How To Set Up A Loadbalanced High-Availability Apache Cluster
Author: Falko Timme
Last edited 04/26/2006
This tutorial shows how to set up a two-node Apache web server cluster that provides high-availability. In front of the Apache cluster we create a load balancer that splits up incoming requests between the two Apache nodes. Because we do not want the load balancer to become another "Single Point Of Failure", we must provide high-availability for the load balancer, too. Therefore our load balancer will in fact consist out of two load balancer nodes that monitor each other using heartbeat, and if one load balancer fails, the other takes over silently.
The advantage of using a load balancer compared to using round robin DNS is that it takes care of the load on the web server nodes and tries to direct requests to the node with less load, and it also takes care of connections/sessions. Many web applications (e.g. forum software, shopping carts, etc.) make use of sessions, and if you are in a session on Apache node 1, you would lose that session if suddenly node 2 served your requests. In addition to that, if one of the Apache nodes goes down, the load balancer realizes that and directs all incoming requests to the remaining node which would not be possible with round robin DNS.
For this setup, we need four nodes (two Apache nodes and two load balancer nodes) and five IP addresses: one for each node and one virtual IP address that will be shared by the load balancer nodes and used for incoming HTTP requests.
I will use the following setup here:
- Apache node 1: webserver1.example.com (webserver1) - IP address: 192.168.0.101; Apache document root: /var/www
- Apache node 2: webserver2.example.com (webserver2) - IP address: 192.168.0.102; Apache document root: /var/www
- Load Balancer node 1: loadb1.example.com (loadb1) - IP address: 192.168.0.103
- Load Balancer node 2: loadb2.example.com (loadb2) - IP address: 192.168.0.104
- Virtual IP Address: 192.168.0.105 (used for incoming requests)
Have a look at the drawing on http://www.linuxvirtualserver.org/docs/ha/ultramonkey.html to understand how this setup looks like.
In this tutorial I will use Debian Sarge for all four nodes. I assume that you have installed a basic Debian installation on all four nodes, and that you have installed Apache on webserver1 and webserver2, with /var/www being the document root of the main web site.
I want to say first that this is not the only way of setting up such a system. There are many ways of achieving this goal but this is the way I take. I do not issue any guarantee that this will work for you!
1 Enable IPVS On The Load Balancers
First we must enable IPVS on our load balancers. IPVS (IP Virtual Server) implements transport-layer load balancing inside the Linux kernel, so called Layer-4 switching.
echo ip_vs_dh >> /etc/modules
echo ip_vs_ftp >> /etc/modules
echo ip_vs >> /etc/modules
echo ip_vs_lblc >> /etc/modules
echo ip_vs_lblcr >> /etc/modules
echo ip_vs_lc >> /etc/modules
echo ip_vs_nq >> /etc/modules
echo ip_vs_rr >> /etc/modules
echo ip_vs_sed >> /etc/modules
echo ip_vs_sh >> /etc/modules
echo ip_vs_wlc >> /etc/modules
echo ip_vs_wrr >> /etc/modules
Then we do this:
If you get errors, then most probably your kernel wasn't compiled with IPVS support, and you need to compile a new kernel with IPVS support (or install a kernel image with IPVS support) now.
2 Install Ultra Monkey On The Load Balancers
Ultra Monkey is a project to create load balanced and highly available services on a local area network using Open Source components on the Linux operating system; the Ultra Monkey package provides heartbeat (used by the two load balancers to monitor each other and check if the other node is still alive) and ldirectord, the actual load balancer.
To install Ultra Monkey, we must edit /etc/apt/sources.list now and add these two lines (don't remove the other repositories):
deb http://www.ultramonkey.org/download/3/ sarge main
Afterwards we do this:
and install Ultra Monkey:
apt-get install ultramonkey
If you see this warning:
¦ libsensors3 not functional ¦
you can ignore it.
During the Ultra Monkey installation you will be asked a few question. Answer as follows:
Do you want to automatically load IPVS rules on boot?
Select a daemon method.