Replication and Binary Logging Options and Variables 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  value for each MySQL server as part of preparing and running MySQL replication, as described earlier in this section. 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: The  auto.cnf  file is automatically generated; do not attempt to write or modify this file. 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” . Issuing a  STOP REPLICA | SLAVE  or  RESET REPLICA | SLAVE  statement does  not  reset the source’s UUID as used on the replica. 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 | SHOW SLAVE HOSTS . Once  START REPLICA | SLAVE  has been executed, the value of the source’s UUID is available on the replica in the output of  SHOW REPLICA | SLAVE STATUS . When starting, the replication I/O 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 I/O 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. 3489 Note Important [auto] server_uuid=8a94f357-aab4-11df-86ab-c80aa9429562 Note System Variable server_uuid Scope Global Dynamic No SET_VAR  Hint Applies No Type String 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.