I know how to run two or more tomcats under one apache server. I want to know how to run two or more apache servers in a cluster and run my application. I know it's possible. can someone suggest a simple tutorial? there are many articles for apache - tomcat integration, but not for apache clustering. :( it would be great if you guys suggested a basic tutorial. thanks.
I'm assuming that you want to run this on multiple machines (running multiple Apache instances on the one machine makes very little sense).
Unfortunately, you can't find a simple tutorial for HA clustering because it's not a simple topic. You've got a lot of different concepts, ranging from low-level network stuff to application-specific items like shared data storage.
I started my clustering adventures at http://linuxvirtualserver.org/. You might like to start at http://www.linuxvirtualserver.org/architecture.html and go from there.
get a software or hardware load balancer and put it in front of the apache servers.
1) install free load balancer: "balance" from http://www.inlab.de/balance-3.42.tar.gz
2) run: balance -f 80 server1:80 server2:80 server3:80
advanced tutorial: The problem with the solution above is that it introduces a single point of failure. If your "balance" server dies, then you cannot access anything behind it.
If you want to go with a home grown solution GPL, then you need to run "vvrpd", "heartbeat" or some other failover cluster solution (or google: linux load balancing).
If you want to stay with a software solution, but have money, you could look at Redat Cluster, Veritas Cluster, or some other vendors cluster software.
Your best/most reliable solution though is to get a pair of hardware load balancers (which will do failover automatically).