the backup. In such a situation, use the backup to rebuild the member, and then add it to the group as a new member. In the steps below, we assume the rebuilt member is named s3, like the failed member, and it is run on the same host as s3 was:

  1. Copy the backup of s2 onto the host for s3 . The exact way to copy the backup depends on the operating system and tools available to you. In this example we assume the hosts are both Linux servers and use SCP to copy the files between them:

    s2/backups> scp my.mbi_2206_1429 s3:/backups
  2. Restore the backup. Connect to the target host (the host for s3 in this case), and restore the backup using MySQL Enterprise Backup. Here are the steps:

    1. Stop the corrupted server, if it is still running. For example, on Linux distributions that use systemd:

      s3> systemctl stop mysqld
    2. Delete all contents in the data directory of s3. For example:

      s3> rm -rf /var/lib/mysql/*

      If the system variables innodb_data_home_dir, innodb_log_group_home_dir, and innodb_undo_directory point to any directories other than the data directory, they should also be made empty; otherwise, the restore operation fails.

    3. Restore the backup of s2 onto the host of s3. With this approach, we are rebuilding s3 as a new member, for which we do not need or do not want to use the old binary and relay logs in the backup; therefore, if these logs have been included in your backup, exclude them using the --skip-binlog and --skip-relaylog options:

      s3> mysqlbackup --defaults-file=/etc/my.cnf \
        --datadir=/var/lib/mysql \
        --backup-image=/backups/my.mbi_2206_1429  \
        --backup-dir=/tmp/restore_`date +%d%m_%H%M` \
        --skip-binlog --skip-relaylog \
      copy-back-and-apply-log
      Notes
      • If you have healthy binary log and relay logs in the backup that you can transfer onto the target host with no issues, you are recommended to follow the easier procedure as described in Restoring a Failed Member above.

      • Do NOT restore manually the corrupted server's auto.cnf file to the data directory of the new member—when the rebuilt s3 joins the group as a new member, it is going to be assigned a new server UUID.

  3. Start the restored server. For example, on Linux distributions that use systemd:

    systemctl start mysqld
    Note

    If the server you are restoring is a primary member, perform the steps described in Restoring a Primary Member before starting the restored server.

  4. Reconfigure the restored member to join Group Replication. Connect to the restored server with a mysql client and reset the source and replica information with the following commands:

    mysql> RESET MASTER;
    mysql> RESET SLAVE ALL;

    For the restored server to be able to recover automatically using Group Replication's built-in mechanism for distributed recovery, configure the server's gtid_executed variable. To do this, use the backup_gtid_executed.sql file included in the backup of s2, which is usually restored under the restored member's data directory. Disable binary logging, use the backup_gtid_executed.sql file to configure gtid_executed, and then re-enable binary logging by issuing the following statements with your mysql client:

    mysql> SET SQL_LOG_BIN=OFF;
    mysql> SOURCE datadir/backup_gtid_executed.sql
    mysql> SET SQL_LOG_BIN=ON;
    

    Then, configure the Group Replication user credentials on the member:

    mysql> CHANGE MASTER TO MASTER_USER='rpl_user', MASTER_PASSWORD='password' /
    		FOR CHANNEL 'group_replication_recovery';
  5. Restart Group Replication. Issue the following command to the restored server with your mysql client:

    mysql> START GROUP_REPLICATION;

    Before the restored instance can become an online member of the group, it needs to apply any transactions that have happened to the group after the backup was taken; this is achieved using Group Replication's distributed recovery mechanism, and the process starts after the START GROUP_REPLICATION statement has been issued. To check the member status of the restored instance, issue:

    mysql> SELECT member_host, member_port, member_state FROM performance_schema.replication_group_members;
    +-------------+-------------+--------------+
    | member_host | member_port | member_state |
    +-------------+-------------+--------------+
    | s3          |        3306 | RECOVERING   |
    | s2          |        3306 | ONLINE       |
    | s1          |        3306 | ONLINE       |
    +-------------+-------------+--------------+
    

    This shows that s3 is applying transactions to catch up with the group. Once it has caught up with the rest of the group, its member_state changes to ONLINE:

    mysql> SELECT member_host, member_port, member_state FROM performance_schema.replication_group_members;
    +-------------+-------------+--------------+
    | member_host | member_port | member_state |
    +-------------+-------------+--------------+
    | s3          |        3306 | ONLINE       |
    | s2          |        3306 | ONLINE       |
    | s1          |        3306 | ONLINE       |
    +-------------+-------------+--------------+
    
    Note

    If the server you are restoring is a primary member, once it has gained synchrony with the group and become ONLINE, perform the steps described at the end of Restoring a Primary Member to revert the configuration changes you had made to the server before you started it.

The member has now been restored to the group as a new member.

Restoring a Primary Member.  If the restored member is a primary in the group, care must be taken to prevent writes to the restored database during the Group Replication recovery phase: Depending on how the group is accessed by clients, there is a possibility of DML statements being executed on the restored member once it becomes accessible on the network, prior to the member finishing its catch-up on the activities it has missed while off the group. To avoid this, before starting the restored server, configure the following system variables in the server option file:

group_replication_start_on_boot=OFF
super_read_only=ON
event_scheduler=OFF

These settings ensure that the member becomes read-only at startup and that the event scheduler is turned off while the member is catching up with the group during the recovery phase. Adequate error handling must also be configured on the clients, as they are prevented temporarily from performing DML operations during this period on the restored member. Once the restore process is fully completed and the restored member is in-sync with the rest of the group, revert those changes; restart the event scheduler:

mysql> SET global event_scheduler=ON;

Edit the following system variables in the member's option file, so things are correctly configured for the next startup:

group_replication_start_on_boot=ON
super_read_only=OFF
event_scheduler=ON

17.5 Group Replication Security

This section explains how to secure a group, securing the connections between members of a group, or by establishing a security perimeter using IP address allowlisting.

17.5.1 Group Replication IP Address Allowlisting

The Group Replication plugin has a configuration option to determine from which hosts an incoming Group Communication System connection can be accepted. This option is called group_replication_ip_whitelist. If you set this option on a server s1, then when server s2 is establishing a connection to s1 for the purpose of engaging group communication, s1 first checks the allowlist before accepting the connection from s2. If s2 is in the allowlist, then s1 accepts the connection, otherwise s1 rejects the connection attempt by s2.

If you do not specify an allowlist explicitly, the group communication engine (XCom) automatically scans active interfaces on the host, and identifies those with addresses on private subnetworks. These addresses and the localhost IP address for IPv4 are used to create an automatic Group Replication allowlist. The automatic allowlist therefore includes any IP addresses found for the host in the following ranges:

10/8 prefix       (10.0.0.0 - 10.255.255.255) - Class A
172.16/12 prefix  (172.16.0.0 - 172.31.255.255) - Class B
192.168/16 prefix (192.168.0.0 - 192.168.255.255) - Class C
127.0.0.1 - localhost for IPv4

An entry is added to the error log stating the addresses that have been allowlisted automatically for the host.

The automatic allowlist of private addresses cannot be used for connections from servers outside the private network, so a server, even if it has interfaces on public IPs, does not by default allow Group Replication connections from external hosts. For Group Replication connections between server instances that are on different machines, you must provide public IP addresses and specify these as an explicit allowlist. If you specify any entries for the allowlist, the private and localhost addresses are not added automatically, so if you use any of these, you must specify them explicitly.

To specify an allowlist manually, use the group_replication_ip_whitelist option. You cannot change the allowlist on a server while it is an active member of a replication group. If the member is active, you must issue a STOP GROUP_REPLICATION statement before changing the allowlist, and a START GROUP_REPLICATION statement afterwards.

In the allowlist, you can specify any combination of the following:

  • IPv4 addresses (for example, 198.51.100.44)

  • IPv4 addresses with CIDR notation (for example, 192.0.2.21/24)

  • Host names, from MySQL 5.7.21 (for example, example.org)

  • Host names with CIDR notation, from MySQL 5.7.21 (for example, www.example.com/24)

IPv6 addresses, and host names that resolve to IPv6 addresses, are not supported in MySQL 5.7. You can use CIDR notation in combination with host names or IP addresses to allowlist a block of IP addresses with a particular network prefix, but do ensure that all the IP addresses in the specified subnet are under your control.

You must stop and restart Group Replication on a member in order to change its allowlist. A comma must separate each entry in the allowlist. For example:

mysql> STOP GROUP_REPLICATION;
mysql> SET GLOBAL group_replication_ip_whitelist="192.0.2.21/24,198.51.100.44,203.0.113.0/24,example.org,www.example.com/24";
mysql> START GROUP_REPLICATION;

The allowlist must contain the IP address or host name that is specified in each member's group_replication_local_address system variable. This address is not the same as the MySQL server SQL protocol host and port, and is not specified in the bind_address system variable for the server instance.

When a replication group is reconfigured (for example, when a new primary is elected or a member joins or leaves), the group members re-establish connections between themselves. If a group member is only allowlisted by servers that are no longer part of the replication group after the reconfiguration, it is unable to reconnect to the remaining servers in the replication group that do not allowlist it. To avoid this scenario entirely, specify the same allowlist for all servers that are members of the replication group.

Note

It is possible to configure different allowlists on different group members according to your security requirements, for example, in order to keep different subnets separate. If you need to configure different allowlists to meet your security requirements, ensure that there is sufficient overlap between the allowlists in the replication group to maximize the possibility of servers being able to reconnect in the absence of their original seed member.

For host names, name resolution takes place only when a connection request is made by another server. A host name that cannot be resolved is not considered for allowlist validation, and a warning message is written to the error log. Forward-confirmed reverse DNS (FCrDNS) verification is carried out for resolved host names.

Warning

Host names are inherently less secure than IP addresses in an allowlist. FCrDNS verification provides a good level of protection, but can be compromised by certain types of attack. Specify host names in your allowlist only when strictly necessary, and ensure that all components used for name resolution, such as DNS servers, are maintained under your control. You can also implement name resolution locally using the hosts file, to avoid the use of external components.

17.5.2 Group Replication Secure Socket Layer (SSL) Support

Group communication connections as well as recovery connections, are secured using SSL. The following sections explain how to configure connections.

