20.7.7.3 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. Up to MySQL 8.0.20, the value of the system variable defaults to 0, so auto-rejoin is not activated by default. From MySQL 8.0.21, the value of the system variable defaults to 3, meaning that the member automatically makes 3 attempts to rejoin the group, with 5 minutes between each.

When auto-rejoin is not activated, a member accepts its expulsion as soon as it resumes communication, and proceeds to the action specified by the group_replication_exit_state_action system variable. After this, manual intervention is needed to bring the member back into the group. Using the auto-rejoin feature is appropriate 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.

With auto-rejoin, 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. The auto-rejoin attempts and the time between them are called the auto-rejoin procedure. 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 and between auto-rejoin attempts, a member remains in super read only mode and displays an ERROR state on its view of the replication group. During this time, the member does not accept writes. However, reads can still be made on the member, with an increasing likelihood of stale reads over time. If you do want to intervene to take the member offline during the auto-rejoin procedure, the member can be stopped manually at any time by using a STOP GROUP_REPLICATION statement or shutting down the server. If you cannot tolerate the possibility of stale reads for any period of time, set the group_replication_autorejoin_tries system variable to 0.

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 column). 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 column). The events_stages_history_long table shows the time each of these auto-rejoin procedures was completed (in the TIMER_END column). While a member is rejoining a replication group, its status can be displayed as OFFLINE or ERROR before the group completes the compatibility checks and accepts it as a member. When the member is catching up with the group's transactions, its status is RECOVERING.