If you need to know about Apache1 modules, the answer’s simple. Buy the O’Reilly book "Writing Apache Modules with Perl and C”" by Lincoln Stein & Doug MacEachern…
Apache 2 modules
Unless you’ve got the patience of a saint, or already know how to do it, give up now!
More seriously, the online documentation available to tell you what the (major) differences between Apache1 and Apache2 modules is extremely limited or non-existent. If anyone finds any good links, please let me know!
The Apache2 website has a comment in its notes about the differences between Apache1 and Apache2 to say that the way modules work has been completely rewritten, but doesn’t actually tell you what has been rewritten or what the new method is. The document which should say how to do it is the old 1.3 version with a comment to say that it needs to be updated for version 2.
Very helpful (not!)
If you can afford to buy a book for just two chapters, then buy (another O’Reilly book) "Apache, the Definitive Guide" by Ben Laurie and Peter Laurie. (Make sure you get the 2003 3rd edition)
and look at Chapter 20 “The Apache API” and Chapter 21 “Writing Apache Modules”.
That explains what’s going on in the code. But it doesn’t give you a lot of detail about how to integrate your new module into Apache, so don’t expect it to give you all the answers…