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MySQL 8.0 Reference Manual
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18.6.5.1 Increasing the cache size

If a member is absent for a period that is not long enough for it to be expelled from the group, it can reconnect and start participating in the group again by retrieving missed transactions from another member's XCom message cache. However, if the transactions that happened during the member's absence have been deleted from the other members' XCom message caches because their maximum size limit was reached, the member cannot reconnect in this way.

Group Replication's Group Communication System (GCS) alerts you, by a warning message, when a message that is likely to be needed for recovery by a member that is currently unreachable is removed from the message cache. This warning message is logged on all the active group members (only once for each unreachable member). Although the group members cannot know for sure what message was the last message seen by the unreachable member, the warning message indicates that the cache size might not be sufficient to support your chosen waiting period before a member is expelled.

In this situation, consider increasing the group_replication_message_cache_size limit with reference to the expected volume of messages in the time period specified by the group_replication_member_expel_timeout system variable plus the 5-second detection period, so that the cache contains all the missed messages required for members to return successfully. You can also consider increasing the cache size limit temporarily if you expect a member to become unreachable for an unusual period of time.