MySQL Essentials
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CHANGE MASTER TO master_def [, master_def] ...

master_def:
    MASTER_HOST = 'host_name'
  | MASTER_USER = 'user_name'
  | MASTER_PASSWORD = 'password'
  | MASTER_PORT = port_num
  | MASTER_CONNECT_RETRY = count
  | MASTER_LOG_FILE = 'master_log_name'
  | MASTER_LOG_POS = master_log_pos
  | RELAY_LOG_FILE = 'relay_log_name'
  | RELAY_LOG_POS = relay_log_pos
  | MASTER_SSL = {0|1}
  | MASTER_SSL_CA = 'ca_file_name'
  | MASTER_SSL_CAPATH = 'ca_directory_name'
  | MASTER_SSL_CERT = 'cert_file_name'
  | MASTER_SSL_KEY = 'key_file_name'
  | MASTER_SSL_CIPHER = 'cipher_list'

CHANGE MASTER TO changes the parameters that the slave server uses for connecting to and communicating with the master server. It also updates the contents of the master.info and relay-log.info files.

MASTER_USER, MASTER_PASSWORD, MASTER_SSL, MASTER_SSL_CA, MASTER_SSL_CAPATH, MASTER_SSL_CERT, MASTER_SSL_KEY, and MASTER_SSL_CIPHER provide information to the slave about how to connect to its master.

The SSL options (MASTER_SSL, MASTER_SSL_CA, MASTER_SSL_CAPATH, MASTER_SSL_CERT, MASTER_SSL_KEY, and MASTER_SSL_CIPHER) can be changed even on slaves that are compiled without SSL support. They are saved to the master.info file, but are ignored until you use a server that has SSL support enabled.

If you don't specify a given parameter, it keeps its old value, except as indicated in the following discussion. For example, if the password to connect to your MySQL master has changed, you just need to issue these statements to tell the slave about the new password:

STOP SLAVE; -- if replication was running
CHANGE MASTER TO MASTER_PASSWORD='new3cret';
START SLAVE; -- if you want to restart replication

There is no need to specify the parameters that do not change (host, port, user, and so forth).

MASTER_HOST and MASTER_PORT are the hostname (or IP address) of the master host and its TCP/IP port. Note that if MASTER_HOST is equal to localhost, then, like in other parts of MySQL, the port number might be ignored (if Unix socket files can be used, for example).

If you specify MASTER_HOST or MASTER_PORT, the slave assumes that the master server is different from before (even if you specify a host or port value that is the same as the current value.) In this case, the old values for the master binary log name and position are considered no longer applicable, so if you do not specify MASTER_LOG_FILE and MASTER_LOG_POS in the statement, MASTER_LOG_FILE='' and MASTER_LOG_POS=4 are silently appended to it.

MASTER_LOG_FILE and MASTER_LOG_POS are the coordinates at which the slave I/O thread should begin reading from the master the next time the thread starts. If you specify either of them, you cannot specify RELAY_LOG_FILE or RELAY_LOG_POS. If neither of MASTER_LOG_FILE or MASTER_LOG_POS are specified, the slave uses the last coordinates of the slave SQL thread before CHANGE MASTER was issued. This ensures that there is no discontinuity in replication, even if the slave SQL thread was late compared to the slave I/O thread, when you merely want to change, say, the password to use.

CHANGE MASTER deletes all relay log files and starts a new one, unless you specify RELAY_LOG_FILE or RELAY_LOG_POS. In that case, relay logs are kept; the relay_log_purge global variable is set silently to 0.

CHANGE MASTER is useful for setting up a slave when you have the snapshot of the master and have recorded the log and the offset corresponding to it. After loading the snapshot into the slave, you can run CHANGE MASTER TO MASTER_LOG_FILE='log_name_on_master', MASTER_LOG_POS=log_offset_on_master on the slave.

The following example changes the master and master's binary log coordinates. This is used when you want to set up the slave to replicate the master:

CHANGE MASTER TO
  MASTER_HOST='master2.mycompany.com',
  MASTER_USER='replication',
  MASTER_PASSWORD='bigs3cret',
  MASTER_PORT=3306,
  MASTER_LOG_FILE='master2-bin.001',
  MASTER_LOG_POS=4,
  MASTER_CONNECT_RETRY=10;

The next example shows an operation that is less frequently employed. It is used when the slave has relay logs that you want it to execute again for some reason. To do this, the master need not be reachable. You need only use CHANGE MASTER TO and start the SQL thread (START SLAVE SQL_THREAD):

CHANGE MASTER TO
  RELAY_LOG_FILE='slave-relay-bin.006',
  RELAY_LOG_POS=4025;

You can even use the second operation in a non-replication setup with a standalone, non-slave server for recovery following a crash. Suppose that your server has crashed and you have restored a backup. You want to replay the server's own binary logs (not relay logs, but regular binary logs), named (for example) myhost-bin.*. First, make a backup copy of these binary logs in some safe place, in case you don't exactly follow the procedure below and accidentally have the server purge the binary logs. Use SET GLOBAL relay_log_purge=0 for additional safety. Then start the server without the --log-bin option, Instead, use the --replicate-same-server-id, --relay-log=myhost-bin (to make the server believe that these regular binary logs are relay logs) and --skip-slave-start options. After the server starts, issue these statements:

CHANGE MASTER TO
  RELAY_LOG_FILE='myhost-bin.153',
  RELAY_LOG_POS=410,
  MASTER_HOST='some_dummy_string';
START SLAVE SQL_THREAD;

The server reads and executes its own binary logs, thus achieving crash recovery. Once the recovery is finished, run STOP SLAVE, shut down the server, delete the master.info and relay-log.info files, and restart the server with its original options.

Specifying the MASTER_HOST option (even with a dummy value) is required to make the server think it is a slave.