Reverse Proxies and Load Balancers

mod_proxy_http mod_backhand http://www.backhand.org/mod_backhand/

So far we have covered vertical scalability, which deals with how to improve the performance of a single server configuration. Distributing the load across multiple web servers is horizontal scalability. In this set of architectures, you can expand your capacity by simply adding new machines, improving the amount of traffic you can serve as well as the reliability of your setup.

Chapter 10 deals with using Apache as a reverse proxy. In this setup, one or several lightweight front-end servers deal with static content and handling SSL requests and slow connections, while relaying requests for specific URLs to specialized back-end servers. A number of companies provide commercial products that implement this functionality using hardware appliances.

Finally, mod_backhand is an Apache 1.3 module that provides dynamic redirection of HTTP requests within a cluster of machines, based on available resources.