Before explaining how to set up replication, let me quickly explain the steps that MySQL goes through to maintain a replicated server. The process is different depending on the version of MySQL. For purposes of this article, my comments will be for version 4.0 or higher, since most systems now are using the later versions.
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When replication is running, basically, as SQL statements are executed on the master server, MySQL records them in a binary log (bin.log) along with a log position identification number. The slave server in turn, through an IO thread, regularly and very often reads the master's binary log for any changes. If it finds a change, it copies the new statements to its relay log (relay.log). It then records the new position identification number in a file (master.info) on the slave server. The slave then goes back to checking the master binary log, using the same IO thread. When the slave server detects a change to its relay log, through an SQL thread the slave executes the new SQL statement recorded in the relay log. As a safeguard, the slave also queries the master server through the SQL thread to compare its data with the master's data. If the comparison shows inconsistency, the replication process is stopped and an error message is recorded in the slave's error log (error.log). If the results of the query match, the new log position identification number is recorded in a file on the slave (relay-log.info) and the slave waits for another change to the relay log file.
This process may seem involved and complicated at first glance, but it all occurs quickly, it isn't a significant drain on the master server, and it ensures reliable replication. Also, it's surprisingly easy to set up. It only requires a few lines of options to be added to the configuration file (i.e., my.cnf) on the master and slave servers. If you're dealing with a new server, you'll need to copy the databases on the master server to the slave to get it caught up. Then it's merely a matter of starting the slave for it to begin replicating.
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