New error codes and messages have been added to MySQL 5.7.6 to provide information about errors encountered in a multi-source replication topology. These error codes and messages are only emitted when multi-source replication is enabled, and provide information related to the channel which generated the error. For example:

Slave is already running and Slave is already stopped have been replaced with Replication thread(s) for channel channel_name are already running and Replication threads(s) for channel channel_name are already stopped respectively.

The server log messages have also been changed to indicate which channel the log messages relate to. This makes debugging and tracing easier.