Mod Perl Icon Mod Perl Icon Introduction. Incentives. Credits.

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The Writing Apache Modules with Perl and C book can be purchased online from O'Reilly and Amazon.com.
Your corrections of the technical and grammatical errors are very welcome. You are encouraged to help me improve this guide. If you have something to contribute please send it directly to me.
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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, and for the rest of the server's life they are served from the cache. Thus the server spends its time only 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 two 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 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.)

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mod_cgi

When you run your CGI scripts by using a configuration like this:

  
  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 ScriptAlias with the ExecCGI configuration option, which we enable so that the script will be executed rather than returned as a plain text 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>

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C API

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Perl API

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Apache::Registry

From the viewpoint of the Perl API, Apache::Registry is simply another handler that's not conceptually different from any other handler. Apache::Registry reads in the script 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 kept in memory until the child dies.

To prevent script name collisions, Apache::Registry creates a unique key for each cached script by prepending Apache::ROOT:: to the mangled path of the script's URI. This key is actually the package name that 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 a 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 is discarded and recompiled using the modified source. However, it doesn't check the modification times of any of the perl libraries the script might use.

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

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

From the viewpoint of the programmer, there is almost no difference between running a script as a plain CGI script 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 to 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";
  }

The first line provides a unique namespace for the code to use, and a unique key by which the code can be referenced from the cache.

The second line imports Apache::exit which over-rides perl's built-in exit.

The sub handler subroutine is wrapped around your code. By default (i.e. if you do not specify an alternative), when you use mod_perl and your code's URI is called, mod_perl will seek to execute the URI's associated handler subroutine.

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Apache::PerlRun

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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 the installation and configuration of both Perl and the Apache web server and delves deeply into the 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 to coverage of 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 administering Apache servers, debugging scripts, using databases, mod_perl related Perl, 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 the other documents listed in Getting Help and Further Learning you feel that your questions remain unanswered, you could try asking 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 most questions have already been asked and answered already! If you ignore this advice, do not be surprised if your question goes unanswered - it bores people when they're asked to answer the same question repeatedly - especially if the answer can be found in the archive or in the documentation. 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...) And if you do ask questions on the mailing list please make your subject line descriptive of the problem, not just ``Help'' - you're far more likely to get replies if people can see the issue you are talking about straight away.

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 .

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High-Profile Sites Running mod_perl

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

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References and Acknowledgments

I have used the following references while writing this guide:

I have quoted many snippets of information from FAQs and emails, but I have not credited each quote in the guide individually. 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 instead of scattering credits thoughout the Guide I have gathered them all together 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:

Credits of course go to ( alphabetically sorted ):

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. [ TOC ]

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Your corrections of the technical and grammatical errors are very welcome. You are encouraged to help me improve this guide. If you have something to contribute please send it directly to me.
The Writing Apache Modules with Perl and C book can be purchased online from O'Reilly and Amazon.com.

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Written by Stas Bekman.
Last Modified at 10/20/2000
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