17.2.3 Replication Channels

Replication channels represent the path of transactions flowing from a master to a slave. This section describes how channels can be used in a replication topology, and the impact they have on single-source replication.

To provide compatibility with previous versions, the MySQL server automatically creates on startup a default channel whose name is the empty string (""). This channel is always present; it cannot be created or destroyed by the user. If no other channels (having nonempty names) have been created, replication statements act on the default channel only, so that all replication statements from older slaves function as expected (see Section 17.2.3.2, “Compatibility with Previous Replication Statements”. Statements applying to replication channels as described in this section can be used only when there is at least one named channel.

A replication channel encompasses the path of transactions transmitted from a master to a slave. In multi-source replication a slave opens multiple channels, one per master, and each channel has its own relay log and applier (SQL) threads. Once transactions are received by a replication channel's receiver (I/O) thread, they are added to the channel's relay log file and passed through to an applier thread. This enables channels to function independently.

A replication channel is also associated with a host name and port. You can assign multiple channels to the same combination of host name and port. In MySQL 8.0, the maximum number of channels that can be added to one slave in a multi-source replication topology is 256. Each replication channel must have a unique (nonempty) name (see Section 17.2.3.4, “Replication Channel Naming Conventions”). Channels can be configured independently.