What is replication
Replication means that data written on a master MySQL will be sent to separate server and executed there.
- spread read access on multiple servers for scalability
- Asynchronous replication (basic master/slave)
- Semi-asynchronous replication (asynchronous replication + enforce 1 slave replication before completing queries)
- standard: master->slave
- dual master: master<->master
In Master-Master replication both hosts are masters and slaves at the same time. ServerA replicates to serverB which replicates to serevrA. There are no consistency checks and even with auto_increment_increment/auto_increment_offset configured both servers should not be used for concurrent writes.
That's the most simple replication. A master writes a binary log file, and slaves can read this log file (possibly selectively) to replay the query statements. It's asynchronous, which mean the master and slaves may have different states at a specific point of time; also this setup can survive a network disconnection.
Configuration on the master
/etc/mysql/my.cnf, in the
- Define a server identifier (detects loops?); customarily we'll use
1for 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
/etc/mysql/my.cnf, in the
- 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 HOSTSbelow):
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):
Start the replication:
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.
Oct 15 21:11:19 builder mysqld: 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
*************************** 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 inspect the asynchronous nature of the replication:
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)
Note that this replication is a simple replay, similar to feeding a
mysqldump output to the
Consequently, to maintain the consistency:
- Avoid writing critical data on the slave
- 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
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
Oct 15 21:11:19 builder mysqld: 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 if there are more errors and
- 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)
To erase the replication:
mysql> RESET SLAVE;
- Note: at this point, MySQL paused the slave and replaced the configuration with default values. The
master.infofile was also removed.
- Restart MySQL to clear all configuration.
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:
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)
Some hints can be placed in comment before each request concerning the replication. For example via the PHP Mysqlnd plugin (for native driver).
- MYSQLND_MS_MASTER_SWITCH: forces the request execution on the master.
- MYSQLND_MS_SLAVE_SWITCH: forces the request execution on the slave.
- MYSQLND_MS_LAST_USED_SWITCH: forces the request execution on the last used server.
As an alternative to the replication and clustering, the MySQL Federated storage engine allows to create a table on a server which synchronizes with the same on another one.