20.7.6.2 Reducing the cache size

The minimum setting for the XCom message cache size is 1 GB up to MySQL 8.0.20. From MySQL 8.0.21, the minimum setting is 134217728 bytes (128 MB), which enables deployment on a host that has a restricted amount of available memory. Having a very low group_replication_message_cache_size setting is not recommended if the host is on an unstable network, because a smaller message cache makes it harder for group members to reconnect after a transient loss of connectivity.

If a reconnecting member cannot retrieve all the messages it needs from the XCom message cache, the member must leave the group and rejoin it, in order to retrieve the missing transactions from another member's binary log using distributed recovery. From MySQL 8.0.21, a member that has left a group makes three auto-rejoin attempts by default, so the process of rejoining the group can still take place without operator intervention. However, rejoining using distributed recovery is a significantly longer and more complex process than retrieving messages from an XCom message cache, so the member takes longer to become available and the performance of the group can be impacted. On a stable network, which minimizes the frequency and duration of transient losses of connectivity for members, the frequency of this occurrence should also be minimized, so the group might be able to tolerate a smaller XCom message cache size without a significant impact on its performance.

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.