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Database Replication in MySQL

May 18, 2004

An introduction to replication

Recently, while having the knots pounded out of my body during a particularly painful shiatsu lesson, I reflected on what put them there in the first place. Yes, 'the database' was once more to blame. A busy database I work with saw one of its tables jump from 3GB to 7GB overnight, as we imported archive data. As expected, this had some performance impact. Unfortunately, I had not expected quite the knock, and it turned out that this single database server could no longer handle the load. No matter how much more I tried to optimize the queries, tweak the variables or bump up the query cache, it was not enough. The machine could not take any more memory, and a hardware upgrade would do little good (at least with the kind of budget I have to play with). However, MySQL does not claim to be enterprise-ready for nothing, and Yahoo and other high-volume users of MySQL certainly do not run on one database server. There are a number of techniques to handle high volumes, one of which I will introduce this month - MySQL replication (I will look at others in future articles).

Replication allows you to take one database, make an exact copy of it on another server, and set one of them (the slave) to take all its updates from the other (the master). The slave reads the master's binary logs, which store all statements that change a database, and repeats these on its database, keeping the two in exact sync. Since a replicating database simply repeats statements, the databases are not necessarily exactly in sync, and advanced users can take advantage of this. That is a topic for another article however, and we will look at simple replication this month - getting one database to be an exact copy of another one.

What replication is not

  • Replication is not a backup policy. A mistyped DELETE statement will be replicated on the slave too, and you could end up with two, perfectly synchronized, empty databases. Replication can help protect against hardware failure though.
  • Replication is not an answer to all performance problems. Although updates on the slave are more optimized than if you ran the updates normally, if you use MyISAM tables, table-locking will still occur, and databases under high-load could still struggle.
  • Replication is not a guarantee that the slave will be in sync with the master at any one point in time. Even assuming the connection is always up, a busy slave may not yet have caught up with the master, so you can't simply interchange SELECT queries across master and slave servers.

How to start replicating - the master server

  • Grant the slave permission to replicate with the REPLICATION SLAVE privilege, for example as follows: GRANT REPLICATION SLAVE ON *.* TO slave_user IDENTIFIED BY 'slave_password'
  • If the master is not using the binary update log, add the following lines to the my.cnf or my.ini configuration file, and restart the server:
     
    log-bin
    server-id=1
    

    By convention, the master is usually server-id 1, and any slaves from 2 onwards, though you can change this if you wish. If the master is already using the binary update log, either take note of the offset at the moment of the backup (the next step), or use the RESET MASTER statement to clear all binary logs and immediately begin the backup. You may want to make a copy of the binary logs before doing this, in case you need to use the binary logs to restore from backup.

  • Make a backup of the database. You will use this to start the slave server. Note the comments about the binary log above. You can also skip this step if you use the LOAD DATA FROM MASTER statement, but see the comments about locking the master below first.







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