Replication Threads

MariaDB's replication implementation requires several types of threads.

Threads on the Master

The master usually only has one type of replication-related thread: the binary log dump thread.

If semisynchronous replication is enabled, then the master also has an ACK receiver thread.

Binary Log Dump Thread

The binary log dump thread runs on the master and dumps the binary log to the slave. This thread can be identified by running the SHOW PROCESSLIST statement and finding the thread where the thread command is "Binlog Dump".

The master creates a separate binary log dump thread for each slave connected to the master. You can identify which slaves are connected to the master by executing the SHOW SLAVE HOSTS statement.

Binary Log Dump Threads and the Shutdown Process

When a master server is shutdown and it goes through the normal shutdown process, the master kills client threads in random order. By default, the master also considers its binary log dump threads to be regular client threads. As a consequence, the binary log dump threads can be killed while client threads still exist, and this means that data can be written on the master during a normal shutdown that won't be replicated. This is true even if semi-synchronous replication is being used.

In MariaDB 10.4 and later, this problem can be solved by shutting down the server using either the mysqladmin utility or the SHUTDOWN command, and providing a special option.

For example, this problem can be solved by shutting down the server with the mysqladmin utility and by providing the --wait-for-all-slaves option to the utility and by executing the shutdown command with the utility:

mysqladmin --wait-for-all-slaves shutdown

Or this problem can be solved by shutting down the server with the SHUTDOWN command and by providing the WAIT FOR ALL SLAVES option to the command:

SHUTDOWN WAIT FOR ALL SLAVES;

When one of these special options is provided, the server only kills its binary log dump threads after all client threads have been killed, and it only completes the shutdown after the last binary log has been sent to all connected slaves.

In MariaDB 10.4 and later, it is still not possible to enable this behavior by default. This means that this behavior is currently inaccessible when shutting down the server using tools like systemd or sysVinit.

In MariaDB 10.3 and before, it is recommended to manually switchover slaves to a new master before shutting down the old master.

ACK Receiver Thread

When semisynchronous replication is enabled, semisynchronous slaves send acknowledgements (ACKs) to their master confirm that they have received some transaction. The master creates an ACK receiver thread to receive these ACKs.

Threads on the Slave

The slave has three types of replication-related threads: the slave I/O thread, the slave SQL thread, and worker threads, which are only applicable when parallel replication is in use.

When multi-source replication is in use, each independent replication connection has its own slave threads of each type.

Slave I/O Thread

The slave's I/O thread receives the binary log events from the master and writes them to its relay log.

Binary Log Position

The binary log position of the slave's I/O thread can be checked by executing the SHOW SLAVE STATUS statement. It will be shown as the Master_Log_File and Read_Master_Log_Pos columns.

The binary log position of the slave's I/O thread can be set by setting the MASTER_LOG_FILE and MASTER_LOG_POS options with the CHANGE MASTER statement.

The binary log position of the slave's I/O thread and the values of most other CHANGE MASTER options are written to either the default master.info file or the file that is configured by the master_info_file option. The slave's I/O thread keeps this binary log position updated as it downloads events. See CHANGE MASTER TO: Option Persistence for more information

Slave SQL Thread

The slave's SQL thread reads events from the relay log. What it does with them depends on whether parallel replication is in use. If parallel replication is not in use, then the SQL thread applies the events to its local copy of the data. If parallel replication is in use, then the SQL thread hands off the events to its worker threads to apply in parallel.

Relay Log Position

The relay log position of the slave's SQL thread can be checked by executing the SHOW SLAVE STATUS statement. It will be shown as the Relay_Log_File and Relay_Log_Pos columns.

The relay log position of the slave's SQL thread can be set by setting the RELAY_LOG_FILE and RELAY_LOG_POS options with the CHANGE MASTER statement.

The relay log position of the slave's SQL thread is written to either the default relay-log.info file or the file that is configured by the relay_log_info_file system variable. The slave's SQL thread keeps this relay log position updated as it applies events. See CHANGE MASTER TO: Option Persistence for more information.

Binary Log Position

The corresponding binary log position of the current relay log position of the slave's SQL thread can be checked by executing the SHOW SLAVE STATUS statement. It will be shown as the Relay_Master_Log_File and Exec_Master_Log_Pos columns.

GTID Position

If the slave is replicating binary log events that contain GTIDs, then the slave's SQL thread will write every GTID that it applies to the mysql.gtid_slave_pos table. This GTID can be inspected and modified through the gtid_slave_pos system variable.

If the slave has the log_slave_updates system variable enabled and if the slave has the binary log enabled, then every write by the slave's SQL thread will also go into the slave's binary log. This means that GTIDs of replicated transactions would be reflected in the value of the gtid_binlog_pos system variable.

See CHANGE MASTER TO: GTID Persistence for more information.

Worker Threads

When parallel replication is in use, then the SQL thread hands off the events to its worker threads to apply in parallel.