Configuring SSL for Group Replication Recovery

Recovery is performed through a regular asynchronous replication connection. Once the donor is selected, the server joining the group establishes an asynchronous replication connection. This is all automatic.

However, a user that requires an SSL connection must have been created before the server joining the group connects to the donor. Typically, this is set up at the time one is provisioning a server to join the group.

donor> SET SQL_LOG_BIN=0;
donor> CREATE USER 'rec_ssl_user'@'%' REQUIRE SSL;
donor> GRANT replication slave ON *.* TO 'rec_ssl_user'@'%';
donor> SET SQL_LOG_BIN=1;

Assuming that all servers already in the group have a replication user set up to use SSL, you configure the server joining the group to use those credentials when connecting to the donor. That is done according to the values of the SSL options provided for the Group Replication plugin.

new_member> SET GLOBAL group_replication_recovery_use_ssl=1;
new_member> SET GLOBAL group_replication_recovery_ssl_ca= '.../cacert.pem';
new_member> SET GLOBAL group_replication_recovery_ssl_cert= '.../client-cert.pem';
new_member> SET GLOBAL group_replication_recovery_ssl_key= '.../client-key.pem';

And by configuring the recovery channel to use the credentials of the user that requires an SSL connection.

new_member> CHANGE MASTER TO MASTER_USER="rec_ssl_user" FOR CHANNEL "group_replication_recovery";
new_member> START GROUP_REPLICATION;

Configuring SSL for Group Communication

Secure sockets can be used to establish communication between members in a group. The configuration for this depends on the server's SSL configuration. As such, if the server has SSL configured, the Group Replication plugin also has SSL configured. For more information on the options for configuring the server SSL, see Command Options for Encrypted Connections. The options which configure Group Replication are shown in the following table.

Table 17.2 SSL Options

Server Configuration

Plugin Configuration Description

ssl_key

Path of key file. To be used as client and server certificate.

ssl_cert

Path of certificate file. To be used as client and server certificate.

ssl_ca

Path of file with SSL Certificate Authorities that are trusted.

ssl_capath

Path of directory containing certificates for SSL Certificate Authorities that are trusted.

ssl_crl

Path of file containing the certificate revocation lists.

ssl_crlpath

Path of directory containing revoked certificate lists.

ssl_cipher

Permitted ciphers to use while encrypting data over the connection.

tls_version

Secure communication uses this version and its protocols.


These options are MySQL Server configuration options which Group Replication relies on for its configuration. In addition there is the following Group Replication specific option to configure SSL on the plugin itself.

Table 17.3 group_replication_ssl_mode configuration values

Value

Description

DISABLED

Establish an unencrypted connection (default).

REQUIRED

Establish a secure connection if the server supports secure connections.

VERIFY_CA

Like REQUIRED, but additionally verify the server TLS certificate against the configured Certificate Authority (CA) certificates.

VERIFY_IDENTITY

Like VERIFY_CA, but additionally verify that the server certificate matches the host to which the connection is attempted.


The following example shows an example my.cnf file section used to configure SSL on a server and how activate it for Group Replication.

[mysqld]
ssl_ca = "cacert.pem"
ssl_capath = "/.../ca_directory"
ssl_cert = "server-cert.pem"
ssl_cipher = "DHE-RSA-AEs256-SHA"
ssl_crl = "crl-server-revoked.crl"
ssl_crlpath = "/.../crl_directory"
ssl_key = "server-key.pem"
group_replication_ssl_mode= REQUIRED

The only plugin specific configuration option that is listed is group_replication_ssl_mode. This option activates the SSL communication between members of the group, by configuring the SSL framework with the ssl_* parameters that are provided to the server.

17.5.3 Group Replication and Virtual Private Networks (VPNs)

There is nothing preventing Group Replication from operating over a virtual private network. At its core, it just relies on an IPv4 socket to establish connections between servers for the purpose of propagating messages between them.

17.6 Group Replication System Variables

This section lists the system variables that are specific to the Group Replication plugin. Every configuration option is prefixed with "group_replication".

Most system variables for Group Replication are described as dynamic, and their values can be changed while the server is running. However, in most cases, the change only takes effect after you stop and restart Group Replication on the group member using a STOP GROUP_REPLICATION statement followed by a START GROUP_REPLICATION statement. Changes to the following system variables take effect without stopping and restarting Group Replication:

Most system variables for Group Replication can have different values on different group members. For the following system variables, it is advisable to set the same value on all members of a group in order to avoid unnecessary rollback of transactions, failure of message delivery, or failure of message recovery:

Some system variables on a Group Replication group member, including some Group Replication-specific system variables and some general system variables, are group-wide configuration settings. These system variables must have the same value on all group members, cannot be changed while Group Replication is running, and require a full reboot of the group (a bootstrap by a server with group_replication_bootstrap_group=ON) in order for the value change to take effect. These conditions apply to the following system variables:

Important
  • A number of system variables for Group Replication are not completely validated during server startup if they are passed as command line arguments to the server. These system variables include group_replication_group_name, group_replication_single_primary_mode, group_replication_force_members, the SSL variables, and the flow control system variables. They are only fully validated after the server has started.

  • System variables for Group Replication that specify IP addresses or host names for group members are not validated until a START GROUP_REPLICATION statement is issued. Group Replication's Group Communication System (GCS) is not available to validate the values until that point.

The system variables that are specific to the Group Replication plugin are as follows:

  • group_replication_allow_local_disjoint_gtids_join

    Command-Line Format --group-replication-allow-local-disjoint-gtids-join[={OFF|ON}]
    Introduced 5.7.17
    Deprecated 5.7.21
    System Variable group_replication_allow_local_disjoint_gtids_join
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Deprecated in version 5.7.21 and scheduled for removal in a future version. Allows the server to join the group even if it has local transactions that are not present in the group.

    Warning

    Use caution when enabling this option as incorrect usage can lead to conflicts in the group and rollback of transactions. The option should only be enabled as a last resort method to allow a server that has local transactions to join an existing group, and then only if the local transactions do not affect the data that is handled by the group (for example, an administrative action that was written to the binary log). The option should not be left enabled on all group members.

  • group_replication_allow_local_lower_version_join

    Command-Line Format --group-replication-allow-local-lower-version-join[={OFF|ON}]
    Introduced 5.7.17
    System Variable group_replication_allow_local_lower_version_join
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Allows the current server to join the group even if it has a lower major version than the group. With the default setting OFF, servers are not permitted to join a replication group if they have a lower major version than the existing group members. For example, a MySQL 5.7 server cannot join a group that consists of MySQL 8.0 servers. This standard policy ensures that all members of a group are able to exchange messages and apply transactions. Set group_replication_allow_local_lower_version_join to ON only in the following scenarios:

    • A server must be added to the group in an emergency in order to improve the group's fault tolerance, and only older versions are available.

    • You want to carry out a downgrade of the replication group members without shutting down the whole group and bootstrapping it again.

    Warning

    Setting this option to ON does not make the new member compatible with the group, and allows it to join the group without any safeguards against incompatible behaviors by the existing members. To ensure the new member's correct operation, take both of the following precautions:

    1. Before the server with the lower major version joins the group, stop all writes on that server.

    2. From the point where the server with the lower major version joins the group, stop all writes on the other servers in the group.

    Without these precautions, the server with the lower major version is likely to experience difficulties and terminate with an error.

  • group_replication_auto_increment_increment

    Command-Line Format --group-replication-auto-increment-increment=#
    Introduced 5.7.17
    System Variable group_replication_auto_increment_increment
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 7
    Minimum Value 1
    Maximum Value 65535

    Determines the interval between successive column values for transactions that execute on this server instance. This system variable should have the same value on all group members. When Group Replication is started on a server, the value of the server system variable auto_increment_increment is changed to this value, and the value of the server system variable auto_increment_offset is changed to the server ID. These settings avoid the selection of duplicate auto-increment values for writes on group members, which causes rollback of transactions. The changes are reverted when Group Replication is stopped. These changes are only made and reverted if auto_increment_increment and auto_increment_offset each have their default value of 1. If their values have already been modified from the default, Group Replication does not alter them. From MySQL 8.0, the system variables are also not modified when Group Replication is in single-primary mode, where only one server writes.

    The default value of 7 represents a balance between the number of usable values and the permitted maximum size of a replication group (9 members). If your group has more or fewer members, you can set this system variable to match the expected number of group members before Group Replication is started. You cannot change the setting while Group Replication is running.

  • group_replication_bootstrap_group

    Command-Line Format --group-replication-bootstrap-group[={OFF|ON}]
    Introduced 5.7.17
    System Variable group_replication_bootstrap_group
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Configure this server to bootstrap the group. This option must only be set on one server and only when starting the group for the first time or restarting the entire group. After the group has been bootstrapped, set this option to OFF. It should be set to OFF both dynamically and in the configuration files. Starting two servers or restarting one server with this option set while the group is running may lead to an artificial split brain situation, where two independent groups with the same name are bootstrapped.

  • group_replication_components_stop_timeout

    Command-Line Format --group-replication-components-stop-timeout=#
    Introduced 5.7.17
    System Variable group_replication_components_stop_timeout
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 31536000
    Minimum Value 2
    Maximum Value 31536000

    Timeout, in seconds, that Group Replication waits for each of the components when shutting down.

  • group_replication_compression_threshold

    Command-Line Format --group-replication-compression-threshold=#
    Introduced 5.7.17
    System Variable group_replication_compression_threshold
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 1000000
    Minimum Value 0
    Maximum Value 4294967295
  • The threshold value in bytes above which compression is applied to messages sent between group members. If this system variable is set to zero, compression is disabled. The value of group_replication_compression_threshold should be the same on all group members.

    Group Replication uses the LZ4 compression algorithm to compress messages sent in the group. Note that the maximum supported input size for the LZ4 compression algorithm is 2113929216 bytes. This limit is lower than the maximum possible value for the group_replication_compression_threshold system variable, which is matched to the maximum message size accepted by XCom. With the LZ4 compression algorithm, do not set a value greater than 2113929216 bytes for group_replication_compression_threshold, because transactions above this size cannot be committed when message compression is enabled.

    For more information, see Section 17.9.7.2, “Message Compression”.

