CHANGE MASTER TO

Sintassi

CHANGE MASTER TO def_master [, def_master] ...

master_def:
    MASTER_BIND = 'nome_interfaccia'
  | MASTER_HOST = 'nome_host'
  | MASTER_USER = 'nome_utente'
  | MASTER_PASSWORD = 'password'
  | MASTER_PORT = num_porta
  | MASTER_CONNECT_RETRY = intervallo
  | MASTER_HEARTBEAT_PERIOD = intervallo
  | MASTER_LOG_FILE = 'nome_log_master'
  | MASTER_LOG_POS = pos_log_master
  | RELAY_LOG_FILE = 'nome_relay_log'
  | RELAY_LOG_POS = pos_relay_log
  | MASTER_SSL = {0|1}
  | MASTER_SSL_CA = 'nome_file_ca'
  | MASTER_SSL_CAPATH = 'nome_dir_ca'
  | MASTER_SSL_CERT = 'nome_file_cert'
  | MASTER_SSL_KEY = 'nome_file_chiave'
  | MASTER_SSL_CIPHER = 'lista_cifrature'
  | MASTER_SSL_VERIFY_SERVER_CERT = {0|1}

Spiegazione

CHANGE MASTER TO modifica i parametri utilizzati dallo slave per connettersi e comunicare con il master. Inoltre aggiorna i contenuti dei file master.info e relay-log.info.

MASTER_USER, MASTER_PASSWORD, MASTER_SSL, MASTER_SSL_CA, MASTER_SSL_CAPATH, MASTER_SSL_CERT, MASTER_SSL_KEY, MASTER_SSL_CIPHER e MASTER_SSL_VERIFY_SERVER_CERT forniscono allo slave informazioni su come connettersi al master. MASTER_SSL_VERIFY_SERVER_CERT è stato aggiunto in MySQL 5.1.18. Si usa come spiegato per l'opzione --ssl-verify-server-cert alla pagina: http://dev.mysql.com/doc/refman/5.1/en/ssl-options.html

MASTER_CONNECT_RETRY specifica quanti secondi deve attendere prima di riprovare a connettersi. Il default è 60. Il numero massimo di tentativi di riconnessione è determinato dall'opzione server --master-retry-count; per ulteriori informazioni, si veda http://dev.mysql.com/doc/refman/5.1/en/replication-options.html.

Le opzioni SSL (MASTER_SSL, MASTER_SSL_CA, MASTER_SSL_CAPATH, MASTER_SSL_CERT, MASTER_SSL_KEY, MASTER_SSL_CIPHER) e MASTER_SSL_VERIFY_SERVER_CERT possono essere impostate anche sugli slave compilati senza il supporto a SSL. Esse vengono salvate nel file master.info file, ma ignorate se non si utilizza un server con il supporto SSL abilitato.

Se non si specifica un certo parametro, esso mantiene il vecchio valore, eccetto dove indicato nella seguente spiegazione. Per esempio, se la password per connettersi al master viene modificata, per comunicare la nuova password allo slave è sufficiente usare le seguenti istruzioni:

STOP SLAVE; -- se la replica è in funzione
CHANGE MASTER TO MASTER_PASSWORD='new3cret';
START SLAVE; -- se si vuole riavviare la replica

Non c'è bisogno di specificare i parametri che non si desidera modificare (host, porta, utente e così via).

MASTER_HOST e MASTER_PORT sono il nome (o l'indirizzo IP) dell'host del master e la sua porta TCP/IP.

The next two options are available only in MySQL Cluster NDB 6.3 and 6.4; they are not supported in mainline MySQL 5.1:

• MASTER_BIND is for use on replication slaves having multiple network interfaces, and determines which of the slave's network interfaces is chosen for connecting to the master. It is also possible to determine which network interface is to be used in such cases by starting the slave mysqld process with the --master-bind option.
  
  The ability to bind a replication slave to specific network interface was added in MySQL Cluster NDB 6.3.4.
• MASTER_HEARTBEAT_PERIOD is used to set the interval in seconds between replication heartbeats. Whenever the master's binlog is updated with an event, the waiting period for the next heartbeat is reset. interval is a decimal value having the range 0 to 4294967 seconds and a resolution to hundredths of a second; the smallest nonzero value is 0.001. Heartbeats are sent by the master only if there are no unsent events in the binlog file for a period longer than interval.
  
  Setting interval to 0 disables heartbeats altogether. The default value for interval is equal to the value of slave_net_timeout divided by 2.
  
  Note: Setting @@global.slave_net_timeout to a value less than that of the current heartbeat interval results in a warning being issued.
  
  Issuing RESET SLAVE resets the heartbeat interval to the default.
  
  MASTER_HEARTBEAT_PERIOD was added in MySQL Cluster NDB 6.3.4.

Note: Replication cannot use Unix socket files. You must be able to connect to the master MySQL server using TCP/IP.

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.

Setting MASTER_HOST='' — that is, setting its value explicitly to an empty string — is not the same as not setting it at all. Setting this option to an empty string causes START SLAVE subsequently to fail. This issue is addressed in MySQL 6.0. (MySQL Bug #28796)

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 TO 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 TO 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 TO 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;