The Apache HTTP server is the most widely-used web server in the world. It’s powerful, portable, and extensible. And with the introduction of filters in Apache HTTP 2.0, you’ll find even more ways to customize the server to suit your site. Find out how.
Historically, the architecture of Apache has been centered around extensibility — allowing third-party modules to add or modify the operation and features of the HTTP server. Consequently, a large number and variety of third-party Apache modules have been built to do everything from dynamic web page generation (for example, Perl and PHP) to secure communication (via mod_ssl), to making web site authoring and maintenance easier (the FrontPage module).
While Apache 1.3 was clearly a success, the Apache maintainers wanted Apache 2.0 to provide an even more stable, flexible, and powerful foundation for third-party enhancements. So, in addition to many other new features (see the sidebar “New features in the Apache 2.0 HTTP Server”), the Apache developers introduced filters. You can think of a filter as a kind of plug-in: Apache modules can register a filter, and at the appropriate point in the processing of an HTTP request or response, the filter is executed.
Apache 2.0 filters can be far less complicated to write and deploy than Apache 1.3 modules, and provide for much more flexible content transformations. In fact, the HTTP protocol logic within the Apache 2.0 web server itself is now (mostly) implemented as several input and output filters. Using filters for HTTP processing allows Apache 2.0 to support new protocols or enhancements simply by plugging in replacement filters.
To use filters properly, you must know what they are, how they fit into the big picture, and how to configure them. Let’s take a…
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