  • group_replication_enforce_update_everywhere_checks

    Command-Line Format --group-replication-enforce-update-everywhere-checks[={OFF|ON}]
    Introduced 5.7.17
    System Variable group_replication_enforce_update_everywhere_checks
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Enable or disable strict consistency checks for multi-primary update everywhere. The default is that checks are disabled. In single-primary mode, this option must be disabled on all group members. In multi-primary mode, when this option is enabled, statements are checked as follows to ensure they are compatible with multi-primary mode:

    • If a transaction is executed under the SERIALIZABLE isolation level, then its commit fails when synchronizing itself with the group.

    • If a transaction executes against a table that has foreign keys with cascading constraints, then the transaction fails to commit when synchronizing itself with the group.

    This system variable is a group-wide configuration setting. It must have the same value on all group members, cannot be changed while Group Replication is running, and requires a full reboot of the group (a bootstrap by a server with group_replication_bootstrap_group=ON) in order for the value change to take effect.

  • group_replication_exit_state_action

    Command-Line Format --group-replication-exit-state-action=value
    Introduced 5.7.24
    System Variable group_replication_exit_state_action
    Scope Global
    Dynamic Yes
    Type Enumeration
    Default Value READ_ONLY
    Valid Values

    ABORT_SERVER

    READ_ONLY

    Configures how Group Replication behaves when a server instance leaves the group unintentionally, for example after encountering an applier error, or in the case of a loss of majority, or when another member of the group expels it due to a suspicion timing out. The timeout period for a member to leave the group in the case of a loss of majority is set by the group_replication_unreachable_majority_timeout system variable. Note that an expelled group member does not know that it was expelled until it reconnects to the group, so the specified action is only taken if the member manages to reconnect, or if the member raises a suspicion on itself and expels itself.

  • When group_replication_exit_state_action is set to ABORT_SERVER, if the member exits the group unintentionally, the instance shuts down MySQL.

    When group_replication_exit_state_action is set to READ_ONLY, if the member exits the group unintentionally, the instance switches MySQL to super read only mode (by setting the system variable super_read_only to ON). This setting is the default in MySQL 5.7.

    Important

    If a failure occurs before the member has successfully joined the group, the specified exit action is not taken. This is the case if there is a failure during the local configuration check, or a mismatch between the configuration of the joining member and the configuration of the group. In these situations, the super_read_only system variable is left with its original value, and the server does not shut down MySQL. To ensure that the server cannot accept updates when Group Replication did not start, we therefore recommend that super_read_only=ON is set in the server's configuration file at startup, which Group Replication changes to OFF on primary members after it has been started successfully. This safeguard is particularly important when the server is configured to start Group Replication on server boot (group_replication_start_on_boot=ON), but it is also useful when Group Replication is started manually using a START GROUP_REPLICATION command.

    If a failure occurs after the member has successfully joined the group, the specified exit action is taken. This is the case if there is an applier error, if the member is expelled from the group, or if the member is set to time out in the event of an unreachable majority. In these situations, if READ_ONLY is the exit action, the super_read_only system variable is set to ON, or if ABORT_SERVER is the exit action, the server shuts down MySQL.

    Table 17.4 Exit actions in Group Replication failure situations

    Failure situation

    Group Replication started with START GROUP_REPLICATION

    Group Replication started with group_replication_start_on_boot =ON

    Member fails local configuration check

    OR

    Mismatch between joining member and group configuration

    super_read_only unchanged

    MySQL continues running

    Set super_read_only=ON at startup to prevent updates

    super_read_only unchanged

    MySQL continues running

    Set super_read_only=ON at startup to prevent updates (Important)

    Applier error on member

    OR

    Member expelled from group

    OR

    Unreachable majority timeout

    super_read_only set to ON

    OR

    MySQL shuts down

    super_read_only set to ON

    OR

    MySQL shuts down


  • group_replication_flow_control_applier_threshold

    Command-Line Format --group-replication-flow-control-applier-threshold=#
    Introduced 5.7.17
    System Variable group_replication_flow_control_applier_threshold
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 25000
    Minimum Value 0
    Maximum Value 2147483647

    Specifies the number of waiting transactions in the applier queue that trigger flow control. This variable can be changed without resetting Group Replication.

  • group_replication_flow_control_certifier_threshold

    Command-Line Format --group-replication-flow-control-certifier-threshold=#
    Introduced 5.7.17
    System Variable group_replication_flow_control_certifier_threshold
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 25000
    Minimum Value 0
    Maximum Value 2147483647

    Specifies the number of waiting transactions in the certifier queue that trigger flow control. This variable can be changed without resetting Group Replication.

  • group_replication_flow_control_hold_percent

    Command-Line Format --group-replication-flow-control-hold-percent=#
    System Variable group_replication_flow_control_hold_percent
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 10
    Minimum Value 0
    Maximum Value 100

    Defines what percentage of the group quota remains unused to allow a cluster under flow control to catch up on backlog. A value of 0 implies that no part of the quota is reserved for catching up on the work backlog.

  • group_replication_flow_control_max_commit_quota

    Command-Line Format --group-replication-flow-control-max-commit-quota=#
    System Variable group_replication_flow_control_max_commit_quota
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 2147483647

    Defines the maximum flow control quota of the group, or the maximum available quota for any period while flow control is enabled. A value of 0 implies that there is no maximum quota set. Cannot be smaller than group_replication_flow_control_min_quota and group_replication_flow_control_min_recovery_quota.

  • group_replication_flow_control_member_quota_percent

    Command-Line Format --group-replication-flow-control-member-quota-percent=#
    System Variable group_replication_flow_control_member_quota_percent
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 100

    Defines the percentage of the quota that a member should assume is available for itself when calculating the quotas. A value of 0 implies that the quota should be split equally between members that were writers in the last period.

  • group_replication_flow_control_min_quota

    Command-Line Format --group-replication-flow-control-min-quota=#
    System Variable group_replication_flow_control_min_quota
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 2147483647

    Controls the lowest flow control quota that can be assigned to a member, independently of the calculated minimum quota executed in the last period. A value of 0 implies that there is no minimum quota. Cannot be larger than group_replication_flow_control_max_commit_quota.

  • group_replication_flow_control_min_recovery_quota

    Command-Line Format --group-replication-flow-control-min-recovery-quota=#
    System Variable group_replication_flow_control_min_recovery_quota
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 2147483647

    Controls the lowest quota that can be assigned to a member because of another recovering member in the group, independently of the calculated minimum quota executed in the last period. A value of 0 implies that there is no minimum quota. Cannot be larger than group_replication_flow_control_max_commit_quota.

  • group_replication_flow_control_mode

    Command-Line Format --group-replication-flow-control-mode=value
    Introduced 5.7.17
    System Variable group_replication_flow_control_mode
    Scope Global
    Dynamic Yes
    Type Enumeration
    Default Value QUOTA
    Valid Values

    DISABLED

    QUOTA

    Specifies the mode used for flow control. This variable can be changed without resetting Group Replication.

  • group_replication_force_members

    Command-Line Format --group-replication-force-members=value
    Introduced 5.7.17
    System Variable group_replication_force_members
    Scope Global
    Dynamic Yes
    Type String

    A list of peer addresses as a comma separated list such as host1:port1,host2:port2. This option is used to force a new group membership, in which the excluded members do not receive a new view and are blocked. (You need to manually kill the excluded servers.) Any invalid host names in the list could cause this action to fail because they could block group membership. For a description of the procedure to follow, see Section 17.4.3, “Network Partitioning”.

    You must specify the address or host name and port as they are given in the group_replication_local_address option for each member. For example:

    "198.51.100.44:33061,example.org:33061"

    After you have used the group_replication_force_members system variable to successfully force a new group membership and unblock the group, ensure that you clear the system variable. group_replication_force_members must be empty in order to issue a START GROUP_REPLICATION statement.

  • group_replication_group_name

    Command-Line Format --group-replication-group-name=value
    Introduced 5.7.17
    System Variable group_replication_group_name
    Scope Global
    Dynamic Yes
    Type String

    The name of the group which this server instance belongs to. Must be a valid UUID. This UUID is used internally when setting GTIDs for Group Replication events in the binary log.

    Important

    A unique UUID must be used.

  • group_replication_group_seeds

    Command-Line Format --group-replication-group-seeds=value
    Introduced 5.7.17
    System Variable group_replication_group_seeds
    Scope Global
    Dynamic Yes
    Type String

    A list of group members to which a joining member can connect to obtain details of all the current group members. The joining member uses these details to select and connect to a group member to obtain the data needed for synchrony with the group. The list consists of the seed member's network addresses specified as a comma separated list, such as host1:port1,host2:port2.

    Important

    These addresses must not be the member's SQL hostname and port.

    Note that the value you specify for this variable is not validated until a START GROUP_REPLICATION statement is issued and the Group Communication System (GCS) is available.

    Usually this list consists of all members of the group, but you can choose a subset of the group members to be seeds. The list must contain at least one valid member address. Each address is validated when starting Group Replication. If the list does not contain any valid host names, issuing START GROUP_REPLICATION fails.

  • group_replication_gtid_assignment_block_size

    Command-Line Format --group-replication-gtid-assignment-block-size=#
    Introduced 5.7.17
    System Variable group_replication_gtid_assignment_block_size
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 1000000
    Minimum Value 1
    Maximum Value (64-bit platforms) 9223372036854775807
    Maximum Value (32-bit platforms) 4294967295

    The number of consecutive GTIDs that are reserved for each member. Each member consumes its blocks and reserves more when needed.

    This system variable is a group-wide configuration setting. It must have the same value on all group members, cannot be changed while Group Replication is running, and requires a full reboot of the group (a bootstrap by a server with group_replication_bootstrap_group=ON) in order for the value change to take effect.

  • group_replication_ip_whitelist

    Command-Line Format --group-replication-ip-whitelist=value
    Introduced 5.7.17
    System Variable group_replication_ip_whitelist
    Scope Global
    Dynamic Yes
    Type String
    Default Value AUTOMATIC

    Specifies the allowlist of hosts that are permitted to connect to the group. The address that you specify for each group member in group_replication_local_address must be allowlisted on the other servers in the replication group. Note that the value you specify for this variable is not validated until a START GROUP_REPLICATION statement is issued and the Group Communication System (GCS) is available.

