The group communication engine for Group Replication (XCom, a Paxos variant) includes a cache for messages (and their metadata) exchanged between the group members as a part of the consensus protocol. Among other functions, the message cache is used for recovery by members that return to the group after a period where they were unable to communicate with the other group members.

From MySQL 8.0.16, a cache size limit can be set for XCom's message cache using the group_replication_message_cache_size system variable. This system variable has a default and minimum setting of 1 GB, which is the size of the message cache in MySQL Server versions prior to MySQL 8.0.16. If the cache size limit is reached, XCom removes the oldest entries that have been decided and delivered. Ensure that sufficient memory is available on your system for your chosen cache size limit, considering the size of MySQL Server's other caches and object pools.

If an unreachable member that is attempting to reconnect requires a message for recovery, but the message has already been removed from the message cache, the member cannot reconnect. This situation is more likely to occur if you have used the group_replication_member_expel_timeout system variable (introduced in MySQL 8.0.13) to specify an additional delay time before suspect members are expelled from a group. 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 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, 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.

If you are considering reducing the cache size limit, you can query the Performance Schema table memory_summary_global_by_event_name using the following statement:

SELECT * FROM performance_schema.memory_summary_global_by_event_name
  WHERE EVENT_NAME LIKE 'memory/group_rpl/GCS_XCom::xcom_cache';

This returns memory usage statistics for the message cache, including the current number of cached entries and current size of the cache. If you reduce the cache size limit, XCom removes the oldest entries that have been decided and delivered until the current size is below the limit. XCom might temporarily exceed the cache size limit while this removal process is ongoing.