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17.1.6 Replication and Binary Logging Options and Variables
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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”.
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.
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:
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
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.
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_uuidexists.The master's
server_uuidhas changed, although noCHANGE MASTER TOstatement has ever been executed.
Document created the 26/06/2006, last modified the 26/10/2018
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