17.1.6 Replication and Binary Logging 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 and global transaction identifiers (GTIDs). 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
`SET_VAR` Hint Applies	No
Type	Integer
Default Value (>= 8.0.3)	`1`
Default Value (<= 8.0.2)	`0`
Minimum Value	`0`
Maximum Value	`4294967295`

Specifies the server ID. The server_id system variable is set to 1 by default. The server can be started with this default ID, but when binary logging is enabled, an informational message is issued if you did not specify a server ID explicitly using the --server-id option.

For servers that are used in a replication topology, you must specify a unique server ID for each replication server, 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 additional information, see Section 17.1.6.2, “Replication Master Options and Variables”, and Section 17.1.6.3, “Replication Slave Options and Variables”.

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.

For more information, see Section 17.1.2.2, “Setting the Replication Slave Configuration”.

server_uuid

The MySQL server generates a true UUID in addition to the default or user-supplied server ID set in the server_id system variable. This is available as the global, read-only variable server_uuid.

Note

The presence of the server_uuid system variable 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.

Property	Value
System Variable	`server_uuid`
Scope	Global
Dynamic	No
`SET_VAR` Hint Applies	No
Type	String

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

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

When using MySQL replication, masters and slaves know each other'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, 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.

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.

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