Since you have Master/Slave, you may want to implement the use of two things

1. Circular Replication
2. Database Virtual IP (aka DBVIP)

Circular Replication

Circular Replication is nothing more than first setting up Master/Slave then performing the same steps using the Slave as the Master's Master and the Master as the Slave's Slave. It just entails

• explicitly user a different server_id on each DB Server
• enabling binary logging on both DB servers
• making sure the replication user is defined on both DB servers

Database Virtual IP (aka DBVIP)

There are products you can download and install to setup a DBVIP. One such product I use is ucarp. Another product is Linux Heartbeat. I normally do not use such things with MySQL Circular Replication or Master/Slave. Why?

Since those products can perform automatic failover, you do not want to do that in the event a Slave is some number of seconds behind in replication lag.

You should perform manual failovers.

Here is a poor man's approach to implementing DBVIP management.

Suppose you have this setup

• DB Server1 has IP 10.1.2.30
• DB Server2 has IP 10.1.2.40
• You want to use DBVIP 10.1.2.50

Create the Script called /usr/local/sbin/MyAppDBVIP like this

echo echo 10.1.2.50 > /usr/local/sbin/MyAppDBVIP 

Create the Script called /usr/local/sbin/dbvip-up

DBVIP=`/usr/local/sbin/MyAppDBVIP` ip addr add ${DBVIP}/24 dev eth1 

Create the Script called /usr/local/sbin/dbvip-down

DBVIP=`/usr/local/sbin/MyAppDBVIP` ip addr del ${DBVIP}/24 dev eth1 

Make sure all scripts are executable

chmod +x /usr/local/sbin/MyAppDBVIP chmod +x /usr/local/sbin/dbvip-up chmod +x /usr/local/sbin/dbvip-down 

Make sure these script exist on both DB Servers

Simply run dbvip on whichever server you choose. .

So the failover process and protocol are the following:

• Run dbvip-down on the DB Server that has the DBVIP. If you cannot
• Run dbvip-up on the DB Server that you want to have the DBVIP
• Just remember you should not run dbvip-up on both machines
• After running dbvip-up, restart apache, JBoss, or any other app server contacting MySQL via the old Master

In what way do you mean 'to use the slave when the master is down'? When the master goes down, the process of switching the slave to become the master is what we call 'failover'.

But once your application starts using the old slave in this failover scenario, it becomes the new master. When you get the old master online again it must be set up as a slave, because your data has changed.

Failover for your situation (a single slave) is going to be mainly handled in your application. You will need to implement logic to check that the application is connecting. If it is not, have the application connect to the slave instead.

A few things to keep in mind:

• Again, once your application connects to the slave and modifies data, you need to ensure it never tries to connect to the master again.
• You will probably want to have some alert monitoring setup (using Nagios or some other suite), so you know when the master fails.