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17.2.2 Replication Implementation Details

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

Binlog dump thread. The master creates a thread to send the binary log contents to a slave when the slave connects. This thread can be identified in the output of SHOW PROCESSLIST on the master as the Binlog Dump thread.
The binary log dump thread acquires a lock on the master's binary log for reading each event that is to be sent to the slave. As soon as the event has been read, the lock is released, even before the event is sent to the slave.
Slave I/O thread. When a START SLAVE statement is issued on a slave server, the slave creates an I/O thread, which connects to the master and asks it to send the updates recorded in its binary logs.
The slave I/O thread reads the updates that the master's Binlog Dump thread sends (see previous item) and copies them to local files that comprise the slave's relay log.
The state of this thread is shown as Slave_IO_running in the output of SHOW SLAVE STATUS or as Slave_running in the output of SHOW STATUS.
Slave SQL thread. The slave creates an SQL thread to read the relay log that is written by the slave I/O thread and execute the events contained therein.

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

A slave uses two threads to separate reading updates from the master 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 slave server has not been running for a while, its I/O thread can quickly fetch all the binary log contents from the master when the slave starts, even if the SQL thread lags far behind. If the slave 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 slave's relay logs, ready for execution the next time that the slave starts.

The SHOW PROCESSLIST statement provides information that tells you what is happening on the master and on the slave regarding replication. For information on master states, see Section 8.14.3, “Replication Master Thread States”. For slave states, see Section 8.14.4, “Replication Slave I/O Thread States”, and Section 8.14.5, “Replication Slave SQL Thread States”.

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

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

On a slave 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 I/O thread that is communicating with the master server, and thread 11 is the 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 slave is compared to the master. See Section A.13, “MySQL 8.0 FAQ: Replication”. If sufficient time elapses on the master side without activity on the Binlog Dump thread, the master determines that the slave 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 slave server. See Section 17.1.7.1, “Checking Replication Status”.

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