Replication Threads

MariaDB's replication implementation requires several types of threads.

Threads on the Primary

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

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

Binary Log Dump Thread

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

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

Binary Log Dump Threads and the Shutdown Process

When a primary server is shutdown and it goes through the normal shutdown process, the primary kills client threads in random order. By default, the primary 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 primary during a normal shutdown that won't be replicated. This is true even if semi-synchronous replication is being used. Data is not lost, it is stored in the primary server's binary log. The replicas on reconnection, after the primary server restarts, will resume at the exact position they where killed off during the primary shutdown. No data is lost.

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

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

mariadb-admin --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 replicas.

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 replicas to a new primary before shutting down the old primary.

ACK Receiver Thread

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

Threads on the Replica

The replica has three types of replication-related threads: the replica I/O thread, the replica 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 replica threads of each type.

Replica I/O Thread

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

Binary Log Position

The binary log position of the replica'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 replica'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 replica'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 replica's I/O thread keeps this binary log position updated as it downloads events only when the MASTER_USE_GTID option is set to NO. Otherwise the file is not updated on a per event basis. See CHANGE MASTER TO: Option Persistence for more information.

Replica SQL Thread

The replica'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 replica'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 replica'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 replica'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 replica'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 replica'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 replica is replicating binary log events that contain GTIDs, then the replica's'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 replica has the log_slave_updates system variable enabled and if the replica has the binary log enabled, then every write by the replica's SQL thread will also go into the replica'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.