    By default, this system variable is set to AUTOMATIC, which permits connections from private subnetworks active on the host. The group communication engine (XCom) automatically scans active interfaces on the host, and identifies those with addresses on private subnetworks. These addresses and the localhost IP address for IPv4 are used to create the Group Replication allowlist. For a list of the ranges from which addresses are automatically allowlisted, see Section 17.5.1, “Group Replication IP Address Allowlisting”.

    The automatic allowlist of private addresses cannot be used for connections from servers outside the private network. For Group Replication connections between server instances that are on different machines, you must provide public IP addresses and specify these as an explicit allowlist. If you specify any entries for the allowlist, the private addresses are not added automatically, so if you use any of these, you must specify them explicitly. The localhost IP address is added automatically.

    As the value of the group_replication_ip_whitelist option, you can specify any combination of the following:

    • IPv4 addresses (for example, 198.51.100.44)

    • IPv4 addresses with CIDR notation (for example, 192.0.2.21/24)

    • Host names, from MySQL 5.7.21 (for example, example.org)

    • Host names with CIDR notation, from MySQL 5.7.21 (for example, www.example.com/24)

    IPv6 addresses, and host names that resolve to IPv6 addresses, are not supported in MySQL 5.7. You can use CIDR notation in combination with host names or IP addresses to allowlist a block of IP addresses with a particular network prefix, but do ensure that all the IP addresses in the specified subnet are under your control.

    A comma must separate each entry in the allowlist. For example:

    192.0.2.22,198.51.100.0/24,example.org,www.example.com/24

    It is possible to configure different allowlists on different group members according to your security requirements, for example, in order to keep different subnets separate. However, this can cause issues when a group is reconfigured. If you do not have a specific security requirement to do otherwise, use the same allowlist on all members of a group. For more details, see Section 17.5.1, “Group Replication IP Address Allowlisting”.

    For host names, name resolution takes place only when a connection request is made by another server. A host name that cannot be resolved is not considered for allowlist validation, and a warning message is written to the error log. Forward-confirmed reverse DNS (FCrDNS) verification is carried out for resolved host names.

    Warning

    Host names are inherently less secure than IP addresses in an allowlist. FCrDNS verification provides a good level of protection, but can be compromised by certain types of attack. Specify host names in your allowlist only when strictly necessary, and ensure that all components used for name resolution, such as DNS servers, are maintained under your control. You can also implement name resolution locally using the hosts file, to avoid the use of external components.

  • group_replication_local_address

    Command-Line Format --group-replication-local-address=value
    Introduced 5.7.17
    System Variable group_replication_local_address
    Scope Global
    Dynamic Yes
    Type String

    The network address which the member provides for connections from other members, specified as a host:port formatted string. This address must be reachable by all members of the group because it is used by the group communication engine for Group Replication (XCom, a Paxos variant) for TCP communication between remote XCom instances. Communication with the local instance is over an input channel using shared memory.

    Warning

    Do not use this address for communication with the member.

    Other Group Replication members contact this member through this host:port for all internal group communication. This is not the MySQL server SQL protocol host and port.

    The address or host name that you specify in group_replication_local_address is used by Group Replication as the unique identifier for a group member within the replication group. You can use the same port for all members of a replication group as long as the host names or IP addresses are all different, and you can use the same host name or IP address for all members as long as the ports are all different. The recommended port for group_replication_local_address is 33061. Note that the value you specify for this variable is not validated until the START GROUP_REPLICATION statement is issued and the Group Communication System (GCS) is available.

  • group_replication_member_weight

    Command-Line Format --group-replication-member-weight=#
    Introduced 5.7.20
    System Variable group_replication_member_weight
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 50
    Minimum Value 0
    Maximum Value 100

    A percentage weight that can be assigned to members to influence the chance of the member being elected as primary in the event of failover, for example when the existing primary leaves a single-primary group. Assign numeric weights to members to ensure that specific members are elected, for example during scheduled maintenance of the primary or to ensure certain hardware is prioritized in the event of failover.

    For a group with members configured as follows:

    • member-1: group_replication_member_weight=30, server_uuid=aaaa

    • member-2: group_replication_member_weight=40, server_uuid=bbbb

    • member-3: group_replication_member_weight=40, server_uuid=cccc

    • member-4: group_replication_member_weight=40, server_uuid=dddd

    during election of a new primary the members above would be sorted as member-2, member-3, member-4, and member-1. This results in member-2 being chosen as the new primary in the event of failover. For more information, see Section 17.4.1.1, “Single-Primary Mode”.

  • group_replication_poll_spin_loops

    Command-Line Format --group-replication-poll-spin-loops=#
    Introduced 5.7.17
    System Variable group_replication_poll_spin_loops
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    The number of times the group communication thread waits for the communication engine mutex to be released before the thread waits for more incoming network messages.

  • group_replication_recovery_retry_count

    Command-Line Format --group-replication-recovery-retry-count=#
    Introduced 5.7.17
    System Variable group_replication_recovery_retry_count
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 10
    Minimum Value 0
    Maximum Value 31536000

    The number of times that the member that is joining tries to connect to the available donors before giving up.

  • group_replication_recovery_reconnect_interval

    Command-Line Format --group-replication-recovery-reconnect-interval=#
    Introduced 5.7.17
    System Variable group_replication_recovery_reconnect_interval
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 60
    Minimum Value 0
    Maximum Value 31536000

    The sleep time, in seconds, between reconnection attempts when no donor was found in the group.

  • group_replication_recovery_use_ssl

    Command-Line Format --group-replication-recovery-use-ssl[={OFF|ON}]
    Introduced 5.7.17
    System Variable group_replication_recovery_use_ssl
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Whether Group Replication recovery connection should use SSL or not.

  • group_replication_recovery_ssl_ca

    Command-Line Format --group-replication-recovery-ssl-ca=value
    Introduced 5.7.17
    System Variable group_replication_recovery_ssl_ca
    Scope Global
    Dynamic Yes
    Type String

    The path to a file that contains a list of trusted SSL certificate authorities.

  • group_replication_recovery_ssl_capath

    Command-Line Format --group-replication-recovery-ssl-capath=value
    Introduced 5.7.17
    System Variable group_replication_recovery_ssl_capath
    Scope Global
    Dynamic Yes
    Type String

    The path to a directory that contains trusted SSL certificate authority certificates.

  • group_replication_recovery_ssl_cert

    Command-Line Format --group-replication-recovery-ssl-cert=value
    Introduced 5.7.17
    System Variable group_replication_recovery_ssl_cert
    Scope Global
    Dynamic Yes
    Type String

    The name of the SSL certificate file to use for establishing a secure connection.

  • group_replication_recovery_ssl_key

    Command-Line Format --group-replication-recovery-ssl-key=value
    Introduced 5.7.17
    System Variable group_replication_recovery_ssl_key
    Scope Global
    Dynamic Yes
    Type String

    The name of the SSL key file to use for establishing a secure connection.

  • group_replication_recovery_ssl_cipher

    Command-Line Format --group-replication-recovery-ssl-cipher=value
    Introduced 5.7.17
    System Variable group_replication_recovery_ssl_cipher
    Scope Global
    Dynamic Yes
    Type String

    The list of permissible ciphers for SSL encryption.

  • group_replication_recovery_ssl_crl

    Command-Line Format --group-replication-recovery-ssl-crl=value
    Introduced 5.7.17
    System Variable group_replication_recovery_ssl_crl
    Scope Global
    Dynamic Yes
    Type File name

    The path to a directory that contains files containing certificate revocation lists.

  • group_replication_recovery_ssl_crlpath

    Command-Line Format --group-replication-recovery-ssl-crlpath=value
    Introduced 5.7.17
    System Variable group_replication_recovery_ssl_crlpath
    Scope Global
    Dynamic Yes
    Type Directory name

    The path to a directory that contains files containing certificate revocation lists.

  • group_replication_recovery_ssl_verify_server_cert

    Command-Line Format --group-replication-recovery-ssl-verify-server-cert[={OFF|ON}]
    Introduced 5.7.17
    System Variable group_replication_recovery_ssl_verify_server_cert
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Make the recovery process check the server's Common Name value in the donor sent certificate.

  • group_replication_recovery_complete_at

    Command-Line Format --group-replication-recovery-complete-at=value
    Introduced 5.7.17
    System Variable group_replication_recovery_complete_at
    Scope Global
    Dynamic Yes
    Type Enumeration
    Default Value TRANSACTIONS_APPLIED
    Valid Values

    TRANSACTIONS_CERTIFIED

    TRANSACTIONS_APPLIED

    Recovery policies when handling cached transactions after state transfer. This option specifies whether a member is marked online after it has received all transactions that it missed before it joined the group (TRANSACTIONS_CERTIFIED) or after it has received and applied them (TRANSACTIONS_APPLIED).

  • group_replication_single_primary_mode

    Command-Line Format --group-replication-single-primary-mode[={OFF|ON}]
    Introduced 5.7.17
    System Variable group_replication_single_primary_mode
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value ON

    Instructs the group to automatically pick a single server to be the one that handles read/write workload. This server is the PRIMARY and all others are SECONDARIES.

    This system variable is a group-wide configuration setting. It must have the same value on all group members, cannot be changed while Group Replication is running, and requires a full reboot of the group (a bootstrap by a server with group_replication_bootstrap_group=ON) in order for the value change to take effect.

  • group_replication_ssl_mode

    Command-Line Format --group-replication-ssl-mode=value
    Introduced 5.7.17
    System Variable group_replication_ssl_mode
    Scope Global
    Dynamic Yes
    Type Enumeration
    Default Value DISABLED
    Valid Values

    DISABLED

    REQUIRED

    VERIFY_CA

    VERIFY_IDENTITY

    Specifies the security state of the connection between Group Replication members.

  • group_replication_start_on_boot

    Command-Line Format --group-replication-start-on-boot[={OFF|ON}]
    Introduced 5.7.17
    System Variable group_replication_start_on_boot
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value ON

    Whether the server should start Group Replication or not during server start.

  • group_replication_transaction_size_limit

    Command-Line Format --group-replication-transaction-size-limit=#
    Introduced 5.7.19
    System Variable group_replication_transaction_size_limit
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 2147483647

    Configures the maximum transaction size in bytes which the replication group accepts. Transactions larger than this size are rolled back by the receiving member and are not broadcast to the group. Large transactions can cause problems for a replication group in terms of memory allocation, which can cause the system to slow down, or in terms of network bandwidth consumption, which can cause a member to be suspected of having failed because it is busy processing the large transaction.

