Apache: The Definitive Guide, 3rd Edition

Modules Under Unix

Apache can do a wide range of things, not all of which are needed on every web site. Those that are needed are often not all needed all the time. The more capability the executable, httpd, has, the bigger it is. Even though RAM is cheap, it isn’t so cheap that the size of the executable has no effect. Apache handles user requests by starting up a new version of itself for each one that comes in. All the versions share the same static executable code, but each one has to have its own dynamic RAM. In most cases this is not much, but in some — as in mod_perl (see Chapter 17) — it can be huge.

The problem is handled by dividing Apache’s functionality into modules and allowing the webmaster to choose which modules to include into the executable. A sensible choice can markedly reduce the size of the program.

There are two ways of doing this. One is to choose which modules you want and then to compile them in permanently. The other is to load them when Apache is run, using the Dynamic Shared Object (DSO) mechanism — which is somewhat like Dynamic Link Libraries (DLL) under Windows. In the two previous editions of this book, we deprecated DSO because:

• It was experimental and not very reliable.

• The underlying mechanism varies strongly from Unix to Unix so it was, to begin with, not available on many platforms.

However, things have moved on, the list of supported platforms is much longer, and the bugs have been ironed out. When we went to press, the following operating systems ...