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MySQL 8.0 Reference Manual  /  ...  /  Replication Implementation Details

17.2.2 Replication Implementation Details

MySQL replication capabilities are implemented using three threads, one on the source server and two on the replica:

  • Binlog dump thread.  The source creates a thread to send the binary log contents to a replica when the replica connects. This thread can be identified in the output of SHOW PROCESSLIST on the source as the Binlog Dump thread.

    The binary log dump thread acquires a lock on the source's binary log for reading each event that is to be sent to the replica. As soon as the event has been read, the lock is released, even before the event is sent to the replica.

  • Replication I/O thread.  When a START SLAVE statement is issued on a replica server, the replica creates an I/O thread, which connects to the source and asks it to send the updates recorded in its binary logs.

    The replication I/O thread reads the updates that the source's Binlog Dump thread sends (see previous item) and copies them to local files that comprise the replica's relay log.

    The state of this thread is shown as Slave_IO_running in the output of SHOW SLAVE STATUS.

  • Replication SQL thread.  The replica creates an SQL thread to read the relay log that is written by the replication I/O thread and execute the events contained in it.

In the preceding description, there are three threads per source/replica connection. A source that has multiple replicas creates one binary log dump thread for each currently connected replica, and each replica has its own replication I/O and SQL threads.

A replica uses two threads to separate reading updates from the source and executing them into independent tasks. Thus, the task of reading statements is not slowed down if statement execution is slow. For example, if the replica server has not been running for a while, its I/O thread can quickly fetch all the binary log contents from the source when the replica starts, even if the SQL thread lags far behind. If the replica stops before the SQL thread has executed all the fetched statements, the I/O thread has at least fetched everything so that a safe copy of the statements is stored locally in the replica's relay logs, ready for execution the next time that the replica starts.

The SHOW PROCESSLIST statement provides information that tells you what is happening on the source and on the replica regarding replication. For information on source states, see Section 8.14.4, “Replication Master Thread States”. For replica states, see Section 8.14.5, “Replication Slave I/O Thread States”, and Section 8.14.6, “Replication Slave SQL Thread States”.

The following example illustrates how the three threads show up in the output from SHOW PROCESSLIST.

On the source server, the output from SHOW PROCESSLIST looks like this:

mysql> SHOW PROCESSLIST\G
*************************** 1. row ***************************
     Id: 2
   User: root
   Host: localhost:32931
     db: NULL
Command: Binlog Dump
   Time: 94
  State: Has sent all binlog to slave; waiting for binlog to
         be updated
   Info: NULL

Here, thread 2 is a Binlog Dump thread that services a connected replica. The State information indicates that all outstanding updates have been sent to the replica and that the source is waiting for more updates to occur. If you see no Binlog Dump threads on a source server, this means that replication is not running; that is, no replicas are currently connected.

On a replica server, the output from SHOW PROCESSLIST looks like this:

mysql> SHOW PROCESSLIST\G
*************************** 1. row ***************************
     Id: 10
   User: system user
   Host:
     db: NULL
Command: Connect
   Time: 11
  State: Waiting for master to send event
   Info: NULL
*************************** 2. row ***************************
     Id: 11
   User: system user
   Host:
     db: NULL
Command: Connect
   Time: 11
  State: Has read all relay log; waiting for the slave I/O
         thread to update it
   Info: NULL

The State information indicates that thread 10 is the replication I/O thread that is communicating with the source server, and thread 11 is the replication SQL thread that is processing the updates stored in the relay logs. At the time that SHOW PROCESSLIST was run, both threads were idle, waiting for further updates.

The value in the Time column can show how late the replica is compared to the source. See Section A.14, “MySQL 8.0 FAQ: Replication”. If sufficient time elapses on the source side without activity on the Binlog Dump thread, the source determines that the replica is no longer connected. As for any other client connection, the timeouts for this depend on the values of net_write_timeout and net_retry_count; for more information about these, see Section 5.1.8, “Server System Variables”.

The SHOW SLAVE STATUS statement provides additional information about replication processing on a replica server. See Section 17.1.7.1, “Checking Replication Status”.