    When this system variable is set to 0, which is the default in MySQL 5.7, there is no limit to the size of transactions the group accepts. From MySQL 8.0, the default setting for this system variable is 150000000 bytes (approximately 143 MB). Adjust the value of this system variable depending on the maximum message size that you need the group to tolerate, bearing in mind that the time taken to process a transaction is proportional to its size. The value of group_replication_transaction_size_limit should be the same on all group members. For further mitigation strategies for large transactions, see Section 17.7.2, “Group Replication Limitations”.

  • group_replication_unreachable_majority_timeout

    Command-Line Format --group-replication-unreachable-majority-timeout=#
    Introduced 5.7.19
    System Variable group_replication_unreachable_majority_timeout
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 31536000

    Configures how long members that suffer a network partition and cannot connect to the majority wait before leaving the group.

    In a group of 5 servers (S1,S2,S3,S4,S5), if there is a disconnection between (S1,S2) and (S3,S4,S5) there is a network partition. The first group (S1,S2) is now in a minority because it cannot contact more than half of the group. While the majority group (S3,S4,S5) remains running, the minority group waits for the specified time for a network reconnection. Any transactions processed by the minority group are blocked until Group Replication is stopped using STOP GROUP REPLICATION on the members of the minority. Note that group_replication_unreachable_majority_timeout has no effect if it is set on the servers in the minority group after the loss of majority has been detected.

    By default, this system variable is set to 0, which means that members that find themselves in a minority due to a network partition wait forever to leave the group. If configured to a number of seconds, members wait for this amount of time after losing contact with the majority of members before leaving the group. When the specified time elapses, all pending transactions processed by the minority are rolled back, and the servers in the minority partition move to the ERROR state. These servers then follow the action specified by the system variable group_replication_exit_state_action, which can be to set themselves to super read only mode or shut down MySQL.

    Warning

    When you have a symmetric group, with just two members for example (S0,S2), if there is a network partition and there is no majority, after the configured timeout all members enter ERROR state.

Group Replication Status Variable

This section describes the status variables which provide information about Group Replication. The variable has the following meaning:

17.7 Requirements and Limitations

This section lists and explains the requirements and limitations of Group Replication.

17.7.1 Group Replication Requirements

Server instances that you want to use for Group Replication must satisfy the following requirements.

Infrastructure

  • InnoDB Storage Engine.  Data must be stored in the InnoDB transactional storage engine. Transactions are executed optimistically and then, at commit time, are checked for conflicts. If there are conflicts, in order to maintain consistency across the group, some transactions are rolled back. This means that a transactional storage engine is required. Moreover, InnoDB provides some additional functionality that enables better management and handling of conflicts when operating together with Group Replication. The use of other storage engines, including the temporary MEMORY storage engine, might cause errors in Group Replication. You can prevent the use of other storage engines by setting the disabled_storage_engines system variable on group members, for example:

    disabled_storage_engines="MyISAM,BLACKHOLE,FEDERATED,ARCHIVE,MEMORY"
    
  • Primary Keys.  Every table that is to be replicated by the group must have a defined primary key, or primary key equivalent where the equivalent is a non-null unique key. Such keys are required as a unique identifier for every row within a table, enabling the system to determine which transactions conflict by identifying exactly which rows each transaction has modified.

  • IPv4 Network.  The group communication engine used by MySQL Group Replication only supports IPv4. Therefore, Group Replication requires an IPv4 network infrastructure.

  • Network Performance.  MySQL Group Replication is designed to be deployed in a cluster environment where server instances are very close to each other. The performance and stabiity of a group can be impacted by both network latency and network bandwidth. Bi-directional communication must be maintained at all times between all group members. If either inbound or outbound communication is blocked for a server instance (for example, by a firewall, or by connectivity issues), the member cannot function in the group, and the group members (including the member with issues) might not be able to report the correct member status for the affected server instance.

Server Instance Configuration

The following options must be configured on server instances that are members of a group.

  • Unique Server Identifier.  Use the server_id system variable to configure the server with a unique server ID, as required for all servers in replication topologies. With the default server ID of 0, servers in a replication topology cannot connect to each other. The server ID must be a positive integer between 1 and (232)−1, and it must be different from every other server ID in use by any other server in the replication topology.

  • Binary Log Active.  Set --log-bin[=log_file_name]. MySQL Group Replication replicates binary log contents, therefore the binary log needs to be on for it to operate. This option is enabled by default. See Section 5.4.4, “The Binary Log”.

  • Replica Updates Logged.  Set --log-slave-updates. Servers need to log binary logs that are applied through the replication applier. Servers in the group need to log all transactions that they receive and apply from the group. This is required because recovery is conducted by relying on binary logs form participants in the group. Therefore, copies of each transaction need to exist on every server, even for those transactions that were not initiated on the server itself.

  • Binary Log Row Format.  Set --binlog-format=row. Group Replication relies on row-based replication format to propagate changes consistently among the servers in the group. It relies on row-based infrastructure to be able to extract the necessary information to detect conflicts among transactions that execute concurrently in different servers in the group. See Section 16.2.1, “Replication Formats”.

  • Binary Log Checksums Off.  Set --binlog-checksum=NONE. Due to a design limitation of replication event checksums, Group Replication cannot make use of them, and they must be disabled.

  • Global Transaction Identifiers On.  Set gtid_mode=ON and enforce_gtid_consistency=ON. Group Replication uses global transaction identifiers to track exactly which transactions have been committed on every server instance and thus be able to infer which servers have executed transactions that could conflict with already committed transactions elsewhere. In other words, explicit transaction identifiers are a fundamental part of the framework to be able to determine which transactions may conflict. See Section 16.1.3, “Replication with Global Transaction Identifiers”.

  • Replication Information Repositories.  Set master_info_repository=TABLE and relay_log_info_repository=TABLE. The replication applier needs to have the source and replica metadata written to the mysql.slave_master_info and mysql.slave_relay_log_info system tables. This ensures the Group Replication plugin has consistent recoverability and transactional management of the replication metadata. See Section 16.2.4.2, “Replication Metadata Repositories”.

  • Transaction Write Set Extraction.  Set --transaction-write-set-extraction=XXHASH64 so that while collecting rows to log them to the binary log, the server collects the write set as well. The write set is based on the primary keys of each row and is a simplified and compact view of a tag that uniquely identifies the row that was changed. This tag is then used for detecting conflicts.

  • Lower Case Table Names.  Set --lower-case-table-names to the same value on all group members. A setting of 1 is correct for the use of the InnoDB storage engine, which is required for Group Replication. Note that this setting is not the default on all platforms.

  • Multithreaded Appliers.  Group Replication members can be configured as multithreaded appliers, enabling transactions to be applied in parallel. Set slave_parallel_workers=N (where N is the number of parallel applier threads), slave_preserve_commit_order=1, and slave_parallel_type=LOGICAL_CLOCK. Setting slave_parallel_workers=N enables the multithreaded applier on the member. Group Replication relies on consistency mechanisms built around the guarantee that all participating members receive and apply committed transaction in the same order, so you must also set slave_preserve_commit_order=1 to ensure that the final commit of parallel transactions is in the same order as the original transactions. Finally, in order to determine which transactions can be executed in parallel, the relay log must contain transaction parent information generated with slave_parallel_type=LOGICAL_CLOCK. Attempting to add a member with slave_parallel_workers set to greater than 0 without also setting the other two options, generates an error and the instance is prevented from joining.

17.7.2 Group Replication Limitations

The following known limitations exist for Group Replication. Note that the limitations and issues described for multi-primary mode groups can also apply in single-primary mode clusters during a failover event, while the newly elected primary flushes out its applier queue from the old primary.

Tip

Group Replication is built on GTID based replication, therefore you should also be aware of Section 16.1.3.6, “Restrictions on Replication with GTIDs”.

  • Gap Locks.  Group Replication's certification process for concurrent transactions does not take into account gap locks, as information about gap locks is not available outside of InnoDB. See Gap Locks for more information.

    Note

    For a group in multi-primary mode, unless you rely on REPEATABLE READ semantics in your applications, we recommend using the READ COMMITTED isolation level with Group Replication. InnoDB does not use gap locks in READ COMMITTED, which aligns the local conflict detection within InnoDB with the distributed conflict detection performed by Group Replication. For a group in single-primary mode, only the primary accepts writes, so the READ COMMITTED isolation level is not important to Group Replication.

  • Table Locks and Named Locks.  The certification process does not take into account table locks (see Section 13.3.5, “LOCK TABLES and UNLOCK TABLES Statements”) or named locks (see GET_LOCK()).

  • Replication Event Checksums.  Due to a design limitation of replication event checksums, Group Replication cannot currently make use of them. Therefore set --binlog-checksum=NONE.

  • SERIALIZABLE Isolation Level.  SERIALIZABLE isolation level is not supported in multi-primary groups by default. Setting a transaction isolation level to SERIALIZABLE configures Group Replication to refuse to commit the transaction.

  • Concurrent DDL versus DML Operations.  Concurrent data definition statements and data manipulation statements executing against the same object but on different servers is not supported when using multi-primary mode. During execution of Data Definition Language (DDL) statements on an object, executing concurrent Data Manipulation Language (DML) on the same object but on a different server instance has the risk of conflicting DDL executing on different instances not being detected.

  • Foreign Keys with Cascading Constraints.  Multi-primary mode groups (members all configured with group_replication_single_primary_mode=OFF) do not support tables with multi-level foreign key dependencies, specifically tables that have defined CASCADING foreign key constraints. This is because foreign key constraints that result in cascading operations executed by a multi-primary mode group can result in undetected conflicts and lead to inconsistent data across the members of the group. Therefore we recommend setting group_replication_enforce_update_everywhere_checks=ON on server instances used in multi-primary mode groups to avoid undetected conflicts.

    In single-primary mode this is not a problem as it does not allow concurrent writes to multiple members of the group and thus there is no risk of undetected conflicts.

  • MySQL Enterprise Audit and MySQL Enterprise Firewall.  Prior to version 5.7.21 MySQL Enterprise Audit and MySQL Enterprise Firewall use MyISAM tables in the mysql system database. Group Replication does not support MyISAM tables.

