MySQL 5.7 Reference Manual Including MySQL NDB Cluster 7.5 and NDB Cluster 7.6

16.2.3 Replication Threads

MySQL replication capabilities are implemented using three main threads, one on the source server and two on the replica:

There are three main threads for each source/replica connection. A source that has multiple replicas creates one binary log dump thread for each currently connected replica, and each replica has its own replication I/O and SQL threads.

A replica uses two threads to separate reading updates from the source and executing them into independent tasks. Thus, the task of reading transactions is not slowed down if the process of applying them is slow. For example, if the replica server has not been running for a while, its I/O thread can quickly fetch all the binary log contents from the source when the replica starts, even if the SQL thread lags far behind. If the replica stops before the SQL thread has executed all the fetched statements, the I/O thread has at least fetched everything so that a safe copy of the transactions is stored locally in the replica's relay logs, ready for execution the next time that the replica starts.

You can enable further parallelization for tasks on a replica by setting the slave_parallel_workers system variable to a value greater than 0 (the default). When this system variable is set, the replica creates the specified number of worker threads to apply transactions, plus a coordinator thread to manage them. If you are using multiple replication channels, each channel has this number of threads. A replica with slave_parallel_workers set to a value greater than 0 is called a multithreaded replica. With this setup, transactions that fail can be retried.


Multithreaded replicas are not currently supported by NDB Cluster, which silently ignores the setting for this variable. See Section 21.7.3, “Known Issues in NDB Cluster Replication” for more information.