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MySQL 8.0 Reference Manual
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17.1.2.2 Setting the Replica Configuration

Each replica must have a unique server ID, as specified by the server_id system variable. If you are setting up multiple replicas, each one must have a unique server_id value that differs from that of the source and from any of the other replicas. If the replica's server ID is not already set, or the current value conflicts with the value that you have chosen for the source or another replica, you must change it.

The default server_id value is 1. You can change the server_id value dynamically by issuing a statement like this:

SET GLOBAL server_id = 21;

Note that a value of 0 for the server ID prevents a replica from connecting to a source. If that server ID value (which was the default in earlier releases) was set previously, you must restart the server to initialize the replica with your new nonzero server ID. Otherwise, a server restart is not needed when you change the server ID, unless you make other configuration changes that require it. For example, if binary logging was disabled on the server and you want it enabled for your replica, a server restart is required to enable this.

If you are shutting down the replica server, you can edit the [mysqld] section of the configuration file to specify a unique server ID. For example:

[mysqld]
server-id=21

Binary logging is enabled by default on all servers. A replica is not required to have binary logging enabled for replication to take place. However, binary logging on a replica means that the replica's binary log can be used for data backups and crash recovery. Replicas that have binary logging enabled can also be used as part of a more complex replication topology. For example, you might want to set up replication servers using this chained arrangement:

A -> B -> C

Here, A serves as the source for the replica B, and B serves as the source for the replica C. For this to work, B must be both a source and a replica. Updates received from A must be logged by B to its binary log, in order to be passed on to C. In addition to binary logging, this replication topology requires the log_slave_updates system variable to be enabled. With replica updates enabled, the replica writes updates that are received from a source and performed by the replica's SQL thread to the replica's own binary log. The log_slave_updates system variable is enabled by default.

If you need to disable binary logging or replica update logging on a replica, you can do this by specifying the --skip-log-bin and --log-slave-updates=OFF options for the replica. If you decide to re-enable these features on the replica, remove the relevant options and restart the server.