Description

An introduction to what mod_perl is all about, its different features, and some explanations of the C API, Apache::Registry, Apache::PerlRun, and the Apache/Perl API.

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 (such as running sub-requests) that are more difficult or impossible in regular CGI programs. 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.

For examples on how you use mod_perl, see our What is mod_perl? section.

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 Technologie Extraordinaire for the facts.)

mod_cgi

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

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

you are running them under a mod_cgi handler, although 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 such as:

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

C API

The Apache C API has been present for a long time, and has been the usual way to program extensions (such as mod_perl) to Apache. When you write C extension modules, you write C code that is not independent, but will be linked into the Apache httpd executable either at build time (if the module is statically linked), or at runtime (if it is compiled as a Dynamic Shared Object, or DSO). Either way, you do as with any C library: you write functions that receive a certain number of arguments and make use of external API functions, provided by Apache or by other libraries.

The difference is that with Apache extension modules, these functions are registered inside a module record: you tell Apache already at compile-time for which phases you wish to run any functions. Of course, you probably won't be handling all the phases. Here is an example of a module handling only the content generation phase:

  /* Dispatch list of content handlers */
  static const handler_rec hello_handlers[] = {
      { "hello", hello_handler },
      { NULL, NULL }
  };
  
  /* Dispatch list for API hooks */
  module MODULE_VAR_EXPORT hello_module = {
      STANDARD_MODULE_STUFF,
      NULL,                  /* module initializer                  */
      NULL,                  /* create per-dir    config structures */
      NULL,                  /* merge  per-dir    config structures */
      NULL,                  /* create per-server config structures */
      NULL,                  /* merge  per-server config structures */
      NULL,                  /* table of config file commands       */
      hello_handlers,        /* [#8] MIME-typed-dispatched handlers */
      NULL,                  /* [#1] URI to filename translation    */
      NULL,                  /* [#4] validate user id from request  */
      NULL,                  /* [#5] check if the user is ok _here_ */
      NULL,                  /* [#3] check access by host address   */
      NULL,                  /* [#6] determine MIME type            */
      NULL,                  /* [#7] pre-run fixups                 */
      NULL,                  /* [#9] log a transaction              */
      NULL,                  /* [#2] header parser                  */
      NULL,                  /* child_init                          */
      NULL,                  /* child_exit                          */
      NULL                   /* [#0] post read-request              */
  };

Using this configuration (and a correctly built hello_handler() function), you'd then be able to use the following configuration to allow your module to handle the requests for the /hello URI.

  <Location /hello>
    SetHandler hello
  </Location>

When Apache sees a request for the /hello URI, it will then figure out what the "hello" handler corresponds to by looking it up in the handler record, and match that to the hello_handler function pointer, which will execute the hello_handler function of your module with a request_rec *r as an argument. From that point, your handler is free to do whatever it wants, returning content, declining the request, or doing other bizarre things based on user input.

It is not the object of this guide to explain how to program C handlers. However, this example lets you in on some of the secrets of the Apache core, which you will probably understand anyway by using mod_perl. If you want more information on writing C modules, you should read the Apache API documentation at http://httpd.apache.org/docs/misc/API.html and more importantly Writing Apache Modules with Perl and C, which will teach you about both mod_perl and C modules!

Perl API

After a while, C modules were found hard to write and difficult to maintain, mostly because code had to be recompiled or just because of the low-level nature of the C language, and because these modules were so intricately linked with Apache that a small bug could put at risk your whole server environment. In comes mod_perl. Programmed in C and using all the techniques described above and more, it allows Perl modules, written in good Perl style, to access the (almost) complete API provided to the conventional C extensions.

However, the structure used for Perl Apache modules is a little different. If you've programmed normal Perl modules (like those found on CPAN) before, you'll be happy to know that programming for mod_perl using the Apache API doesn't involve anything else than writing a Perl module that defines a handler subroutine (that is the convention--we'll see that that doesn't necessarily have to be the name). This subroutine accepts an argument, $r, which is the Perl API equivalent of the C API request_rec *r.

$r is your entry point to the whole Perl Apache API. Through it you access methods in good object-oriented fashion, which makes it slightly easier than with C, and looks a lot more familiar to Perl programmers.

Furthermore, Perl Apache modules do not define handler records like C modules. You only need to create your handler subroutine(s), and then control which requests they should handle solely with mod_perl configuration directives inside your Apache configuration.

Let's look at a sample handler that returns a greeting and the current local time.

  file:My/Greeting.pm
  -------------------
  package My::Greeting;
  use strict;
  
  use Apache::Constants qw(OK);
  
  sub handler {
      my $r = shift;
      my $now = scalar localtime;
      my $server_name = $r->server->server_hostname;
  
      $r->send_http_header('text/plain');
  
      print <<EOT;
  Thanks for visiting $server_name.
  The local time is $now.
  EOT
  
      return OK;
  }
  1; # modules must return true

As you can see, we're mixing Perl standard functions (like localtime()) with Apache functions ($r->send_http_header()). To return the above greeting when accessing the /hello URI, you would configure Apache like this:

  <Location /hello>
      SetHandler perl-script
      PerlHandler My::Greeting
  </Location>

When it sees this configuration, mod_perl loads the My::Greeting module, finds the handler() subroutine, and calls it to allow it to return the appropriate content. There are equivalent Perl*Handler directives for the different phases we saw were available to C handlers.

The Perl API gives you an incredible number of possibilities, which you can then use to be more productive or creative. mod_perl is an enabling technology; it won't make you smarter or more creative, but it will do its best to make you lose less time because of "accidental difficulties" of programming, and let you concentrate more on the important parts.

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.

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.

Apache::Registry is usually configured in this way:

  Alias /perl/ /usr/local/apache/bin/
  <Location /perl>
    SetHandler perl-script
    PerlHandler Apache::Registry
  </Location>

In short, we see that Apache::Registry is just another mod_perl handler, which is executed when requests are made for the /perl directory, and then does some special handling of the Perl scripts in that directory to turn them into Apache handlers.

Apache::PerlRun

Apache::PerlRun is very similar to Apache::Registry. It uses the same basic concepts, i.e. it runs CGI scripts under mod_perl for additional speed. However, unlike Apache::Registry, Apache::PerlRun will not cache scripts. The reason for this is that it's designed for use with CGI scripts that may have been "dirty", which might cause problems when run persistently under mod_perl. Apart from that, the configuration is the same. We discuss Apache::PerlRun in Apache::PerlRun, a closer look.

What you will 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 Offsite resources section of this site. 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.

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 documentation 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. Often you will find the answer to your question by searching the mailing list archive, since most questions have 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. When asking your question, be sure to have read the email-etiquette and How to report problems

If you find errors in these documents, please contact the maintainer, after having read about how to submit documentation patches.

Maintainers

Maintainer is the person(s) you should contact with updates, corrections and patches.

• Stas Bekman [http://stason.org/]

Authors

• Stas Bekman [http://stason.org/]

Only the major authors are listed above. For contributors see the Changes file.