You have to use Slave database for read (select queries) operations and Master database for write( insert and update queries). Make changes in the following config file of magento: app/etc/local.xml
<default_setup>
<connection>
<host><![CDATA[Master-host]]></host>
<username><![CDATA[user]]></username>
<password><![CDATA[pass]]></password>
<dbname><![CDATA[magentodb]]></dbname>
<active>1</active>
</connection>
</default_setup>
<default_read>
<connection>
<use/>
<host><![CDATA[Slave-host]]></host>
<username><![CDATA[user]]></username>
<password><![CDATA[pass]]></password>
<dbname><![CDATA[magento]]></dbname>
<type>pdo_mysql</type>
<model>mysql4</model>
<initStatements>SET NAMES utf8</initStatements>
<active>1</active>
</connection>
</default_read>
Prior to this setup , you must already have configured your mysql master and slave server. To configure master server, edit /etc/my.cnf and add below content in the file:
[mysqld]
server-id = 1
log_bin = /var/log/mysql/mysql-bin.log
expire_logs_days = 10
max_binlog_size = 100M
binlog_do_db = magento_demo
binlog_ignore_db = mysql
then restart your mysql server.
Configuration for slave server
edit /etc/my.cnf
add below content in the file:
[mysqld]
server-id=2
log-bin=mysql-bin
master-host=192.168.1.2
master-user=username
master-password=111111
master-port=3306
replicate-do-db=magento_demo
replicate-ignore-db=mysql
master-connect-retry=60
then restart your mysql server.