  • Multi-primary Mode Deadlock.  When a group is operating in multi-primary mode, SELECT .. FOR UPDATE statements can result in a deadlock. This is because the lock is not shared across the members of the group, therefore the expectation for such a statement might not be reached.

  • Replication Filters.  Replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state.

Limit on Group Size

The maximum number of MySQL servers that can be members of a single replication group is 9. If further members attempt to join the group, their request is refused. This limit has been identified from testing and benchmarking as a safe boundary where the group performs reliably on a stable local area network.

Limits on Transaction Size

If an individual transaction results in message contents which are large enough that the message cannot be copied between group members over the network within a 5-second window, members can be suspected of having failed, and then expelled, just because they are busy processing the transaction. Large transactions can also cause the system to slow due to problems with memory allocation. To avoid these issues use the following mitigations:

  • Where possible, try and limit the size of your transactions. For example, split up files used with LOAD DATA into smaller chunks.

  • Use the system variable group_replication_transaction_size_limit to specify the maximum transaction size that the group accepts. In MySQL 5.7, this system variable defaults to zero, but in MySQL 8.0, it defaults to a maximum transaction size of 150000000 bytes (approximately 143 MB). Transactions above this limit are rolled back and are not sent to Group Replication's Group Communication System (GCS) for distribution to the group. Adjust the value of this variable depending on the maximum message size that you need the group to tolerate, bearing in mind that the time taken to process a transaction is proportional to its size.

  • Use the system variable group_replication_compression_threshold to specify a message size above which compression is applied. This system variable defaults to 1000000 bytes (1 MB), so large messages are automatically compressed. Compression is carried out by Group Replication's Group Communication System (GCS) when it receives a message that was permitted by the group_replication_transaction_size_limit setting but exceeds the group_replication_compression_threshold setting. If you set the system variable value to zero, compression is deactivated. For more information, see Section 17.9.7.2, “Message Compression”.

If you have deactivated message compression and do not specify a maximum transaction size, the upper size limit for a message that can be handled by the applier thread on a member of a replication group is the value of the member's slave_max_allowed_packet system variable, which has a default and maximum value of 1073741824 bytes (1 GB). A message that exceeds this limit fails when the receiving member attempts to handle it. The upper size limit for a message that a group member can originate and attempt to transmit to the group is 4294967295 bytes (approximately 4 GB). This is a hard limit on the packet size that is accepted by the group communication engine for Group Replication (XCom, a Paxos variant), which receives messages after GCS has handled them. A message that exceeds this limit fails when the originating member attempts to broadcast it.

17.8 Frequently Asked Questions

This section provides answers to frequently asked questions.

What is the maximum number of MySQL servers in a group?

A group can consist of maximum 9 servers. Attempting to add another server to a group with 9 members causes the request to join to be refused. This limit has been identified from testing and benchmarking as a safe boundary where the group performs reliably on a stable local area network.

How are servers in a group connected?

Servers in a group connect to the other servers in the group by opening a peer-to-peer TCP connection. These connections are only used for internal communication and message passing between servers in the group. This address is configured by the group_replication_local_address variable.

What is the group_replication_bootstrap_group option used for?

The bootstrap flag instructs a member to create a group and act as the initial seed server. The second member joining the group needs to ask the member that bootstrapped the group to dynamically change the configuration in order for it to be added to the group.

A member needs to bootstrap the group in two scenarios. When the group is originally created, or when shutting down and restarting the entire group.

How do I set credentials for the recovery procedure?

You pre-configure the Group Replication recovery channel credentials using the CHANGE MASTER TO statement.

Can I scale-out my write-load using Group Replication?

Not directly, but MySQL Group replication is a shared nothing full replication solution, where all servers in the group replicate the same amount of data. Therefore if one member in the group writes N bytes to storage as the result of a transaction commit operation, then roughly N bytes are written to storage on other members as well, because the transaction is replicated everywhere.

However, given that other members do not have to do the same amount of processing that the original member had to do when it originally executed the transaction, they apply the changes faster. Transactions are replicated in a format that is used to apply row transformations only, without having to re-execute transactions again (row-based format).

Furthermore, given that changes are propagated and applied in row-based format, this means that they are received in an optimized and compact format, and likely reducing the number of IO operations required when compared to the originating member.

To summarize, you can scale-out processing, by spreading conflict free transactions throughout different members in the group. And you can likely scale-out a small fraction of your IO operations, since remote servers receive only the necessary changes to read-modify-write changes to stable storage.

Does Group Replication require more network bandwidth and CPU, when compared to simple replication and under the same workload?

Some additional load is expected because servers need to be constantly interacting with each other for synchronization purposes. It is difficult to quantify how much more data. It also depends on the size of the group (three servers puts less stress on the bandwidth requirements than nine servers in the group).

Also the memory and CPU footprint are larger, because more complex work is done for the server synchronization part and for the group messaging.

Can I deploy Group Replication across wide-area networks?

Yes, but the network connection between each member must be reliable and have suitable perfomance. Low latency, high bandwidth network connections are a requirement for optimal performance.

If network bandwidth alone is an issue, then Section 17.9.7.2, “Message Compression” can be used to lower the bandwidth required. However, if the network drops packets, leading to re-transmissions and higher end-to-end latency, throughput and latency are both negatively affected.

Warning

When the network round-trip time (RTT) between any group members is 5 seconds or more you could encounter problems as the built-in failure detection mechanism could be incorrectly triggered.

Do members automatically rejoin a group in case of temporary connectivity problems?

This depends on the reason for the connectivity problem. If the connectivity problem is transient and the reconnection is quick enough that the failure detector is not aware of it, then the server may not be removed from the group. If it is a "long" connectivity problem, then the failure detector eventually suspects a problem and the server is removed from the group.

Once a server is removed from the group, you need to join it back again. In other words, after a server is removed explicitly from the group you need to rejoin it manually (or have a script doing it automatically).

When is a member excluded from a group?

If the member becomes silent, the other members remove it from the group configuration. In practice this may happen when the member has crashed or there is a network disconnection.

The failure is detected after a given timeout elapses for a given member and a new configuration without the silent member in it is created.

What happens when one node is significantly lagging behind?

There is no method for defining policies for when to expel members automatically from the group. You need to find out why a member is lagging behind and fix that or remove the member from the group. Otherwise, if the server is so slow that it triggers the flow control, then the entire group slows down as well. The flow control can be configured according to the your needs.

Upon suspicion of a problem in the group, is there a special member responsible for triggering a reconfiguration?

No, there is no special member in the group in charge of triggering a reconfiguration.

Any member can suspect that there is a problem. All members need to (automatically) agree that a given member has failed. One member is in charge of expelling it from the group, by triggering a reconfiguration. Which member is responsible for expelling the member is not something you can control or set.

Can I use Group Replication for sharding?

Group Replication is designed to provide highly available replica sets; data and writes are duplicated on each member in the group. For scaling beyond what a single system can provide, you need an orchestration and sharding framework built around a number of Group Replication sets, where each replica set maintains and manages a given shard or partition of your total dataset. This type of setup, often called a sharded cluster, allows you to scale reads and writes linearly and without limit.

How do I use Group Replication with SELinux?

If SELinux is enabled, which you can verify using sestatus -v, then you need to enable the use of the Group Replication communication port. See Setting the TCP Port Context for Group Replication.

How do I use Group Replication with iptables?

If iptables is enabled, then you need to open up the Group Replication port for communication between the machines. To see the current rules in place on each machine, issue iptables -L. Assuming the port configured is 33061, enable communication over the necessary port by issuing iptables -A INPUT -p tcp --dport 33061 -j ACCEPT.

How do I recover the relay log for a replication channel used by a group member?

The replication channels used by Group Replication behave in the same way as replication channels used in source to replica replication, and as such rely on the relay log. In the event of a change of the relay_log variable, or when the option is not set and the host name changes, there is a chance of errors. See Section 16.2.4.1, “The Relay Log” for a recovery procedure in this situation. Alternatively, another way of fixing the issue specifically in Group Replication is to issue a STOP GROUP_REPLICATION statement and then a START GROUP_REPLICATION statement to restart the instance. The Group Replication plugin creates the group_replication_applier channel again.

Why does Group Replication use two bind addresses?

Group Replication uses two bind addresses in order to split network traffic between the SQL address, used by clients to communicate with the member, and the group_replication_local_address, used internally by the group members to communicate. For example, assume a server with two network interfaces assigned to the network addresses 203.0.113.1 and 198.51.100.179. In such a situation you could use 203.0.113.1:33061 for the internal group network address by setting group_replication_local_address=203.0.113.1:33061. Then you could use 198.51.100.179 for hostname and 3306 for the port. Client SQL applications would then connect to the member at 198.51.100.179:3306. This enables you to configure different rules on the different networks. Similarly, the internal group communication can be separated from the network connection used for client applications, for increased security.

How does Group Replication use network addresses and hostnames?

Group Replication uses network connections between members and therefore its functionality is directly impacted by how you configure hostnames and ports. For example, the Group Replication recovery procedure is based on asynchronous replication which uses the server's hostname and port. When a member joins a group it receives the group membership information, using the network address information that is listed at performance_schema.replication_group_members. One of the members listed in that table is selected as the donor of the missing data from the group to the new member.

This means that any value you configure using a hostname, such as the SQL network address or the group seeds address, must be a fully qualified name and resolvable by each member of the group. You can ensure this for example through DNS, or correctly configured /etc/hosts files, or other local processes. If a you want to configure the MEMBER_HOST value on a server, specify it using the --report-host option on the server before joining it to the group.

Important

The assigned value is used directly and is not affected by the skip_name_resolve system variable.

To configure MEMBER_PORT on a server, specify it using the report_port system variable.

Why did the auto increment setting on the server change?

When Group Replication is started on a server, the value of auto_increment_increment is changed to the value of group_replication_auto_increment_increment, which defaults to 7, and the value of auto_increment_offset is changed to the server ID. The changes are reverted when Group Replication is stopped. These settings avoid the selection of duplicate auto-increment values for writes on group members, which causes rollback of transactions. The default auto increment value of 7 for Group Replication represents a balance between the number of usable values and the permitted maximum size of a replication group (9 members).

The changes are only made and reverted if auto_increment_increment and auto_increment_offset each have their default value of 1. If their values have already been modified from the default, Group Replication does not alter them.

