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Excerpts from this Manual Expel Timeout

You can use the group_replication_member_expel_timeout system variable, which is available from MySQL 8.0.13, to allow additional time between the creation of a suspicion and the expulsion of the suspect member.

A group member is expelled when another member's suspicion of it (or its own suspicion of itself) times out. By default, group_replication_member_expel_timeout is set to 0, meaning that there is no waiting period and a suspected member is liable for expulsion immediately after the 5-second detection period ends. An additional short period of time might elapse before the expelling mechanism detects and implements the expulsion. If a group member is at an older MySQL Server version that does not support this setting, this is its behavior towards other members or itself.

To avoid unnecessary expulsions on slower networks, or in the case of expected transient network failures or machine slowdowns, you can specify a timeout value greater than zero, up to a maximum of 3600 seconds (1 hour). Suspect members in this state are listed as UNREACHABLE, but are not removed from the group's membership list. If a suspect member becomes active again before the suspicion times out, it rejoins the group, applies all the messages that were buffered by the remaining group members, and enters ONLINE state.

If the timeout is exceeded, the suspect member is liable for expulsion immediately after the suspicion times out. If the member is able to resume communications and receives a view where it is expelled, it realises it was expelled. By default, the expelled member then follows the exit action specified by group_replication_exit_state_action. Alternatively you can use the group_replication_autorejoin_tries system variable, which is available from MySQL 8.0.16, to make the member automatically try to rejoin the group.

The waiting period before expelling a member only applies to members that have previously been active in the group. Non-members that were never active in the group do not get this waiting period and are removed after the initial detection period because they took too long to join.

If any members in a group are currently under suspicion, the group membership cannot be reconfigured (by adding or removing members or electing a new leader). If group membership changes need to be implemented while one or more members are under suspicion, and you want the suspect members to remain in the group, take any actions required to make the members active again, if that is possible. If you cannot make the members active again and you want them to be expelled from the group, you can force the suspicions to time out immediately. Do this by changing the value of group_replication_member_expel_timeout on any active members to a value lower than the time that has already elapsed since the suspicions were created. The suspect members then become liable for expulsion immediately.

If a replication group member stops unexpectedly and is immediately restarted (for example, because it was started with mysqld_safe), it automatically attempts to rejoin the group if group_replication_start_on_boot=on is set. In this situation, it is possible for the restart and rejoin attempt to take place before the member's previous incarnation has been expelled from the group, in which case the member cannot rejoin. From MySQL 8.0.19, Group Replication automatically uses a Group Communication System (GCS) feature to retry the rejoin attempt for the member 10 times, with a 5-second interval between each retry. This should cover most cases and allow enough time for the previous incarnation to be expelled from the group, letting the member rejoin. Note that if the group_replication_member_expel_timeout system variable is set to specify a longer waiting period before the member is expelled, the automatic rejoin attempts might still not succeed.

For alternative mitigation strategies to avoid unnecessary expulsions where the group_replication_member_expel_timeout system variable is not available, see Section 18.9.2, “Group Replication Limitations”.