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16.1.4.2 Pausing Replication on the Slave »
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16.1.4.1. 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 slave and the master.

The primary statement for this is SHOW SLAVE STATUS which you must execute on each slave: 

mysql> SHOW SLAVE STATUS\G
*************************** 1. row ***************************
             Slave_IO_State: Waiting for master to send event
                Master_Host: master1
                Master_User: root
                Master_Port: 3306
              Connect_Retry: 60
            Master_Log_File: mysql-bin.000004
        Read_Master_Log_Pos: 931
             Relay_Log_File: slave1-relay-bin.000056
              Relay_Log_Pos: 950
      Relay_Master_Log_File: mysql-bin.000004
           Slave_IO_Running: Yes
          Slave_SQL_Running: Yes
            Replicate_Do_DB:
        Replicate_Ignore_DB:
         Replicate_Do_Table:
     Replicate_Ignore_Table:
    Replicate_Wild_Do_Table:
Replicate_Wild_Ignore_Table:
                 Last_Errno: 0
                 Last_Error:
               Skip_Counter: 0
        Exec_Master_Log_Pos: 931
            Relay_Log_Space: 1365
            Until_Condition: None
             Until_Log_File:
              Until_Log_Pos: 0
         Master_SSL_Allowed: No
         Master_SSL_CA_File:
         Master_SSL_CA_Path:
            Master_SSL_Cert:
          Master_SSL_Cipher:
             Master_SSL_Key:
      Seconds_Behind_Master: 0
1 row in set (0.01 sec)

The key fields from the status report to examine are:

  • Slave_IO_State — indicates the current status of the slave. See Section 7.5.6.5, “Replication Slave I/O Thread States”, and Section 7.5.6.6, “Replication Slave SQL Thread States”, for more information.

  • Slave_IO_Running — shows whether the IO thread for the reading the master's binary log is running.

  • Slave_SQL_Running — shows whether the SQL thread for the executing events in the relay log is running.

  • Last_Error — shows the last error registered when processing the relay log. Ideally this should be blank, indicating no errors.

  • Seconds_Behind_Master — shows the number of seconds that the slave SQL thread is behind processing the master binary log. A high number (or an increasing one) can indicate that the slave is unable to cope with the large number of statements from the master.

    A value of 0 for Seconds_Behind_Master can usually be interpreted as meaning that the slave has caught up with the master, but there are some cases where this is not strictly true. For example, this can occur if the network connection between master and slave is broken but the slave I/O thread has not yet noticed this — that is, slave_net_timeout has not yet elapsed.

    It is also possible that transient values for Seconds_Behind_Master may not reflect the situation accurately. When the slave SQL thread has caught up on I/O, Seconds_Behind_Master displays 0; but when the slave I/O thread is still queuing up a new event, Seconds_Behind_Master may show a large value until the SQL thread finishes executing the new event. This is especially likely when the events have old timestamps; in such cases, if you execute SHOW SLAVE STATUS several times in a relatively short peiod, you may see this value change back and forth repeatedly between 0 and a relatively large value.

On the master, you can check the status of slaves by examining the list of running processes. Slaves execute the Binlog Dump command: 

mysql> SHOW PROCESSLIST \G;
*************************** 4. row ***************************
     Id: 10
   User: root
   Host: slave1:58371
     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 slave that drives the core of the replication process, very little information is available in this report.

If you have used the --report-host option, then the SHOW SLAVE HOSTS statement will show basic information about connected slaves: 

mysql> SHOW SLAVE HOSTS;
+-----------+--------+------+-------------------+-----------+
| Server_id | Host   | Port | Rpl_recovery_rank | Master_id |
+-----------+--------+------+-------------------+-----------+
|        10 | slave1 | 3306 |                 0 |         1 |
+-----------+--------+------+-------------------+-----------+
1 row in set (0.00 sec)

The output includes the ID of the slave server, the value of the --report-host option, the connecting port, master ID and the priority of the slave for receiving binary log updates.