By default, the group communication engine for Group Replication (XCom, a Paxos variant) operates using every member of the replication group as a leader. From MySQL 8.0.27, the group communication engine can use a single leader to drive consensus when the group is in single-primary mode. Operating with a single consensus leader improves performance and resilience in single-primary mode, particularly when some of the group’s secondary members are currently unreachable.
To use a single consensus leader, the group must be configured as follows:
The group must be in single-primary mode.
group_replication_paxos_single_leadersystem variable must be set to
ON. With the default setting
OFF, the behavior is disabled. You must carry out a full reboot of the replication group (bootstrap) for Group Replication to pick up a change to this setting.
The Group Replication communication protocol version must be set to 8.0.27 or above. Use the
group_replication_get_communication_protocol()function to view the group's communication protocol version. If a lower version is in use, the group cannot use this behavior. You can use the
group_replication_set_communication_protocol()function to set the group's communication protocol to a higher version if all group members support it. MySQL InnoDB Cluster manages the communication protocol version automatically. For more information, see Section 22.214.171.124, “Setting a Group's Communication Protocol Version”.
When this configuration is in place, Group Replication instructs
the group communication engine to use the group’s primary as the
single leader to drive consensus. When a new primary is elected,
Group Replication tells the group communication engine to use it
instead. If the primary is currently unhealthy, the group
communication engine uses an alternative member as the consensus
leader. The Performance Schema table
shows the current preferred and actual consensus leader, with the
preferred leader being Group Replication’s choice, and the
actual leader being the one selected by the group communication
If the group is in multi-primary mode, has a lower communication
protocol version, or the behavior is disabled by the
setting, all members are used as leaders to drive consensus. In
this situation, the Performance Schema table
shows all of the members as both the preferred and actual leaders.