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MySQL 8.0 Reference Manual  /  ...  /  Replication I/O (Receiver) Thread States

8.14.5 Replication I/O (Receiver) Thread States

The following list shows the most common states you see in the State column for a replication I/O (receiver) thread on a replica server. This state also appears in the Replica_IO_State column displayed by SHOW REPLICA STATUS (or before MySQL 8.0.22, SHOW REPLICA STATUS), so you can get a good view of what is happening by using that statement.

In MySQL 8.0.26, incompatible changes were made to instrumentation names, including the names of thread stages, containing the terms master, which is changed to source, slave, which is changed to replica, and mts (for multithreaded slave), which is changed to mta (for multithreaded applier). Monitoring tools that work with these instrumentation names might be impacted. If the incompatible changes have an impact for you, set the terminology_use_previous system variable to BEFORE_8_0_26 to make MySQL Server use the old versions of the names for the objects specified in the previous list. This enables monitoring tools that rely on the old names to continue working until they can be updated to use the new names.

Set the terminology_use_previous system variable with session scope to support individual functions, or global scope to be a default for all new sessions. When global scope is used, the slow query log contains the old versions of the names.

  • Checking master version

    From MySQL 8.0.26: Checking source version

    A state that occurs very briefly, after the connection to the source is established.

  • Connecting to master

    From MySQL 8.0.26: Connecting to source

    The thread is attempting to connect to the source.

  • Queueing master event to the relay log

    From MySQL 8.0.26: Queueing source event to the relay log

    The thread has read an event and is copying it to the relay log so that the SQL thread can process it.

  • Reconnecting after a failed binlog dump request

    The thread is trying to reconnect to the source.

  • Reconnecting after a failed master event read

    From MySQL 8.0.26: Reconnecting after a failed source event read

    The thread is trying to reconnect to the source. When connection is established again, the state becomes Waiting for master to send event.

  • Registering slave on master

    From MySQL 8.0.26: Registering replica on source

    A state that occurs very briefly after the connection to the source is established.

  • Requesting binlog dump

    A state that occurs very briefly, after the connection to the source is established. The thread sends to the source a request for the contents of its binary logs, starting from the requested binary log file name and position.

  • Waiting for its turn to commit

    A state that occurs when the replica thread is waiting for older worker threads to commit if replica_preserve_commit_order or slave_preserve_commit_order is enabled.

  • Waiting for master to send event

    From MySQL 8.0.26: Waiting for source to send event

    The thread has connected to the source and is waiting for binary log events to arrive. This can last for a long time if the source is idle. If the wait lasts for replica_net_timeout or slave_net_timeout seconds, a timeout occurs. At that point, the thread considers the connection to be broken and makes an attempt to reconnect.

  • Waiting for master update

    From MySQL 8.0.26: Waiting for source update

    The initial state before Connecting to master or Connecting to source.

  • Waiting for slave mutex on exit

    From MySQL 8.0.26: Waiting for replica mutex on exit

    A state that occurs briefly as the thread is stopping.

  • Waiting for the slave SQL thread to free enough relay log space

    From MySQL 8.0.26: Waiting for the replica SQL thread to free enough relay log space

    You are using a nonzero relay_log_space_limit value, and the relay logs have grown large enough that their combined size exceeds this value. The I/O (receiver) thread is waiting until the SQL (applier) thread frees enough space by processing relay log contents so that it can delete some relay log files.

  • Waiting to reconnect after a failed binlog dump request

    If the binary log dump request failed (due to disconnection), the thread goes into this state while it sleeps, then tries to reconnect periodically. The interval between retries can be specified using the the CHANGE REPLICATION SOURCE TO statement (from MySQL 8.0.23) or CHANGE MASTER TO statement (before MySQL 8.0.23).

  • Waiting to reconnect after a failed master event read

    From MySQL 8.0.26: Waiting to reconnect after a failed source event read

    An error occurred while reading (due to disconnection). The thread is sleeping for the number of seconds set by the the CHANGE REPLICATION SOURCE TO statement (from MySQL 8.0.23) or CHANGE MASTER TO statement (before MySQL 8.0.23), which defaults to 60, before attempting to reconnect.