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• group_replication_message_cache_size 

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This system variable should have the same value on all group members. The value of this system variable can be changed while Group Replication is running. The change takes effect on each group member after you stop and restart Group Replication on the member. During this process, the value of the system variable is permitted to differ between group members, but members might be unable to reconnect in the event of a disconnection. 

group_replication_message_cache_size  sets the maximum amount of memory that is available for the message cache in the group communication engine for Group Replication (XCom). The XCom message cache holds messages (and their metadata) that are exchanged between the group members as a part of the consensus protocol. Among other functions, the message cache is used for recovery of missed messages by members that reconnect with the group after a period where they were unable to communicate with the other group members. 

The group_replication_member_expel_timeout  system variable determines the waiting period (up to an hour) that is allowed in addition to the initial 5-second detection period for members to return to the group rather than being expelled. The size of the XCom message cache should be set with reference to the expected volume of messages in this time period, so that it contains all the missed messages required for members to return successfully. Up to MySQL 8.0.20, the default is only the 5-second detection period, but from MySQL 8.0.21, the default is a 5-second waiting period after the 5-second detection period, for a total time period of 10 seconds. 

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. The default setting is 1073741824 bytes (1 GB). The minimum setting is also 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, and good network connectivity to minimize the frequency and duration of transient losses of connectivity for group members. Note that the limit set using 

