Each member in a replication group certifies and applies transactions received by the group. Statistics regarding the certifier and applier procedures are useful to understand how the applier queue is growing, how many conflicts have been found, how many transactions were checked, which transactions are committed everywhere, and so on.
table provides group-level information related to the
certification process, and also statistics for the transactions
received and originated by each individual member of the
replication group. The information is shared between all the
server instances that are members of the replication group, so
information on all the group members can be queried from any
member. Note that refreshing of statistics for remote members is
controlled by the message period specified in the
option, so these can differ slightly from the locally collected
statistics for the member where the query is made. To use this
table to monitor a Group Replication member, issue the following
mysql> SELECT * FROM performance_schema.replication_group_member_stats\G
Beginning with MySQL 8.0.19, you can also use the following statement:
mysql> TABLE performance_schema.replication_group_member_stats\G
These columns are important for monitoring the performance of the members connected in the group. Suppose that one of the group's members always reports a large number of transactions in its queue compared to other members. This means that the member is delayed and is not able to keep up to date with the other members of the group. Based on this information, you could decide to either remove the member from the group, or delay the processing of transactions on the other members of the group in order to reduce the number of queued transactions. This information can also help you to decide how to adjust the flow control of the Group Replication plugin, see Section 18.7.2, “Flow Control”.