MySQL replication capabilities are implemented using the following types of threads:
Binary log 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 PROCESSLISTon the source as the
Replication I/O receiver thread. When a
START REPLICAstatement is issued on a replica server, the replica creates an I/O (receiver) thread, which connects to the source and asks it to send the updates recorded in its binary logs.
The replication receiver thread reads the updates that the source's
Binlog Dumpthread 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_runningin the output of
SHOW REPLICA STATUS.
Replication SQL applier thread. When
replica_parallel_workers(in MySQL 8.0.26 and earlier, use
slave_parallel_workers) is equal to 0, the replica creates an SQL (applier) thread to read the relay log that is written by the replication receiver thread and execute the transactions contained in it. When
, there are
Napplier threads and one coordinator thread, which reads transactions sequentially from the relay log, and schedules them to be applied by worker threads. Each worker applies the transactions that the coordinator has assigned to it.
You can enable further parallelization for tasks on a replica by
setting the system variable
replica_parallel_workers (MySQL 8.0.26 or
slave_parallel_workers (prior to
MySQL 8.0.26) to a value greater than 0. When this is done, the
replica creates the specified number of worker threads to apply
transactions, plus a coordinator thread which reads transactions
from the relay log and assigns them to workers. A replica with
slave_parallel_workers) set to a value greater
than 0 is called a multithreaded replica. If you are using
multiple replication channels, each channel has the number of
threads specified using this variable.
Multithreaded replicas are supported by NDB Cluster beginning
with NDB 8.0.33. (Previously,
ignored any setting for
NDB Cluster Replication Using the Multithreaded Applier, for more