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MySQL 5.7 Reference Manual  /  ...  /  Group Replication Requirements

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.

  • 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.  Group Replication is designed to be deployed in a cluster environment where server instances are very close to each other, and is impacted by both network latency as well as network bandwidth.

Server Instance Configuration

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

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

  • Slave 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”.

  • Global Transaction Identifiers On.  Set --gtid-mode=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 master information and relay log 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, “Slave Status Logs”.

  • 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.

  • Multi-threaded Appliers.  Group Replication members can be configured as multi-threaded 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 multi-threaded 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.


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