17.1.4 Replication and Binary Logging Options and Variables

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17.1.4.1 Replication and Binary Logging Option and Variable Reference

17.1.4.2 Replication Master Options and Variables

17.1.4.3 Replication Slave Options and Variables

17.1.4.4 Binary Log Options and Variables

17.1.4.5 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.

Property	Value
Command-Line Format	--server-id=#
System Variable	server_id
Scope	Global
Dynamic	Yes
Type	Integer
Default Value	0
Minimum Value	0
Maximum Value	4294967295

This option specifies the server ID that is set in the server_id system variable.

On a replication master and each replication slave, you must specify 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. For additional information, see Section 17.1.4.2, “Replication Master Options and Variables”, and Section 17.1.4.3, “Replication Slave Options and Variables”.

If you omit --server-id, the default server ID is 0. If the server ID is set to 0, binary logging takes place, but a master with a server ID of 0 refuses any connections from slaves, and a slave with a server ID of 0 refuses to connect to a master. Note that although you can change the server ID dynamically to a nonzero value, doing so does not enable replication to start immediately. You must change the server ID and then restart the server to initialize the replication slave.

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 17.1.1.2, “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.

Property	Value
System Variable	server_uuid
Scope	Global
Dynamic	No
Type	String

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:

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

Important

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.

Note

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:

• No master having the expected server_uuid exists.

• The master's server_uuid has changed, although no CHANGE MASTER TO statement has ever been executed.

Note

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.