MySQL Shell 8.0 Release Notes
MySQL Router 8.0 User Guide
MySQL Router 8.0 Release Notes
From version 8.0.23 instances support and enable parallel replication applier threads, sometimes referred to as a multi-threaded replica. Using multiple replica applier threads in parallel improves the throughput of both the replication applier and incremental recovery.
This means that on instances running 8.0.23 and later, the following system variables must be configured:
By default, the number of applier threads (configured by the
slave_parallel_workers system
variable) is set to 4.
When you upgrade a cluster that has been running a version of
MySQL server and MySQL Shell earlier than 8.0.23, the instances
are not configured to use the parallel replication applier. If
the parallel applier is not enabled, the output of the
Cluster.status()instanceErrors field, for example:
...
"instanceErrors": [
"NOTE: The required parallel-appliers settings are not enabled on
the instance. Use dba.configureInstance() to fix it."
...
In this situation you should reconfigure your instances, so that
they use the parallel replication applier. For each instance
that belongs to the InnoDB Cluster, update the configuration
by issuing dba.configureInstance(instance).
Note that usually dba.configureInstance() is
used before adding the instance to a cluster, but in this
special case there is no need to remove the instance and the
configuration change is made while it is online.
Information about the parallel replication applier is displayed
in the output of the
Cluster.status(extended=1)topology section output for
the instance shows the number of threads under
applierWorkerThreads. The system variables
configured for the parallel replication applier are shown in the
output of the
Cluster.options()
You can configure the number of threads which an instance uses
for the parallel replication applier with the
applierWorkerThreads option, which defaults
to 4 threads. The option accepts integers in the range of 0 to
1024 and can only be used with the
dba.configureInstance() and
dba.configureReplicaSetInstance() operations.
For example, to use 8 threads, issue:
mysql-js> dba.configureInstance(instance, {applierWorkerThreads: 8, restart: true})Note
The change to the number of threads used by the parallel replication applier only occurs after the instance is restarted and has rejoined the cluster.
To disable the parallel replication applier, set the
applierWorkerThreads option to 0.