MySQL/Réplication

MySQL

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2. PhpMyAdmin
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4. Spécifier les noms
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7. Alias
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9. Manipulation de base
   • mysqldump
10. Manipulation de table
11. Manipulation de données
12. Requêtes
13. NULL
14. Opérateurs
15. Fonctions
16. Procédures stockées
17. Regex
18. Bases de données spatiales
19. Importer et exporter
20. Réplication
21. Optimisation
22. API
23. Sécurité
24. Mots réservés
25. Débogage

Principe

La réplication signifie que les données écrites sur le master MySQL sont envoyées à des slaves faisant office de copies.

Applications :

• sauvegardes
• accès en lecture de la même base depuis plusieurs serveurs : augmentation des performances
• failover

Il y a deux types de réplication :

• Asynchrone (master/slave)
• Semi-asynchrone (réplication asynchrone plus avec un slave avant de terminer la requête)

Configurations des réplications :

• standard : master->slave
• double maître : master<->master

En Master-Master les deux hôtes sont tour à tour master et slave : le serveur A se réplique sur le serveur B qui se réplique sur le serveur A. Il n'y a pas de vérification de consistance des données, même si auto_increment_increment/auto_increment_offset est configuré les deux serveurs ne doivent pas être utilisés pour des accès concurrents.

Réplication asynchrone

C'est le cas le plus simple, un master écrit un fichier de log binaire, et les slaves peuvent lire ce dernier (potentiellement sélectivement) pour rejouer les commandes de la requête.

Étant asynchrone, le master et les slaves peuvent avoir différents états au même moment. Cette configuration peut résister aux coupures réseau.

Configuration du master

In /etc/mysql/my.cnf, in the [mysqld] section:

• Define a server identifier (detects loops?); customarily we'll use 1 for the server, but it can be different:

server-id = 1

• Replication is based on binary logs, so enable them:

log-bin
# or log-bin = /var/log/mysql/mysql-bin.log

Create a new user for the slave to connect with:

 CREATE USER 'myreplication';
 SET PASSWORD FOR 'myreplication' = PASSWORD('mypass');
 GRANT REPLICATION SLAVE ON *.* to 'myreplication';

Verify your server identifier:

 SHOW VARIABLES LIKE 'server_id';

Configuration on each slave

In /etc/mysql/my.cnf, in the [mysqld] section:

• Define a server identifier, different than the master (and different than the other slaves):

server-id = 2

• Verify with:

 SHOW VARIABLES LIKE 'server_id';

• You can also declare the slave hostname to the master (cf. SHOW SLAVE HOSTS below):

report-host=slave1

Declare the master:

 CHANGE MASTER TO MASTER_HOST='master_addr', MASTER_USER='myreplication', MASTER_PASSWORD='mypass';

If setting up replication from backup, specify start point (add to previous command):

 MASTER_LOG_FILE='<binary_log_from_master>', MASTER_LOG_POS=<master_binary_log_position>;

Start the replication:

 START SLAVE;

This will create a file named master.info in your data directory, typically /var/lib/mysql/master.info; this file will contain the slave configuration and status.

TODO:

Oct 15 21:11:19 builder mysqld[4266]: 101015 21:11:19 [Warning] Neither --relay-log nor --relay-log-index were used; so
  replication may break when this MySQL server acts as a slave and has his hostname changed!! Please use
  '--relay-log=mysqld-relay-bin' to avoid this problem.

Check the replication

On the slave

On a slave, type:

 SHOW SLAVE STATUS;

Or more for a more readable (line-based) output:

 SHOW SLAVE STATUS\G

Example:

*************************** 1. row ***************************
             Slave_IO_State: 
                Master_Host: master_addr
                Master_User: myreplication
                Master_Port: 3306
...

Check in particular:

           Slave_IO_Running: Yes
          Slave_SQL_Running: Yes

You can suspect the asynchronous nature of the replication:

      Seconds_Behind_Master: 0

See also:

mysql> SHOW GLOBAL VARIABLES LIKE "%SLAVE%";

On the master

You can see a connection from the slave in the process list.

mysql> SHOW PROCESSLIST\G
[...]
*************************** 6. row ***************************
     Id: 14485
   User: myreplication
   Host: 10.1.0.106:33744
     db: NULL
Command: Binlog Dump
   Time: 31272
  State: Has sent all binlog to slave; waiting for binlog to be updated
   Info: NULL

If you enabled report-host, the slave is also visible in:

mysql> SHOW SLAVE HOSTS;
+-----------+---------+------+-------------------+-----------+
| Server_id | Host    | Port | Rpl_recovery_rank | Master_id |
+-----------+---------+------+-------------------+-----------+
|         2 | myslave | 3306 |                 0 |         1 | 
+-----------+---------+------+-------------------+-----------+
1 row in set (0.00 sec)

Consistency

Note that this replication is a simple replay, similar to feeding a mysqldump output to the mysql client. Consequently, to maintain the consistency:

• Do not write on the slave (this is possible!!)
• Start the replication with identical initial data on both the master and the slave
• To test: we suspect it would be best to use the same version of MySQL on the master and slaves

Fixing

By default, replicate will stop if it meets an error. This can happen if your master and slaves were not consistent in the beginning, or due to a network error causing a malformed query.

In this case, you'll get a trace in the system log (typically /var/log/syslog):

Oct 15 21:11:19 builder mysqld[4266]: 101015 21:11:19 [ERROR] Slave: Error 'Table 'mybase.form'
  doesn't exist' on query. Default database: 'mybase'.  Query:
  'INSERT INTO `form` (`form_id`,`timestamp`,`user_id`) VALUES ('abed',1287172429,0)',
  Error_code: 1146

The best way is to reset the replication entirely.

You can also fix the mistake manually, and then ask MySQL to skip 1 statement this way:

STOP SLAVE;
SET GLOBAL SQL_SLAVE_SKIP_COUNTER = 1;
START SLAVE;

You can set SQL_SLAVE_SKIP_COUNTER to any number, e.g. 100. Beware that in this case, it will skip both valid and invalid statements, not only errors.

Another way to fix broken replication is to use Maatkit tools.

• mk-slave-restart (to restart replication on slave it there are more errors and SQL_SLAVE_SKIP_COUNTER can't help)
• mk-table-checksum (to perform checksumming of tables on master and slave)
• mk-table-sync (to sync slave with master based on stats generated by mk-table-checksum)

Uninstalling

To erase the replication:

• Type:

mysql> RESET SLAVE;

• Note: at this point, MySQL paused the slave and replaced the configuration with default values. The master.info file was also removed.
• Restart MySQL to clear all configuration.

Warning: STOP SLAVE will stop replication. It can be started manually again or (by default) it will automatically resume if you restart the MySQL server. To avoid auto start of replication during process of startup, add to your configuration file:

slave-skip-start 

If you want to stop the replication for good (and use the server for another purpose), you need to reset the configuration as explained above.

At this point your slave configuration should be completely empty:

mysql> SHOW SLAVE STATUS;
Empty set (0.00 sec)