MySQL 8.0 Reference Manual Including MySQL NDB Cluster 8.0 Checking Replication Status

The most common task when managing a replication process is to ensure that replication is taking place and that there have been no errors between the replica and the source.

The SHOW REPLICA STATUS statement, which you must execute on each replica, provides information about the configuration and status of the connection between the replica server and the source server. From MySQL 8.0.22, SHOW SLAVE STATUS is deprecated, and SHOW REPLICA STATUS is available to use instead. The Performance Schema has replication tables that provide this information in a more accessible form. See Section 27.12.11, “Performance Schema Replication Tables”.

The replication heartbeat information shown in the Performance Schema replication tables lets you check that the replication connection is active even if the source has not sent events to the replica recently. The source sends a heartbeat signal to a replica if there are no updates to, and no unsent events in, the binary log for a longer period than the heartbeat interval. The MASTER_HEARTBEAT_PERIOD setting on the source (set by the CHANGE MASTER TO statement) specifies the frequency of the heartbeat, which defaults to half of the connection timeout interval for the replica (specified by the system variable replica_net_timeout or slave_net_timeout). The replication_connection_status Performance Schema table shows when the most recent heartbeat signal was received by a replica, and how many heartbeat signals it has received.

If you are using the SHOW REPLICA STATUS statement to check on the status of an individual replica, the statement provides the following information:

*************************** 1. row ***************************
             Replica_IO_State: Waiting for source to send event
                  Source_Host: source1
                  Source_User: root
                  Source_Port: 3306
                Connect_Retry: 60
              Source_Log_File: mysql-bin.000004
          Read_Source_Log_Pos: 931
               Relay_Log_File: replica1-relay-bin.000056
                Relay_Log_Pos: 950
        Relay_Source_Log_File: mysql-bin.000004
           Replica_IO_Running: Yes
          Replica_SQL_Running: Yes
                   Last_Errno: 0
                 Skip_Counter: 0
          Exec_Source_Log_Pos: 931
              Relay_Log_Space: 1365
              Until_Condition: None
                Until_Log_Pos: 0
           Source_SSL_Allowed: No
        Seconds_Behind_Source: 0
Source_SSL_Verify_Server_Cert: No
                Last_IO_Errno: 0
               Last_SQL_Errno: 0
  Replicate_Ignore_Server_Ids: 0

The key fields from the status report to examine are:

Several pairs of fields provide information about the progress of the replica in reading events from the source binary log and processing them in the relay log:

On the source, you can check the status of connected replicas using SHOW PROCESSLIST to examine the list of running processes. Replica connections have Binlog Dump in the Command field:

*************************** 4. row ***************************
     Id: 10
   User: root
   Host: replica1:58371
     db: NULL
Command: Binlog Dump
   Time: 777
  State: Has sent all binlog to slave; waiting for binlog to be updated
   Info: NULL

Because it is the replica that drives the replication process, very little information is available in this report.

For replicas that were started with the --report-host option and are connected to the source, the SHOW REPLICAS (or before MySQL 8.0.22, SHOW SLAVE HOSTS) statement on the source shows basic information about the replicas. The output includes the ID of the replica server, the value of the --report-host option, the connecting port, and source ID:

| Server_id | Host     | Port | Rpl_recovery_rank | Source_id |
|        10 | replica1 | 3306 |                 0 |         1 |
1 row in set (0.00 sec)