25.12.11 Performance Schema Replication Tables

The Performance Schema provides tables that expose replication information. This is similar to the information available from the SHOW SLAVE STATUS statement, but representation in table form is more accessible and has usability benefits:

SHOW SLAVE STATUS output is useful for visual inspection, but not so much for programmatic use. By contrast, using the Performance Schema tables, information about replica status can be searched using general SELECT queries, including complex WHERE conditions, joins, and so forth.
Query results can be saved in tables for further analysis, or assigned to variables and thus used in stored procedures.
The replication tables provide better diagnostic information. For multithreaded replica operation, SHOW SLAVE STATUS reports all coordinator and worker thread errors using the Last_SQL_Errno and Last_SQL_Error fields, so only the most recent of those errors is visible and information can be lost. The replication tables store errors on a per-thread basis without loss of information.
The last seen transaction is visible in the replication tables on a per-worker basis. This is information not avilable from SHOW SLAVE STATUS.
Developers familiar with the Performance Schema interface can extend the replication tables to provide additional information by adding rows to the tables.

Replication Table Descriptions

The Performance Schema provides the following replication-related tables:

Tables that contain information about the connection of a replica to the replication source server:
- replication_connection_configuration: Configuration parameters for connecting to the source
- replication_connection_status: Current status of the connection to the source
Tables that contain general (not thread-specific) information about the transaction applier:
- replication_applier_configuration: Configuration parameters for the transaction applier on the replica.
- replication_applier_status: Current status of the transaction applier on the replica.
Tables that contain information about specific threads responsible for applying transactions received from the source:
- replication_applier_status_by_coordinator: Status of the coordinator thread (empty unless the replica is multithreaded).
- replication_applier_status_by_worker: Status of the applier thread or worker threads if the replica is multithreaded.
Tables that contain information about replication group members:
- replication_group_members: Provides network and status information for group members.
- replication_group_member_stats: Provides statistical information about group members and transaction in which they participate.

The following sections describe each replication table in more detail, including the correspondence between the columns produced by SHOW SLAVE STATUS and the replication table columns in which the same information appears.

The remainder of this introduction to the replication tables describes how the Performance Schema populates them and which fields from SHOW SLAVE STATUS are not represented in the tables.

Replication Table Life Cycle

The Performance Schema populates the replication tables as follows:

Prior to execution of CHANGE MASTER TO, the tables are empty.
After CHANGE MASTER TO, the configuration parameters can be seen in the tables. At this time, there are no active replica threads, so the THREAD_ID columns are NULL and the SERVICE_STATE columns have a value of OFF.
After START SLAVE, non-NULL THREAD_ID values can be seen. Threads that are idle or active have a SERVICE_STATE value of ON. The thread that connects to the source has a value of CONNECTING while it establishes the connection, and ON thereafter as long as the connection lasts.
After STOP SLAVE, the THREAD_ID columns become NULL and the SERVICE_STATE columns for threads that no longer exist have a value of OFF.
The tables are preserved after STOP SLAVE or threads dying due to an error.
The replication_applier_status_by_worker table is nonempty only when the replica is operating in multithreaded mode. That is, if the slave_parallel_workers system variable is greater than 0, this table is populated when START SLAVE is executed, and the number of rows shows the number of workers.

`SHOW SLAVE STATUS` Information Not In the Replication Tables

The information in the Performance Schema replication tables differs somewhat from the information available from SHOW SLAVE STATUS because the tables are oriented toward use of global transaction identifiers (GTIDs), not file names and positions, and they represent server UUID values, not server ID values. Due to these differences, several SHOW SLAVE STATUS columns are not preserved in the Performance Schema replication tables, or are represented a different way:

The following fields refer to file names and positions and are not preserved:

Master_Log_File
Read_Master_Log_Pos
Relay_Log_File
Relay_Log_Pos
Relay_Master_Log_File
Exec_Master_Log_Pos
Until_Condition
Until_Log_File
Until_Log_Pos

The Master_Info_File field is not preserved. It refers to the master.info file, which has been superseded by crash-safe tables.
The following fields are based on server_id, not server_uuid, and are not preserved:
```
Master_Server_Id
Replicate_Ignore_Server_Ids
```
The Skip_Counter field is based on event counts, not GTIDs, and is not preserved.
These error fields are aliases for Last_SQL_Errno and Last_SQL_Error, so they are not preserved:
```
Last_Errno
Last_Error
```
In the Performance Schema, this error information is available in the LAST_ERROR_NUMBER and LAST_ERROR_MESSAGE columns of the replication_applier_status_by_worker table (and replication_applier_status_by_coordinator if the replica is multithreaded). Those tables provide more specific per-thread error information than is available from Last_Errno and Last_Error.

Fields that provide information about command-line filtering options is not preserved:

Replicate_Do_DB
Replicate_Ignore_DB
Replicate_Do_Table
Replicate_Ignore_Table
Replicate_Wild_Do_Table
Replicate_Wild_Ignore_Table

The Slave_IO_State and Slave_SQL_Running_State fields are not preserved. If needed, these values can be obtained from the process list by using the THREAD_ID column of the appropriate replication table and joining it with the ID column in the INFORMATION_SCHEMA PROCESSLIST table to select the STATE column of the latter table.
The Executed_Gtid_Set field can show a large set with a great deal of text. Instead, the Performance Schema tables show GTIDs of transactions that are currently being applied by the replica. Alternatively, the set of executed GTIDs can be obtained from the value of the gtid_executed system variable.
The Seconds_Behind_Master and Relay_Log_Space fields are in to-be-decided status and are not preserved.

Status Variables Moved to Replication Tables

As of MySQL version 5.7.5, the following status variables (previously monitored using SHOW STATUS) were moved to the Perfomance Schema replication tables:

These status variables are now only relevant when a single replication channel is being used because they only report the status of the default replication channel. When multiple replication channels exist, use the Performance Schema replication tables described in this section, which report these variables for each existing replication channel.

Replication Channels

The first column of the replication Performance Schema tables is CHANNEL_NAME. This enables the tables to be viewed per replication channel. In a non-multisource replication setup there is a single default replication channel. When you are using multiple replication channels on a replica, you can filter the tables per replication channel to monitor a specific replication channel. See Section 16.2.2, “Replication Channels” and Section 16.1.5.8, “Multi-Source Replication Monitoring” for more information.