MySQL :: MySQL Replication :: 2.7.1 Checking Replication Status

version 8.0

5.7
5.6

MySQL Replication/.../ Checking Replication Status

2.7.1 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 replica and the source.

The显示REPLICA | SLAVE STATUSstatement, 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,显示SLAVE STATUSis deprecated, and显示REPLICA STATUSis available to use instead. The Performance Schema has replication tables that provide this information in a more accessible form. SeePerformance 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. TheMASTER_HEARTBEAT_PERIODsetting on the source (set by theCHANGE MASTER TOstatement) specifies the frequency of the heartbeat, which defaults to half of the connection timeout interval for the replica (slave_net_timeout). Thereplication_connection_statusPerformance 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显示REPLICA | SLAVE STATUSstatement to check on the status of an individual replica, the statement provides the following information:

mysql> SHOW REPLICA STATUS\G *************************** 1. row *************************** Replica_IO_State: Waiting for master 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 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_Source_Log_Pos: 931 Relay_Log_Space: 1365 Until_Condition: None Until_Log_File: Until_Log_Pos: 0 Source_SSL_Allowed: No Source_SSL_CA_File: Source_SSL_CA_Path: Source_SSL_Cert: Source_SSL_Cipher: Source_SSL_Key: Seconds_Behind_Source: 0 Source_SSL_Verify_Server_Cert: No Last_IO_Errno: 0 Last_IO_Error: Last_SQL_Errno: 0 Last_SQL_Error: Replicate_Ignore_Server_Ids: 0

The key fields from the status report to examine are:

Replica_IO_State: The current status of the replica. SeeReplication I/O Thread States, andReplication SQL Thread States, for more information.
Replica_IO_Running: Whether the I/O thread for reading the source's binary log is running. Normally, you want this to beYesunless you have not yet started replication or have explicitly stopped it withSTOP REPLICA | SLAVE.
Replica_SQL_Running: Whether the SQL thread for executing events in the relay log is running. As with the I/O thread, this should normally beYes.
Last_IO_Error,Last_SQL_Error: The last errors registered by the I/O and SQL threads when processing the relay log. Ideally these should be blank, indicating no errors.
Seconds_Behind_Source: The number of seconds that the replication SQL thread is behind processing the source binary log. A high number (or an increasing one) can indicate that the replica is unable to handle events from the source in a timely fashion.
A value of 0 forSeconds_Behind_Sourcecan usually be interpreted as meaning that the replica has caught up with the source, but there are some cases where this is not strictly true. For example, this can occur if the network connection between source and replica is broken but the replication I/O thread has not yet noticed this; that is,slave_net_timeouthas not yet elapsed.
It is also possible that transient values forSeconds_Behind_Sourcemay not reflect the situation accurately. When the replication SQL thread has caught up on I/O,Seconds_Behind_Sourcedisplays 0; but when the replication I/O thread is still queuing up a new event,Seconds_Behind_Sourcemay show a large value until the replication SQL thread finishes executing the new event. This is especially likely when the events have old timestamps; in such cases, if you execute显示REPLICA | SLAVE STATUSseveral times in a relatively short period, you may see this value change back and forth repeatedly between 0 and a relatively large value.

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:

(Master_Log_file,Read_Master_Log_Pos): Coordinates in the source binary log indicating how far the replication I/O thread has read events from that log.
(Relay_Master_Log_File,Exec_Master_Log_Pos): Coordinates in the source binary log indicating how far the replication SQL thread has executed events received from that log.
(Relay_Log_File,Relay_Log_Pos): Coordinates in the replica relay log indicating how far the replication SQL thread has executed the relay log. These correspond to the preceding coordinates, but are expressed in replica relay log coordinates rather than source binary log coordinates.

On the source, you can check the status of connected replicas using显示PROCESSLISTto examine the list of running processes. Replica connections haveBinlog Dumpin theCommandfield:

mysql> SHOW PROCESSLIST \G; *************************** 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-hostoption and are connected to the source, the显示REPLICAS | SHOW SLAVE HOSTSstatement on the source shows basic information about the replicas. The output includes the ID of the replica server, the value of the--report-hostoption, the connecting port, and source ID:

mysql> SHOW REPLICAS; +-----------+----------+------+-------------------+-----------+ | Server_id | Host | Port | Rpl_recovery_rank | Source_id | +-----------+----------+------+-------------------+-----------+ | 10 | replica1 | 3306 | 0 | 1 | +-----------+----------+------+-------------------+-----------+ 1 row in set (0.00 sec)

PREV HOME UP NEXT