MySQL 8.0 Reference Manual Including MySQL NDB Cluster 8.0
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
PROCESSLIST on the source as the Binlog
Dump thread.
Replication I/O receiver thread.
When a START REPLICA
statement 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 Dump thread 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_running in 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 replica_parallel_workers is
N >= 1N applier 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
later) 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
replica_parallel_workers
(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, NDB silently
ignored any setting for
replica_parallel_workers.) See
Section 25.7.11, “NDB Cluster Replication Using the Multithreaded Applier”, for more
information.