How do I find the primary?

If the group is operating in single-primary mode, it can be useful to find out which member is the primary. See Section 17.4.1.3, “Finding the Primary”

17.9 Group Replication Technical Details

This section provides more technical details about MySQL Group Replication.

17.9.1 Group Replication Plugin Architecture

MySQL Group Replication is a MySQL plugin and it builds on the existing MySQL replication infrastructure, taking advantage of features such as the binary log, row-based logging, and global transaction identifiers. It integrates with current MySQL frameworks, such as the performance schema or plugin and service infrastructures. The following figure presents a block diagram depicting the overall architecture of MySQL Group Replication.

Figure 17.9 Group Replication Plugin Block Diagram

The text following the figure describes the content of the diagram.

The MySQL Group Replication plugin includes a set of APIs for capture, apply, and lifecycle, which control how the plugin interacts with MySQL Server. There are interfaces to make information flow from the server to the plugin and vice versa. These interfaces isolate the MySQL Server core from the Group Replication plugin, and are mostly hooks placed in the transaction execution pipeline. In one direction, from server to the plugin, there are notifications for events such as the server starting, the server recovering, the server being ready to accept connections, and the server being about to commit a transaction. In the other direction, the plugin instructs the server to perform actions such as committing or aborting ongoing transactions, or queuing transactions in the relay log.

The next layer of the Group Replication plugin architecture is a set of components that react when a notification is routed to them. The capture component is responsible for keeping track of context related to transactions that are executing. The applier component is responsible for executing remote transactions on the database. The recovery component manages distributed recovery, and is responsible for getting a server that is joining the group up to date by selecting the donor, orchestrating the catch up procedure and reacting to donor failures.

Continuing down the stack, the replication protocol module contains the specific logic of the replication protocol. It handles conflict detection, and receives and propagates transactions to the group.

The final two layers of the Group Replication plugin architecture are the Group Communication System (GCS) API, and an implementation of a Paxos-based group communication engine (XCom). The GCS API is a high level API that abstracts the properties required to build a replicated state machine (see Section 17.1, “Group Replication Background”). It therefore decouples the implementation of the messaging layer from the remaining upper layers of the plugin. The group communication engine handles communications with the members of the replication group.

17.9.2 The Group

In MySQL Group Replication, a set of servers forms a replication group. A group has a name, which takes the form of a UUID. The group is dynamic and servers can leave (either voluntarily or involuntarily) and join it at any time. The group adjusts itself whenever servers join or leave.

If a server joins the group, it automatically brings itself up to date by fetching the missing state from an existing server. This state is transferred by means of Asynchronous MySQL replication. If a server leaves the group, for instance it was taken down for maintenance, the remaining servers notice that it has left and reconfigure the group automatically. The group membership service described at Section 17.1.3.1, “Group Membership” powers all of this.

17.9.3 Data Manipulation Statements

As there are no primary servers (sources) for any particular data set, every server in the group is allowed to execute transactions at any time, even transactions that change state (RW transactions).

Any server may execute a transaction without any a priori coordination. But, at commit time, it coordinates with the rest of the servers in the group to reach a decision on the fate of that transaction. This coordination serves two purposes: (i) check whether the transaction should commit or not; (ii) and propagate the changes so that other servers can apply the transaction as well.

As a transaction is sent through an atomic broadcast, either all servers in the group receive the transaction or none do. If they receive it, then they all receive it in the same order with respect to other transactions that were sent before. Conflict detection is carried out by inspecting and comparing write sets of transactions. Thus, they are detected at the row level. Conflict resolution follows the first committer wins rule. If t1 and t2 execute concurrently at different sites, because t2 is ordered before t1, and both changed the same row, then t2 wins the conflict and t1 aborts. In other words, t1 was trying to change data that had been rendered stale by t2.

Note

If two transactions are bound to conflict more often than not, then it is a good practice to start them on the same server. They then have a chance to synchronize on the local lock manager instead of aborting later in the replication protocol.

17.9.4 Data Definition Statements

In a Group Replication topology, care needs to be taken when executing data definition statements also commonly known as data definition language (DDL). Given that MySQL does not support atomic or transactional DDL, one cannot optimistically execute DDL statements and later roll back if needs be. Consequently, the lack of atomicity does not fit directly into the optimistic replication paradigm that Group Replication is based on.

Therefore, more care needs to be taken when replicating data definition statements. Schema changes and changes to the data that the object contains need to be handled through the same server while the schema operation has not yet completed and replicated everywhere. Failure to do so can result in data inconsistency.

Note

If the group is deployed in single-primary mode, then this is not a problem, because all changes are performed through the same server, the primary.

Warning

MySQL DDL execution is not atomic or transactional. The server executes and commits without securing group agreement first. As such, you must route DDL and DML for the same object through the same server, while the DDL is executing and has not replicated everywhere yet.

17.9.5 Distributed Recovery

This section describes the process through which a member joining a group catches up with the remaining servers in the group, called distributed recovery. Distributed recovery can be summarized as the process through which a server gets missing transactions from the group so that it can then join the group having processed the same set of transactions as the other group members.

17.9.5.1 Distributed Recovery Basics

Whenever a member joins a replication group, it connects to an existing member to carry out state transfer. The server joining the group transfers all the transactions that took place in the group before it joined, which are provided by the existing member (called the donor). Next, the server joining the group applies the transactions that took place in the group while this state transfer was in progress. When the server joining the group has caught up with the remaining servers in the group, it begins to participate normally in the group. This process is called distributed recovery.

Phase 1

In the first phase, the server joining the group selects one of the online servers on the group to be the donor of the state that it is missing. The donor is responsible for providing the server joining the group all the data it is missing up to the moment it has joined the group. This is achieved by relying on a standard asynchronous replication channel, established between the donor and the server joining the group, see Section 16.2.2, “Replication Channels”. Through this replication channel, the donor's binary logs are replicated until the point that the view change happened when the server joining the group became part of the group. The server joining the group applies the donor's binary logs as it receives them.

While the binary log is being replicated, the server joining the group also caches every transaction that is exchanged within the group. In other words it is listening for transactions that are happening after it joined the group and while it is applying the missing state from the donor. When the first phase ends and the replication channel to the donor is closed, the server joining the group then starts phase two: the catch up.

Phase 2

In this phase, the server joining the group proceeds to the execution of the cached transactions. When the number of transactions queued for execution finally reaches zero, the member is declared online.

Resilience

The recovery procedure withstands donor failures while the server joining the group is fetching binary logs from it. In such cases, whenever a donor fails during phase 1, the server joining the group fails over to a new donor and resumes from that one. When that happens the server joining the group closes the connection to the failed server joining the group explicitly and opens a connection to a new donor. This happens automatically.

17.9.5.2 Recovering From a Point-in-time

To synchronize the server joining the group with the donor up to a specific point in time, the server joining the group and donor make use of the MySQL Global Transaction Identifiers (GTIDs) mechanism. See Section 16.1.3, “Replication with Global Transaction Identifiers”. However, GTIDS only provide a means to realize which transactions the server joining the group is missing, they do not help marking a specific point in time to which the server joining the group must catch up, nor do they help conveying certification information. This is the job of binary log view markers, which mark view changes in the binary log stream, and also contain additional metadata information, provisioning the server joining the group with missing certification related data.

View and View Changes

To explain the concept of view change markers, it is important to understand what a view and a view change are.

A view corresponds to a group of members participating actively in the current configuration, in other words at a specific point in time. They are correct and online in the system.

A view change occurs when a modification to the group configuration happens, such as a member joining or leaving. Any group membership change results in an independent view change communicated to all members at the same logical point in time.

A view identifier uniquely identifies a view. It is generated whenever a view change happens

At the group communication layer, view changes with their associated view ids are then boundaries between the data exchanged before and after a member joins. This concept is implemented through a new binary log event: the"view change log event". The view id thus becomes a marker as well for transactions transmitted before and after changes happen in the group membership.

The view identifier itself is built from two parts: (i) one that is randomly generated and (ii) a monotonically increasing integer. The first part is generated when the group is created, and remains unchanged while there is at least one member in the group. The second part is incremented every time a view change happens.

The reason for this heterogeneous pair that makes up the view id is the need to unambiguously mark group changes whenever a member joins or leaves but also whenever all members leave the group and no information remains of what view the group was in. In fact, the sole use of monotonic increasing identifiers could lead to the reuse of the same id after full group shutdowns, destroying the uniqueness of the binary log data markers that recovery depends on. To summarize, the first part identifies whenever the group was started from the beginning and the incremental part when the group changed from that point on.

17.9.5.3 View Changes

This section explains the process which controls how the view change identifier is incorporated into a binary log event and written to the log, The following steps are taken:

Begin: Stable Group

All servers are online and processing incoming transactions from the group. Some servers may be a little behind in terms of transactions replicated, but eventually they converge. The group acts as one distributed and replicated database.

Figure 17.10 Stable Group

Servers S1, S2, and S3 are members of the group. The most recent item in all of their binary logs is transaction T20.

View Change: a Member Joins

Whenever a new member joins the group and therefore a view change is performed, every online server queues a view change log event for execution. This is queued because before the view change, several transactions can be queued on the server to be applied and as such, these belong to the old view. Queuing the view change event after them guarantees a correct marking of when this happened.

Meanwhile, the server joining the group selects the donor from the list of online servers as stated by the membership service through the view abstraction. A member joins on view 4 and the online members write a View change event to the binary log.

Figure 17.11 A Member Joins

Server S4 joins the group and looks for a donor. Servers S1, S2, and S3 each queue the view change entry VC4 for their binary logs. Meanwhile, server S1 is receiving new transaction T21.

State Transfer: Catching Up

Once the server joining the group has chosen which server in the group is to be the donor, a new asynchronous replication connection is established between the two and the state transfer begins (phase 1). This interaction with the donor continues until the server joining the group's applier thread processes the view change log event that corresponds to the view change triggered when the server joining the group came into the group. In other words, the server joining the group replicates from the donor, until it gets to the marker with the view identifier which matches the view marker it is already in.

Figure 17.12 State Transfer: Catching Up

Server S4 has chosen server S2 as the donor. State transfer is executed from server S2 to server S4 until the view change entry VC4 is reached (view_id = VC4). Server S4 uses a temporary applier buffer for state transfer, and its binary log is currently empty.

