Running CGI Scripts with mod_perl

What most people want to do with mod_perl is improve CGI performance. The mod_perl installation assumes this request by enabling the PerlHandler callback hook by default and by installing the Apache::Registry module. PerlHandler is the handler used for the content retrieval stage of the server transaction. Apache::Registry is the Perl module that emulates the CGI environment so you can use "standard" Perl CGI scripts with mod_perl without having to rewrite them (much). This is by far the cheapest way to get improved CGI performance.

With Apache::Registry, each individual CGI program is compiled and cached the first time it is called (or whenever it is changed) and remains available for all subsequent instances of that CGI script. This process avoids the costs of startup time.

Whereas most CGI scripts are kept in cgi-bin, scripts that use Apache::Registry are placed in a separate directory, e.g., perl-bin. The access.conf Apache configuration file needs to point to this directory by setting an alias and defining a handler for this new location: 

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

Instead of using the cgi-script handler, we use the perl-script handler to give control to mod_perl. Next, the PerlHandler directive tells mod_perl that the Apache::Registry module should be used for serving all files in that directory.PerlSendHeader is another mod_perl-specific directive; in this case, it tells mod_perl to send response lines and common headers-by default, none are sent. (For NPH scripts, you'll want to turn this feature off again.) Options ExecCGI is a standard Apache header that is needed to tell Apache to treat the script as a CGI script.

If you want to load Perl modules in addition to Apache::Registry, you can use the PerlModule directive: 

PerlModule CGI

If you include this line, you shouldn't need to explicitly use CGI in each Perl CGI script anymore, since CGI.pm will be loaded directly from the Apache server. Up to ten modules can be listed with the PerlModule directive.

CGI scripts in the new directory should work now. However, if you have problems, the mod_perl manpage offers some words of wisdom:

  • Always use use strict. Standard CGI scripts start with a clean slate every time. When switching to mod_perl, CGI developers are often surprised to learn how often they take advantage of this fact. use strict tells you when your variables haven't been properly declared and might inherit values from previous invocations of the script.
  • Don't call exit( ). Calling exit( ) at the end of every program is a habit of many developers. While often totally unnecessary, it usually doesn't hurt . . . except with mod_perl. If you're using mod_perl without Apache::Registry, exit( ) kills the server process. If exit( ) is the last function call, you can just remove it. If the structure of your program is such that it is called from the middle of the script, you can just put a label at the end of the script and use goto( ). There's also an Apache->exit( ) call you can use if you're really attached to exit( ) s.

    If you're using Apache::Registry, you don't have to worry about this problem. Apache::Registry is smart enough to override all exit( ) calls with Apache->exit( ).

In addition, it is recommended that you should use recent versions of Perl and CGI.pm. You should scan the mod_perl documentation for the latest compatibility news.