### 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 sources and replicas 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` system variable.

Command-Line Format | `--server-id=#`
System Variable | `server_id`
Scope | Global
Dynamic | Yes
`SET_VAR` Hint Applies | No
Type | Integer
Default Value | `1`
Minimum Value | `0`
Maximum Value | `4294967295`

This variable specifies the server ID. `server_id` 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 set `server_id` explicitly to specify a server ID.

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 source or replica in the replication topology. For additional information, see Section 17.1.6.2, “Replication Source Options and Variables”, and Section 17.1.6.3, “Replica Server Options and Variables”.

If the server ID is set to 0, binary logging takes place, but a source with a server ID of 0 refuses any connections from replicas, and a replica with a server ID of 0 refuses to connect to a source. 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 replica.

For more information, see Section 17.1.2.2, “Setting the Replica 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` value for each MySQL server as part of preparing and running MySQL replication, as described earlier in this section.

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:

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. `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:

    Press CTRL+C to copy [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, sources and replicas know each other's UUIDs. The value of a replica's UUID can be seen in the output of `SHOW REPLICAS` (or before MySQL 8.0.22, `SHOW SLAVE HOSTS`). Once `START REPLICA` has been executed, the value of the source's UUID is available on the replica in the output of `SHOW REPLICA STATUS`. As of 8.0.22, the keyword SLAVE was replaced by REPLICA.

Note

Issuing a `STOP REPLICA` or `RESET REPLICA` statement does not reset the source's UUID as used on the replica.

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

- No source having the expected `server_uuid` exists.

- The source's `server_uuid` has changed, although no `CHANGE REPLICATION SOURCE TO` | `CHANGE MASTER TO` statement has ever been executed.