18.6.6.3 Auto-Rejoin

The group_replication_autorejoin_tries system variable, which is available from MySQL 8.0.16, makes a member that has been expelled or reached its unreachable majority timeout try to rejoin the group automatically. You can specify a number of attempts that the member makes to rejoin the group, instead of just accepting its expulsion as soon as it resumes communication. Activate auto-rejoin if you can tolerate the possibility of stale reads and want to minimize the need for manual intervention, especially where transient network issues fairly often result in the expulsion of members.

When the member's expulsion or unreachable majority timeout is reached, it makes an attempt to rejoin (using the current plugin option values), then continues to make further auto-rejoin attempts up to the specified number of tries. After an unsuccessful auto-rejoin attempt, the member waits 5 minutes before the next try. If the specified number of tries is exhausted without the member rejoining or being stopped, the member proceeds to the action specified by the group_replication_exit_state_action system variable.

During the auto-rejoin procedure, the expelled member remains in super read only mode and displays an ERROR state on its view of the replication group. Bear in mind that while a member remains in super read only mode, although writes cannot be made on the member, reads can, with an increasing likelihood of stale reads over time. If you do want to intervene to take the member offline, the member can be stopped manually at any time by using a STOP GROUP_REPLICATION statement or shutting down the server.

You can monitor the auto-rejoin procedure using the Performance Schema. While an auto-rejoin procedure is taking place, the Performance Schema table events_stages_current shows the event “Undergoing auto-rejoin procedure”, with the number of retries that have been attempted so far during this instance of the procedure (in the WORK_COMPLETED field). The events_stages_summary_global_by_event_name table shows the number of times the server instance has initiated the auto-rejoin procedure (in the COUNT_STAR field). The events_stages_history_long table shows the time each of these auto-rejoin procedures was completed (in the TIMER_END field).