What is mod_perl

The Apache/Perl integration project brings together the full power of the Perl programming language and the Apache HTTP server. With mod_perl it is possible to write Apache modules entirely in Perl, letting you easily do things that are more difficult or impossible in regular CGI programs, such as running sub requests. In addition, the persistent Perl interpreter embedded in the server saves the overhead of starting an external interpreter, i.e. the penalty of Perl start-up time. And not the least important feature is code caching, where modules and scripts are loaded and compiled only once, then for the rest of the server's life they are served from the cache, thus the server spends its time only on running already loaded and compiled code, which is very fast.

The primary advantages of mod_perl are power and speed. You have full access to the inner workings of the web server and can intervene at any stage of request-processing. This allows for customized processing of (to name just a few of the phases) URI->filename translation, authentication, response generation, and logging. There is very little run-time overhead. In particular, it is not necessary to start a separate process, as is often done with web-server extensions. The most wide-spread such extension, the Common Gateway Interface (CGI), can be replaced entirely with Perl code that handles the response generation phase of request processing. mod_perl includes 2 general purpose modules for this purpose: Apache::Registry, which can transparently run existing perl CGI scripts and Apache::PerlRun, which does a similar job but allows you to run "dirtier" (to some extent) scripts.

You can configure your httpd server and handlers in Perl (using PerlSetVar, and <Perl> sections). You can even define your own configuration directives.

Many people wonder and ask "How much of a performance improvement does mod_perl give?" Well, it all depends on what you are doing with mod_perl and possibly who you ask. Developers report speed boosts from 200% to 2000%. The best way to measure is to try it and see for yourself! (See http://perl.apache.org/tidbits.html and http://perl.apache.org/stories/ for the facts.)

mod_cgi

When you run your CGI scripts by using a configuration of:

  ScriptAlias /cgi-bin/ /home/httpd/cgi-bin/

you run it under a mod_cgi handler, you never define it explicitly. Apache does all the configuration work behind the scenes, when you use a ScriptAlias.

By the way, don't confuse it with a ExecCGI configuration option, it's being enabled so the script will be executed and not returned as a plain file. For example for mod_perl and Apache::Registry you would use a configuration like:

  <Location /perl>
    SetHandler perl-script
    PerlHandler Apache::Registry
    Options ExecCGI
    PerlSendHeader On
  </Location>

C API

META: complete

Perl API

META: complete

Apache::Registry

From the viewpoint of the Perl API, Apache::Registry is just yet another handler that's not conceptually different from any other handler. It reads in the file, compiles, executes it and stores into the cache. Since the perl interpreter keeps running from child process' creation to its death, any code compiled by the interpreter is not removed from memory until the child dies.

To keep the script names from collisions, it prepends Apache::ROOT:: and the mangled path of the URI to the key of the cached script. This key is actually a package name, the script resides in. So if you have requested a script /perl/project/test.pl, the scripts would be wrapped in code which starts with package declaration of:

  package Apache::ROOT::perl::project::test_e2pl;

Apache::Registry also stores the script's last modification time. Everytime the script changes, the cached code would be discarded and recompiled using the modified source. However, it doesn't check any of the perl libraries the script might use.

Apache::Registry overrides the CORE::exit() with <Apache::exit()>, so the CGI scripts that used the <exit()> will run correctly. We will talk about all these details in depth later.

The last thing Apache::Registry does, is emulation of the mod_cgi's environment variables. Like $ENV{SERVER_NAME}, $ENV{REMOTE_USER} and so on. PerlSetupEnv Off disables this feature and saves some memory bits and CPU clocks.

From the viewpoint of the programmer, there is almost no difference between running a script as a plain CGI under mod_cgi and running it under mod_perl. There is however a great speed improvement, but at the expense of much heavier memory usage (there is no free lunch :).

When they run under mod_cgi, your CGI scripts are loaded each time they are called and then they exit. Under mod_perl they are loaded once and cached. This gives a big performance boost. But because the code is cached and doesn't exit, it won't cleanup memory as it would under mod_cgi. This can have unexpected effects.

Your scripts will be recompiled and reloaded by mod_perl when it detects that you have changed them, but remember that any libraries that your scripts might require() or use() will not be recompiled when they are changed. You will have to take action yourself to ensure that they are recompiled.

Of course the guide will answer all these issues in depth.

