Apache Cookbook, 2nd Edition

Load Balancing with mod_proxy_balancer

Problem

You want to balance the load between several backend servers.

Solution

Use mod_proxy_balancer to create a load-balanced cluster:

    <Proxy balancer://mycluster>
        BalancerMember http://192.168.1.50:80 
        BalancerMember http://192.168.1.51:80
    </Proxy>
    ProxyPass /application balancer://mycluster/

Discussion

mod_proxy_balancer is an exciting new module that provides load balancing between multiple backend servers. This kind of functionality has traditionally been associated with expensive and complex commercial solutions. This module makes this simple to configure, and it’s included in the standard installation of the Apache Web server.

The example given above sets up a two-member balanced cluster and proxies the URL /application to that cluster.

mod_proxy_balancer offers a wide variety of options, which you can find in detail in the documentation, available here: http://httpd.apache.org/docs/2.2/mod/mod_proxy_balancer.html.

For example, you can indicate that a particular server is more powerful than another, and so should be allowed to assume more of the load than other machines in the cluster. In the following configuration line, we indicate that one particular machine should receive twice as much traffic as other machines:

    BalancerMember http://192.168.1.51:80 loadfactor=2

Traffic may be balanced by traffic (bytes transferred) or by request (number of requests made per host) by putting additional arguments on the ProxyPass directive:

 ProxyPass /application ...