17.1.4 Replication and Binary Logging Options and Variables Replication and Binary Logging Option and Variable Reference Replication Master Options and Variables Replication Slave Options and Variables Binary Log Options and Variables Global Transaction ID Options and Variables

The following 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.

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

Command-Line Format--server-id=#
System VariableNameserver_id
Variable ScopeGlobal
Dynamic VariableYes
Permitted ValuesTypeinteger
Min Value0
Max Value4294967295

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, “Replication Master Options and Variables”, and Section, “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. For example, server-id=3.

If you omit --server-id, the default ID is 0, in which case the master refuses connections from all slaves, and slaves refuse to connect to the master. In MySQL 5.6, whether the server ID is set to 0 explicitly or the default is allowed to be used, the server sets the server_id system variable to 1; this is a known issue that is fixed in MySQL 5.7.

For more information, see Section, “Setting the Replication Slave Configuration”.


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.

System VariableNameserver_uuid
Variable ScopeGlobal
Dynamic VariableNo
Permitted ValuesTypestring

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

  1. Attempt to read and use the UUID written in the file data_dir/auto.cnf (where data_dir is the server's data directory).

  2. If data_dir/auto.cnf is not found, generate a new UUID and save it to this file, creating the file if necessary.

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:


The auto.cnf file is automatically generated; do not attempt to write or modify this file.

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.


Issuing a STOP SLAVE or RESET SLAVE statement does not reset the master's UUID as used on the slave.

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 17.1.3, “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:


The addition of the server_uuid system variable in MySQL 5.6 does not change the requirement for setting a unique --server-id for each MySQL server as part of preparing and running MySQL replication, as described earlier in this section.