Let's see what happens with your script when it's being executed under Apache::Registry. If we take the simplest code of (URI /perl/project/test.pl)

  print "Content-type: text/html\n\n";
  print "It works\n";

Apache::Registry will convert it into the following:

  package Apache::ROOT::perl::project::test_e2pl;
  use Apache qw(exit);
  sub handler {
    print "Content-type: text/html\n\n";
    print "It works\n";
  }

META: Complete

Apache::PerlRun

META: Complete

What will you learn

This document was written in an effort to help you start using Apache's mod_perl extension as quickly and easily as possible. It includes information about installation and configuration of Perl and the Apache web server and delves deeply into issues of writing and porting existing Perl scripts to run under mod_perl. Note that it does not attempt to enter the big world of using the Perl API or C API. You will find pointers covering these topics in the Getting Help and Further Learning section of this document. This guide tries to cover the most of the Apache::Registry and Apache::PerlRun modules. Along with mod_perl related topics, there are many more issues related to administrating apache servers, debugging scripts, using databases, Perl reference, code snippets and more. The Guide's Overview will help you to find your way through the guide.

It is assumed that you know at least the basics of building and installing Perl and Apache. (If you do not, just read the INSTALL documents which are part of the distribution of each package.) However, in this guide you will find specific Perl and Apache installation and configuration notes, which will help you successfully complete the mod_perl installation and get the server running in a short time.

If after reading this guide and other documents listed in Getting Help and Further Learning you feel that your question is yet not answered, please ask the apache/mod_perl mailing list to help you. But first try to browse the mailing list archive (located at http://forum.swarthmore.edu/epigone/modperl ). Often you will find the answer to your question by searching the mailing list archive, since there is a good chance someone else has already encountered the problem and found a solution. If you ignore this advice, do not be surprised if your question goes unanswered - it bores people to answer the same question more than once (twice?). This does not mean that you should avoid asking questions, just do not abuse the available help and RTFM before you call for HELP. (You have certainly heard the infamous fable of the shepherd boy and the wolves...)

If you find incorrect details or mistakes in my grammar, or you want to contribute to this document please feel free to send me an email at stas@stason.org .

High-Profile Sites Running mod_perl

A report prepared by Rex Staples at Thu, 14 Oct 1999:

• Macromedia

  4,273,000 unique visitors/month Aug-1999

  http://www.macromedia.com http://www.mediametrix.com/TopRankings/TopRankings.html

  Apache/1.3.4 (Unix) mod_perl/1.18 on Solaris

• ValueClick: Results-based advertising network

  60 million page views/day Oct-1999

  http://valueclick.com

  Apache/1.3.9-dev (Unix) mod_perl/1.21_01 on FreeBSD

• Deja.com

  130 million pageviews/month Oct-1999

  http://www.deja.com

  Apache/1.3b5 mod_perl/1.08 on Linux

• MP3.com, Inc.

  77 million page views/month Aug-1999

  408,000 unique visitors/day Aug-1999

  http://www.mp3.com http://www.mp3.com/pr/990914-keymetrics.html

  Apache/1.3.4-9 (Unix) mod_perl/1.18-21 on Linux/FreeBSD

• IMDB: Internet Movie Database

  1.25 million page views/day Mar-1998

  http://www.imdb.com

  * They are now an Amazon.com company

  Apache/1.3.7-dev (Unix) mod_perl/1.19_01-dev

• Flash.net: Internet Service Provider

  1,603,000 unique visitors/month Aug-1999

  http://www.flash.net http://www.mediametrix.com/TopRankings/TopRankings.html

  Apache/1.2.4 mod_perl/1.00 on Solaris

• At Hand Network Yellow Pages

  917,000 unique visitors/month Aug-1999

  http://www.athand.com http://www.mediametrix.com/TopRankings/TopRankings.html

  Stronghold/2.3 Apache/1.2.6 (Unix) mod_perl/1.15 on Solaris

• Commissioner.com: Subscription Fantasy Football

  12 million page views/day Oct-1999

  http://www.commissioner.com

  Apache/1.35b mod_perl/1.10 on Linux

• Slashdot: News For Nerds

  400,000 page views/day Oct-1999

  http://www.slashdot.org

  Apache/1.3.6 (Unix) mod_perl/1.21 on Linux

• Hot Bot mail and member web pages:

  http://members.hotbot.com

  Also widely used on HotWired, WiredNews, Webmonkey, and Suck.com

  Apache/1.3.4 (Unix) mod_perl/1.21 on Solaris

• Art Today: subscription clip-art service

  250k hits/day

  http://www.arttoday.com

  Oracle 7 + 1 Sun Ultra w/150GB storage Apache/1.3.4 (Unix) mod_perl/1.17 on Solaris

• CMPnet: a technology information network

  500k hits/day

  http://www.cmpnet.com

  Apache/1.3.9 (Unix) mod_perl/1.16

References and Acknowledgments

I have used the following references while writing this guide:

• mod_perl FAQ by Frank Cringle at http://perl.apache.org/faq/ .

