Group Replication Apply Queue Buildup
warningGroup Replication replica falling behind in applying transactions from the group. Apply queue grows as transaction receive rate exceeds apply rate, leading to data staleness on the lagging member and potential flow control activation.
mysql.replication.group.transactions_in_applier_queue > 100 and increasing, with mysql.replication.group.transactions_applied rate lower than group average
Check Group Replication parallel applier configuration: group_replication_slave_parallel_workers should be > 0 for parallel apply. Compare CPU usage between members - lagging member may be resource-constrained. Review mysql.replication.group.member_status to ensure member is ONLINE. If apply queue grows continuously, consider: 1) Scale up lagging member instance class, 2) Enable parallel apply if single-threaded, 3) Review write load distribution in multi-primary mode. Monitor group_replication_flow_control_* status variables - flow control may activate to prevent queue growth.