As view identifiers are transmitted to all members in the group at the same logical time, the server joining the group knows at which view identifier it should stop replicating. This avoids complex GTID set calculations because the view id clearly marks which data belongs to each group view.

While the server joining the group is replicating from the donor, it is also caching incoming transactions from the group. Eventually, it stops replicating from the donor and switches to applying those that are cached.

Figure 17.13 Queued Transactions

State transfer is complete. Server S4 has applied the transactions up to T20 and written them to its binary log. Server S4 has cached transaction T21, which arrived after the view change, in a temporary applier buffer while recovering.

Finish: Caught Up

When the server joining the group recognizes a view change log event with the expected view identifier, the connection to the donor is terminated and it starts applying the cached transactions. An important point to understand is the final recovery procedure. Although it acts as a marker in the binary log, delimiting view changes, the view change log event also plays another role. It conveys the certification information as perceived by all servers when the server joining the group entered the group, in other words the last view change. Without it, the server joining the group would not have the necessary information to be able to certify (detect conflicts) subsequent transactions.

The duration of the catch up (phase 2) is not deterministic, because it depends on the workload and the rate of incoming transactions to the group. This process is completely online and the server joining the group does not block any other server in the group while it is catching up. Therefore the number of transactions the server joining the group is behind when it moves to phase 2 can, for this reason, vary and thus increase or decrease according to the workload.

When the server joining the group reaches zero queued transactions and its stored data is equal to the other members, its public state changes to online.

Figure 17.14 Instance Online

Server S4 is now an online member of the group. It has applied cached transaction T21, so its binary log shows the same items as the binary logs of the other group members, and it no longer needs the temporary applier buffer. New incoming transaction T22 is now received and applied by all group members.

17.9.5.4 Usage Advice and Limitations of Distributed Recovery

Distributed recovery does have some limitations. It is based on classic asynchronous replication and as such it may be slow if the server joining the group is not provisioned at all or is provisioned with a very old backup image. This means that if the data to transfer is too big at phase 1, the server may take a very long time to recover. As such, the recommendation is that before adding a server to the group, one should provision it with a fairly recent snapshot of a server already in the group. This minimizes the length of phase 1 and reduces the impact on the donor server, since it has to save and transfer less binary logs.

Warning

It is recommended that a server is provisioned before it is added to a group. That way, one minimizes the time spent on the recovery step.

17.9.6 Observability

There is a lot of automation built into the Group Replication plugin. Nonetheless, you might sometimes need to understand what is happening behind the scenes. This is where the instrumentation of Group Replication and Performance Schema becomes important. The entire state of the system (including the view, conflict statistics and service states) can be queried through performance_schema tables. The distributed nature of the replication protocol and the fact that server instances agree and thus synchronize on transactions and metadata makes it simpler to inspect the state of the group. For example, you can connect to a single server in the group and obtain both local and global information by issuing select statements on the Group Replication related Performance Schema tables. For more information, see Section 17.3, “Monitoring Group Replication”.

17.9.7 Group Replication Performance

This section explains how to use the available configuration options to gain the best performance from your group.

17.9.7.1 Fine Tuning the Group Communication Thread

The group communication thread (GCT) runs in a loop while the Group Replication plugin is loaded. The GCT receives messages from the group and from the plugin, handles quorum and failure detection related tasks, sends out some keep alive messages and also handles the incoming and outgoing transactions from/to the server/group. The GCT waits for incoming messages in a queue. When there are no messages, the GCT waits. By configuring this wait to be a little longer (doing an active wait) before actually going to sleep can prove to be beneficial in some cases. This is because the alternative is for the operating system to switch out the GCT from the processor and do a context switch.

To force the GCT do an active wait, use the group_replication_poll_spin_loops option, which makes the GCT loop, doing nothing relevant for the configured number of loops, before actually polling the queue for the next message.

For example:

mysql> SET GLOBAL group_replication_poll_spin_loops= 10000;

17.9.7.2 Message Compression

For messages sent between online group members, Group Replication enables message compression by default. Whether a specific message is compressed depends on the threshold that you configure using the group_replication_compression_threshold system variable. Messages that have a payload larger than the specified number of bytes are compressed.

The default compression threshold is 1000000 bytes. You could use the following statements to increase the compression threshold to 2MB, for example:

STOP GROUP_REPLICATION;
SET GLOBAL group_replication_compression_threshold = 2097152;
START GROUP_REPLICATION;

If you set group_replication_compression_threshold to zero, message compression is disabled.

Group Replication uses the LZ4 compression algorithm to compress messages sent in the group. Note that the maximum supported input size for the LZ4 compression algorithm is 2113929216 bytes. This limit is lower than the maximum possible value for the group_replication_compression_threshold system variable, which is matched to the maximum message size accepted by XCom. The LZ4 maximum input size is therefore a practical limit for message compression, and transactions above this size cannot be committed when message compression is enabled. With the LZ4 compression algorithm, do not set a value greater than 2113929216 bytes for group_replication_compression_threshold.

The value of group_replication_compression_threshold is not required by Group Replication to be the same on all group members. However, it is advisable to set the same value on all group members in order to avoid unnecessary rollback of transactions, failure of message delivery, or failure of message recovery.

Compression for messages sent in the group happens at the group communication engine level, before the data is handed over to the group communication thread, so it takes place within the context of the mysql user session thread. If the message payload size exceeds the threshold set by group_replication_compression_threshold, the transaction payload is compressed before being sent out to the group, and decompressed when it is received. Upon receiving a message, the member checks the message envelope to verify whether it is compressed or not. If needed, then the member decompresses the transaction, before delivering it to the upper layer. This process is shown in the following figure.

Figure 17.15 Compression Support

The MySQL Group Replication plugin architecture is shown as described in an earlier topic, with the five layers of the plugin positioned between the MySQL server and the replication group. Compression and decompression are handled by the Group Communication System API, which is the fourth layer of the Group Replication plugin. The group communication engine (the fifth layer of the plugin) and the group members use the compressed transactions with the smaller data size. The MySQL Server core and the three higher layers of the Group Replication plugin (the APIs, the capture, applier, and recovery components, and the replication protocol module) use the original transactions with the larger data size.

When network bandwidth is a bottleneck, message compression can provide up to 30-40% throughput improvement at the group communication level. This is especially important within the context of large groups of servers under load. The TCP peer-to-peer nature of the interconnections between N participants in the group makes the sender send the same amount of data N times. Furthermore, binary logs are likely to exhibit a high compression ratio. This makes compression a compelling feature for Group Replication workloads that contain large transactions.

17.9.7.3 Flow Control

Group Replication ensures that a transaction only commits after a majority of the members in a group have received it and agreed on the relative order between all transactions that were sent concurrently.

This approach works well if the total number of writes to the group does not exceed the write capacity of any member in the group. If it does and some of the members have less write throughput than others, particularly less than the writer members, those members can start lagging behind of the writers.

Having some members lagging behind the group brings some problematic consequences, particularly, the reads on such members may externalize very old data. Depending on why the member is lagging behind, other members in the group may have to save more or less replication context to be able to fulfil potential data transfer requests from the slow member.

There is however a mechanism in the replication protocol to avoid having too much distance, in terms of transactions applied, between fast and slow members. This is known as the flow control mechanism. It tries to address several goals:

  1. to keep the members close enough to make buffering and de-synchronization between members a small problem;

  2. to adapt quickly to changing conditions like different workloads or more writers in the group;

  3. to give each member a fair share of the available write capacity;

  4. to not reduce throughput more than strictly necessary to avoid wasting resources.

Given the design of Group Replication, the decision whether to throttle or not may be decided taking into account two work queues: (i) the certification queue; (ii) and on the binary log applier queue. Whenever the size of one of these queues exceeds the user-defined threshold, the throttling mechanism is triggered. Only configure: (i) whether to do flow control at the certifier or at the applier level, or both; and (ii) what is the threshold for each queue.

The flow control depends on two basic mechanisms:

  1. the monitoring of members to collect some statistics on throughput and queue sizes of all group members to make educated guesses on what is the maximum write pressure each member should be subjected to;

  2. the throttling of members that are trying to write beyond their fair-share of the available capacity at each moment in time.

17.9.7.3.1 Probes and Statistics

The monitoring mechanism works by having each member deploying a set of probes to collect information about its work queues and throughput. It then propagates that information to the group periodically to share that data with the other members.

Such probes are scattered throughout the plugin stack and allow one to establish metrics, such as:

  • the certifier queue size;

  • the replication applier queue size;

  • the total number of transactions certified;

  • the total number of remote transactions applied in the member;

  • the total number of local transactions.

Once a member receives a message with statistics from another member, it calculates additional metrics regarding how many transactions were certified, applied and locally executed in the last monitoring period.

Monitoring data is shared with others in the group periodically. The monitoring period must be high enough to allow the other members to decide on the current write requests, but low enough that it has minimal impact on group bandwidth. The information is shared every second, and this period is sufficient to address both concerns.

17.9.7.3.2 Group Replication Throttling

Based on the metrics gathered across all servers in the group, a throttling mechanism kicks in and decides whether to limit the rate a member is able to execute/commit new transactions.

Therefore, metrics acquired from all members are the basis for calculating the capacity of each member: if a member has a large queue (for certification or the applier thread), then the capacity to execute new transactions should be close to ones certified or applied in the last period.

The lowest capacity of all the members in the group determines the real capacity of the group, while the number of local transactions determines how many members are writing to it, and, consequently, how many members should that available capacity be shared with.

This means that every member has an established write quota based on the available capacity, in other words a number of transactions it can safely issue for the next period. The writer quota is enforced by the throttling mechanism if the queue size of the certifier or the binary log applier exceeds a user defined threshold.

The quota is reduced by the number of transactions that were delayed in the last period, and then also further reduced by 10% to allow the queue that triggered the problem to reduce its size. In order to avoid large jumps in throughput once the queue size goes beyond the threshold, the throughput is only allowed to grow by the same 10% per period after that.

The current throttling mechanism does not penalize transactions below quota, but delays finishing those transactions that exceed it until the end of the monitoring period. As a consequence, if the quota is very small for the write requests issued some transactions may have latencies close to the monitoring period.