Can magento use multiple MySQL slaves?

Question

The following is part of my config for using a single slave.

<default_read>                 
    <connection>               
        <use/>                 
        <host><![CDATA[slavedb1.amazonaws.com]]></host>
        <username><![CDATA[username]]></username>
        <password><![CDATA[Password]]></password>
        <dbname><![CDATA[shop]]></dbname>
        <initStatements><![CDATA[SET NAMES utf8]]></initStatements>
        <model><![CDATA[mysql4]]></model>
        <type><![CDATA[pdo_mysql]]></type>
        <pdoType><![CDATA[]]></pdoType>
        <active>1</active>  
    </connection>
</default_read>

However, I want to use multiple slaves. Is this possible with Magento?

To clarify, I have already got a single master/slave setup working with Magento already. I want to add another slave so that I have two slaves. I am wondering how the config will change to make use of this second slave.

Answer 1

Given your comments above and assuming you have correct replication setup of your Databases.

Solution :-

- Step 1:

In File

app/etc/config.xml

Find "core_read" Closing tag

<resources>
    ....
    <core_read>
            <connection>
                <use>default_read</use>
            </connection>
        </core_read>
    ....
</resources>

Add After the closing tag ( as many as databases you want to use ) it should look like below:

<resources>
    ....
        <core_read>
            <connection>
                <use>default_read</use>
            </connection>
        </core_read>
        <slave_db_1>
            <connection>
                <use>slave_one_db</use>
            </connection>
        </slave_db_1>
        <slave_db_2>
            <connection>
                <use>slave_two_db</use>
            </connection>
        </slave_db_2>
    ....
</resources>

- Step 2:

And Add the new connection in your apt/etc/local.xml after ( "/default_setup>" closing tag )

<resources>
    ....            
        <slave_one_db>
            <connection>
                <host><![CDATA[localhost]]></host>
                <username><![CDATA[slave_one_db_user]]></username>
                <password><![CDATA[slave_one_db_password]]></password>
                <dbname><![CDATA[slave_db_one_name]]></dbname>
                <initStatements><![CDATA[SET NAMES utf8]]></initStatements>
                <model><![CDATA[mysql4]]></model>
                <type><![CDATA[pdo_mysql]]></type>
                <pdoType><![CDATA[]]></pdoType>
                <active>1</active>
            </connection>
        </slave_one_db>
        <slave_two_db>
            <connection>
                <host><![CDATA[localhost]]></host>
                <username><![CDATA[slave_tow_db_user]]></username>
                <password><![CDATA[slave_tow_db_password]]></password>
                <dbname><![CDATA[slave_db_one_tow]]></dbname>
                <initStatements><![CDATA[SET NAMES utf8]]></initStatements>
                <model><![CDATA[mysql4]]></model>
                <type><![CDATA[pdo_mysql]]></type>
                <pdoType><![CDATA[]]></pdoType>
                <active>1</active>
            </connection>
        </slave_two_db>
    ....
</resources>

- Step 3:

Copy the File From " app/code/core/Mage/Core/Model/Resource.php " == TO ==> " app/code/local/Mage/Core/Model/Resource.php "

1- Find protected $_mappedTableNames;

2- Add this method below :

public function getSlaveDb()
{
    $prefix = 'slave_db_'; // prefix for the slaves databased in the xml file
    $cookieExpireTime = 1209600; // 2 weeks Cookie ( database selection ) expire time
    $dbArray = array(1,2); // All slaves Db in-case the cookie has invalid value
    $slaveDb = array_rand( array_flip($dbArray),1 ); // How to alternate between databases ( in this demo i just use 2 database ) adjust the selection of the database to fit hoe many database you want to use !
    if(!isset($_COOKIE['read_db']) || !in_array($_COOKIE['read_db'],$dbArray)) // Check for the cookie values
    {
        setcookie("read_db", $slaveDb, time()+$cookieExpireTime); // set the current database to the user in cookie so next time user use same connection to database ! to avoid jumping or hopping on different databases in short time
    }else{
        $slaveDb = $_COOKIE['read_db']; // return the database selected if the user has it in the cookies
    }
    return $prefix.$slaveDb;
}

3- Modify the method " public function getConnection($name) " to Look like below :

public function getConnection($name)
{
    if($name =='core_read') // Only applied for READ Connections !!!
    {
        $name = $this->getSlaveDb(); // change the name of the connection to the one we get from our main method
    }
    //....... Leave the rest of the function as it is !!
}

This will allow you to use as many as databases you specify in the XML and PHP CODE for core_read connection and the default_setup connection for all other connections in magento ( core_write, core_setup )

Hope this solve your problem.

Answer 2

As far as I am aware it is not possible to use multiple slaves with Magento.

If you are using AWS you could maybe use larger instances for your master/slave setup?