Writing Apache Modules with Perl and C

Thanks to darrn chamberlain for an excellent review of the Lincoln Stein and Doug MacEachern book Writing Apache Modules with Perl and C. This is an excellent book for those considering working with Apache and mod_perl, and helpful for C programmers. Click below for more details.

Writing Apache Modules with Perl and C
author	Li
pages	724
publisher	O'Reilly, ISBN: 156592567X
rating	9.5/10
reviewer	darren chamberlain
ISBN
summary	Absolutely essential for anyone who is considering using Apache and mod_perl. C programmers may need more.

The Scenario

If you're like me, your first introduction to Perl ^[?] was in the form of CGI ^[?] scripts. A few years ago, I inherited a few dozen ancient CGI scripts (Perl and otherwise) that required Immediate Attention. CGI led to Perl, and to Apache ^[?] ; Perl and Apache led, naturally enough, to mod_perl ^[?] , once I started hitting the performance bottlenecks inherenent in CGI programming. After researching mod_perl, building a mod_perl-enalbed Apache, and reading all the available online documentation, I got it up and running--and I was suitably impressed.

So, when O'Reilly ^[?] announced a book devoted to programming Apache with Perl, I was extremely excited. The book starts with an introduction and history of web programming, introduces CGI and other types of web programming (server API ^[?] 's, such as ISAPI and NSAPI; embedded processors, such as mod_perl, mod_dtcl, and mod_pyapache; FastCGI; Java ^[?] servlets ^[?] ; ActiveX ^[?] ; and client-side scripting languages, such as VBScript ^[?] and JavaScript ^[?] ), and then describes the Apache module architecture, using some simple examples ("Hello, World" in Perl and in C). Then it gets good, covering dynamically generated content; the hobgoblin of HTTP, state; and all the other stuff that gives CGI programmer nightmares (like authentication and authorization).

What's Bad?

Although the title reads '... with Perl and C', the emphasis is very obviously on Perl. The C API reference chapters (chapters 10 and 11, pages 505 through 631) are very thorough, but almost all the examples are in Perl only. In fact, the authors go so far as to recommend that almost all Apache modules be written in Perl, and not C, except for very small modules or modules that need that extra speed boost or small memory footprint of being compiled into the server (page 13: "Anything you can do with the C API you can do with mod_perl with less fuss and bother."). Their reasoning is sound: mod_perl modules and scripts require a server restart at most, and often not even that, while for C modules, Apache itself must be recompiled; but I was expecting more in this area, perhaps a larger section on using DSO. After the book was published, however, several of the Perl-only examples were ported to the C API, and are available for download.

A few of these examples have already been published, and in these cases the book is mostly redundant. Notably, the Apache::NavBar module (which Lincoln uses on the server in his lab) and the Apache::AdBlocker module (chapters 4 and 7), appeared in The Perl Journal last year (issues 12 and 11). This is not that big a deal, since both of these modules are incredibly useful and probably deserve to be published in a few more places, but two brand new modules would have been most welcome, especially since the book's target audience probably also reads The Perl Journal.

What's Good?

There's a lot to like here. Since I'm a Perl programmer by trade and disposition, I personally liked the fact that 99.9% of the examples were written in Perl. With only a few exceptions, the modules could be copied into the right locations and run immediately; the exceptions were the modules that made use of either other programs (Chapter 5's Hangman program which uses a relational database to store state information) or specialized Apache features (Chapter 7's Apache::AdBlocker module, which requires proxy functionality).

Much of the text and all of the source code is available on the web at www.modperl.com. Chapters 6, 7, 8, and 9 can be found on the web site for the book, as can all the Perl modules and some of the examples in functional form (Apache::Magic and hangman).

Chapter 9 is the key chapter, and the heart of the book. It describes in great detail all the Apache:: modules. If you use mod_perl at all, download and print this chapter. Memorize it. Use your favorite indexing script to make it searchable. Everything you need to know about mod_perl is here in this chapter.

The appendices are also excellent, although, because it is an Apache book, I would have figured that several of the sections would be regular chapters, and not relegated to the end. The appendices are divided pretty evenly between concentrating on Perl and on C, unlike most of the rest of the book.

So What's In It For Me?

Fortunately for people like me, there is a lot of information about mod_perl on the web; The Perl Journal has had several articles on it, WebMonkey has had an article or two, and so on. There is a comprehensive mod_perl developer's guide on the offical Apache/Perl site. Lincoln Stein uses it a lot on his site and in his software. And, of course, we have the man pages and perldocs. So why do we need a book?

A few reasons. First and foremost, few of those sources go into the kind of detail that this book does, while still being approachable. Second, the book focuses on Apache, programming Apache, and (to a lesser extent) programming applications on the web; Perl and C are the means here, not the end. The in-depth technical discussions are about Apache: how it translates URI's to filenames, how it handles subrequests and internal redirects, how it maps files to MIME types. It then presents techniques for usurping these functions, customizing each phase of the reponse process, and explains when and why you would want to do this, instead of letting Apache do it's own thing. Creating checksums on the fly, compressing and decompressing data, creating extremely flexible HTML preprocessors, and modifying outgoing and incoming headers are some just some of the given examples.

The reference chapters are probably the single most valuable thing about the book. If you are a Perl programmer on a budget, you can download chapter 9 from the web site, but the C programmers out there have to buy the book to get the C API refernce. The C reference is 2 chapters (126 pages) long, and covers all the functions in precise detail.

For those among you who are using Microsoft operating systems, the book pays special attention to building, installing, and configuring mod_perl and Apache on Win32 systems, where it is different from Unix and Unix-like systems. Most of the actual modules are very similar (except for the obvious ones, such as scripts that call sendmail and the scripts that access MySQL), but the installation and building of mod_perl (or ApacheModulePerl.dll) are very different. The process is described in enough detail to make it possible, without boring those readers to whom it is irrelevant.

Conclusion

Programming Apache/mod_perl without this book is like writing Perl without the camel book. It can be done, but it is much easier and more enjoyable with the book. The writing is clear, informative, straight-forward, and, at times, amusing. The authors are the definitive sources for information on mod_perl and CGI programming, and this is reflected in every aspect of the book. While not as definitive for C programmers, it is still the best Apache API reference out there, other than the actual source code itself.

Purchase this book at Amazon.

Errata

Table of Contents

1. Server-Side Programming with Apache
2. A First Module
3. The Apache Module Architecture and API
4. Content Handlers
5. Maintaining State
6. Authentication and Authorization
7. Other Request Phases
8. Customizing the Apache Configuration Process
9. Perl API Reference Guide
10. C API Reference Guide, Part I
11. API Reference Guide, Part II

1. Standard Noncore Modules
2. Building and Installing mod_perl
3. Building Multifile C API Modules
4. Apache:: Modules Available on CPAN
5. Third-Party C Modules
6. HTML::Embperl--Embedding Perl Code in HTML