• mod_perl performance tuning guide by Vivek Khera at http://perl.apache.org/tuning/ .

• mod_perl plugin reference guide by Doug MacEachern at http://perl.apache.org/src/mod_perl.html .

• Quick guide for moving from CGI to mod_perl at http://perl.apache.org/dist/cgi_to_mod_perl.html .

• mod_perl_traps, common traps and solutions for mod_perl users at http://perl.apache.org/dist/mod_perl_traps.html .

• mod_perl mailing list emails. Answers to some of the questions posted to modperl@apache.org Apache/Perl mailing list.

• My personal experience with mod_perl.

As I said, I have quoted many information snippets from FAQs and emails, and I did not credit people after each quote in the guide. I did not mean to take the credit for myself, it's just that I tried to keep track of names, and became lost, so I preferred not to put credit throughout the guide, but rather to centralize it here. If you want your name to show up under your original quote, please tell me and I'll add it for you.

Major contributors:

• Doug MacEachern. A large part of this guide is built upon his email replies to users' questions.

• Frank Cringle. Parts of his mod_perl FAQ have been used in this guide.

• Vivek Khera. For his mod_perl performance tuning guide.

• Steve Reppucci, who made a thorough review of the stuff I wrote. He fixed lots of spelling and grammar errors, and made the guide readable to English speakers :)

• Eric Cholet, who wrote complete sections for the guide, and pointed out technical errors in it.

• Ken Williams, who reviewed a lot of stuff in the guide. Many snippets from his emails are included in the guide.

• Wesley Darlington for contributing a big section for scenario chapter.

• Geoffrey S Young and David Harris for contributing a big sections about mod_perl and RPM packages.

• Andreas J. Koenig for contributing his "Correct HTTP headers" document.

• Ged W. Haywood for reviewing and fixing a big part of the guide, providing lots of constructive critics and helping to reorganize the guide to make it more user friendly.

• Jeffrey W. Baker for his "guide to mod_perl database performance".

• Richard A. Wells for reviewing a correcting a large part of the guide.

• Randy Harmon for rewriting the mod_perl advocacy chapter

• Dean Fitz for reviewing the "Operating System and Hardware Demands" chapter.

Credits of course go to ( alphabetically sorted ):

• Andreas J. Koenig

• Andrei A. Voropaev

• Ask Bjoern Hansen

• Brian Moseley

• Chad K. Lewis

• Christof Damian

• Cliff Rayman

• Daniel Koch

• Daniel W. Burke

• Dave Hodgkinson

• David Harris

• David Landgren

• Dean Fitz

• Doug Bagley

• Ed Park

• Edmund Mergl

• Eric Cholet

• Eric Strovink

• Evan A. Zacks

• Frank Schoeters

• G.Richter

• Garr Updegraff

• Ged W. Haywood

• Geoffrey S Young

• Gerald Richter

• Gunther Birznieks

• Henrique Pantarotto

• Howard Jones

• James G Smith

• Jan Peter Hecking

• Jason Bodnar

• Jauder Ho

• Jay J

• Jeff Baker

• Jeff Rowe

• Jeffrey W. Baker

• Jie Gao

• John Milton

• John Walker

• Jon Orwant

• Jonathan Peterson

• Joshua Chamas

• Leslie Mikesell

• Lincoln Stein

• Mark Mills

• Marshall Dudley

• Matthew Darwin

• Michael Hall

• Mike Fletcher

• Nancy Lin

• Nathan Torkington

• Nathan Vonnahme

• Oleg Bartunov

• Perrin Harkins

• Peter Galbavy

• Peter Skov

• Philip Jacob

• Radu Greab

• Ralf Engelschall

• Randal L. Schwartz

• Randy Harmon

• Rauznitz Balazs

• Richard A. Wells

• Richard Dice

• Rick Myers

• Robin Berjon

• Steve Reppucci

• Tom Hughes

• Tzvetan Stoyanov

• Ulrich Pfeifer

• Vivek Khera

• Wesley Darlington

• Did I miss your name? Tell me!

I want to thank all the people who donated their time and efforts to make this amazing idea of mod_perl a reality. This includes Doug MacEachern, the author of mod_perl, and all the developers who contributed bug patches, modules and help. And of course the numerous unseen users around the world who help to promote mod_perl and to make it a better tool.

cpanm mod_perl_guide

perl -MCPAN -e shell
install mod_perl_guide