The next few sections contain information about mysqld options and server variables that are used in replication and for controlling the binary log. Options and variables for use on replication masters and replication slaves are covered separately, as are options and variables relating to binary logging. A set of quick-reference tables providing basic information about these options and variables is also included (in the next section following this one).

Of particular importance is the --server-id option.

This option is common to both master and slave replication servers, and is used in replication to enable master and slave servers to identify themselves uniquely. For additional information, see Section 16.1.4.2, "Replication Master Options and Variables", and Section 16.1.4.3, "Replication Slave Options and Variables".

On the master and each slave, you must use the --server-id option to establish a unique replication ID in the range from 1 to 2³² – 1. "Unique", means that each ID must be different from every other ID in use by any other replication master or slave. Example: server-id=3.

If you omit --server-id, the default ID is 0, in which case a master refuses connections from all slaves, and a slave refuses to connect to a master. For more information, see Section 16.1.1.2, "Setting the Replication Slave Configuration".

server_uuid

Beginning with MySQL 5.6, the server generates a true UUID in addition to the --server-id supplied by the user. This is available as the global, read-only variable server_uuid.

When starting, the MySQL server automatically obtains a UUID as follows:

The auto.cnf file has a format similar to that used for my.cnf or my.ini files. In MySQL 5.6, auto.cnf has only a single [auto] section containing a single server_uuid setting and value; the file's contents appear similar to what is shown here:

[auto]server_uuid=8a94f357-aab4-11df-86ab-c80aa9429562

Also beginning with MySQL 5.6, when using MySQL replication, masters and slaves know one another's UUIDs. The value of a slave's UUID can be seen in the output of SHOW SLAVE HOSTS. Once START SLAVE has been executed (but not before), the value of the master's UUID is available on the slave in the output of SHOW SLAVE STATUS.

In MySQL 5.6.5 and later, a server's server_uuid is also used in GTIDs for transactions originating on that server. For more information, see Section 16.1.3, "Replication with Global Transaction Identifiers".

When starting, the slave I/O thread generates an error and aborts if its master's UUID is equal to its own unless the --replicate-same-server-id option has been set. In addition, the slave I/O thread generates a warning if either of the following is true: