Configuring load balancing between multiple web servers may seem challenging, but it doesn't have to be. HAProxy is a great lightweight tool that is easy to setup and has a large number of rich features for managing and serving content.

Lets look at a simple scenario using two application or web servers n2 and n3, running on 10.0.0.12 and 10.0.0.13 respectively. Each web server is running apache2, serving a simple html page, but it could be any form of static or rich content.

We use a third server with a minimal install of Debian 6.0 (Squeeze) 64bit, listening on 10.0.0.11 and a publicly accessible IP that we will install HAProxy onto.

root@n1:~# apt-get install haproxy
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
haproxy
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 521 kB of archives.
After this operation, 1,118 kB of additional disk space will be used.
Get:1 http://ftp.us.debian.org/debian/ squeeze/main haproxy amd64 1.4.8-1 [521 kB]
Fetched 521 kB in 0s (541 kB/s)
Selecting previously deselected package haproxy.
(Reading database ... 21199 files and directories currently installed.)
Unpacking haproxy (from .../haproxy_1.4.8-1_amd64.deb) ...
Processing triggers for man-db ...
Setting up haproxy (1.4.8-1) ...
Adding group `haproxy' (GID 105) ...
Done.
Adding system user `haproxy' (UID 102) ...
Adding new user `haproxy' (UID 102) with group `haproxy' ...
Not creating home directory `/home/haproxy'.
root@n1:~#

You can verify the installed version of HAProxy easily...

root@n1:~# haproxy -v
HA-Proxy version 1.4.8 2010/06/16
Copyright 2000-2010 Willy Tarreau

Next lets enable that to start, edit /etc/default/haproxy and change the ENABLED value from 0 to 1. Or use the following snippet.

sed -i 's/ENABLED=0/ENABLED=1/' /etc/default/haproxy

Debian comes with a helpful example configuration. By tweaking that a little the following will work for us...

root@n1:~# cat /etc/haproxy/haproxy.cfg
# this config needs haproxy-1.1.28 or haproxy-1.2.1

global
        log /dev/log   local0
        log 127.0.0.dd1   local1 notice
        maxconn 4096
        user haproxy
        group haproxy
        daemon

defaults
        log     global
        mode    http
        option  httplog
        option  dontlognull
        retries 3
        option redispatch
        maxconn 2000
        contimeout      5000
        clitimeout      50000
        srvtimeout      50000

listen  web-farm 0.0.0.0:80
        cookie  SERVERID rewrite
        balance roundrobin
        server  app1 10.0.0.12:80 cookie app1inst1 check inter 2000 rise 2 fall 5
        server  app2 10.0.0.13:80 cookie app1inst2 check inter 2000 rise 2 fall 5

And now we start that up...

service haproxy start

You should now be able to point your browser at the 'n1' server and see the content from one of the two backend servers.

What else can you do now? Lots of goodness can be gleaned from the main documentation, some ideas include...

• take one or both web servers offline to test what happens when you access HAProxy
• configure HAProxy to serve a custom maintenance page
• configure the web interface so you can visually monitor HAProxy statistics
• change the scheduler to something other than round-robin
• configure prioritization/weights for particular servers
• tweak caching to optimize site load time
• configure cookie pass-through so customers get served predictable content
• setup internal monitoring of HAProxy and HAProxy logs using Monit.

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