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17.3. mod_perl

mod_perl is an Apache server extension that embeds Perl within Apache, providing a Perl interface to the Apache API. This allows us to develop full-blown Apache modules in Perl to handle particular stages of a client request. It was written by Doug MacEachern, and since it was introduced, its popularity has grown quickly.

The most popular Apache/Perl module is Apache::Registry, which emulates the CGI environment, allowing us to write CGI applications that run under mod_perl. Since Perl is embedded within the server, we avoid the overhead of starting up an external interpreter. In addition, we can load and compile all the external Perl modules we want to use at server startup, and not during the execution of our application. Apache::Registry also caches compiled versions of our CGI applications, thereby providing a further boost. Users have reported performance gains of up to 2000 percent in their CGI applications using a combination of mod_perl and Apache::Registry.

Apache::Registry is a response handler, which means that it is responsible for generating the response that will be sent back to the client. It forms a layer over our CGI applications; it executes our applications and sends the resulting output back to the client. If you don't want to use Apache::Registry, you can implement your own response handler to take care of the request. However, these handlers are quite different from standard CGI scripts, so we won't discuss how to create handlers with mod_perl. To learn about handlers along with anything else you might want to know about mod_perl, refer to Writing Apache Modules with Perl and C by Lincoln Stein and Doug MacEachern (O'Reilly & Associates, Inc.).

17.3.1. Installation and Configuration

Before we go any further, let's install mod_perl. You can obtain it from CPAN at http://www.cpan.org/modules/by-module/Apache/. The Apache namespace is used by modules that are specific to mod_perl. The installation is relatively simple and should proceed well:

$ cd mod_perl-1.22
$ perl Makefile.PL \
> APACHE_PREFIX=/usr/local/apache  \
> APACHE_SRC=../apache-1.3.12/src  \
> DO_HTTPD=1                       \
> USE_APACI=1                      \
$ make
$ make test
$ su
# make install

Refer to the installation directions that came with Apache and mod_perl if you want to perform a custom installation. If you're not interested in possibly developing and implementing the various Apache/Perl handlers, then you do not need the EVERYTHING=1 directive, in which case, you can implement only a PerlHandler.

Once that's complete, we need to configure Apache. Here's a simple setup:

PerlRequire      /usr/local/apache/conf/startup.pl
PerlTaintCheck   On
PerlWarn         On

Alias /perl/ /usr/local/apache/perl/

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

As you can see, this is very similar to the manner in which we configured FastCGI. We use the PerlRequire directive to execute a startup script. Generally, this is where you would pre-load all the modules that you intend to use (see Example 17-3).

However, if you are interested in loading only a small set of modules (a limit of ten), you can use the PerlModule directive instead:

PerlModule  CGI  DB_File  MLDBM  Storable

For Apache::Registry to honor taint mode and warnings, we must add directive the PerlTaintMode and PerlWarn directives. Otherwise, they won't be enabled. We do this globally. Then we configure the directory we are setting up to run our scripts.

All requests for resources in the /perl directory go through the perl-script (mod_perl) handler, which then passes the request off to the Apache::Registry module. We also need to enable the ExecCGI option. Otherwise, Apache::Registry will not execute our CGI applications.

Now, here's a sample configuration file in Example 17-3.

Example 17-3. startup.pl

#!/usr/bin/perl -wT

use Apache::Registry;

use CGI;

## any other modules that you may need for your
## other mod_perl applications running ...

print "Finished loading modules. Apache is ready to go!\n";


It is really a very simple program, which does nothing but load the modules. We also want Apache::Registry to be pre-loaded since it'll be handling all of our requests. A thing to note here is that each of Apache's child processes will have access to these modules.

If we do not load a module at startup, but use it in our applications, then that module will have to be loaded once for each child process. The same applies for our CGI applications running under Apache::Registry. Each child process compiles and caches the CGI application once, so the first request that is handled by that child will be relatively slow, but all subsequent requests will be much faster.

17.3.2. mod_perl Considerations

In general, Apache::Registry, does provide a good emulation of a standard CGI environment. However, there are some differences you need to keep in mind:

If it's too much of a hassle to convert your application to run effectively under Apache::Registry, then you should investigate the Apache::PerlRun module. This module uses the Perl interpreter embedded within Apache, but doesn't cache compiled versions of your code. As a result, it can run sloppy CGI scripts, but without the full performance improvement of Apache::Registry. It will, nonetheless, be faster than a typical CGI application.

Increasing the speed of CGI scripts is only part of what mod_perl can do. It also allows you do write code in Perl that interacts with the Apache response cycle, so you can do things like handle authentication and authorization yourself. A full discussion of mod_perl is certainly beyond the scope of this book. If you want to learn more about mod_perl, then you should definitely start with Stas Bekman's mod_perl guide, available at http://perl.apache.org/guide/. Then look at Writing Apache Modules with Perl and C, which provides a very thorough, although technical, overview of mod_perl.

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