Who's Your Daddy?

The Apache web server behaves like Wilt Chamberlain, spawning many children. It is a single parent, expected to raise enough children to handle the average load of incoming HTTP requests. Unlike human babies, an Apache child is "born" with all the knowledge of its parent. So, the more know-how, or compiled Perl in this case, we stuff into the parent server, the more its children know at birth. This can be a good thing, but it can also be taken too far -- if daddy has a big full head, then every baby also has a big full head, and with many babies that can eat up a lot of RAM.

Apache's parent process is created at the time the Apache server is started, and its httpd.conf configuration file is evaluated. This is our opportunity to decide what knowledge the parent will pass onto its children. You may recall a typical set of mod_perl configuration directives placed in the httpd.conf file of a mod_perl-compiled Apache server:

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

Under the above configuration, all scripts in /usr/local/apache/cgi-perl/ will be handled by mod_perl. As a simple mod_perl configuration, the Apache parent server will contain the Perl interpreter, but that's the only knowledge it will have or pass onto its spawned children. The children themselves will have to compile any Perl scripts and any modules that those Perl scripts require. It is worth remembering that a child process will only need to compile its Perl scripts once during its lifetime -- the first time -- but there may be many child processes, spawning and dying, and so this may still result in quite a few compilations of all scripts and modules.

The PerlRequire directive lets us add more knowledge to the Apache parent server, which will be passed wholly onto its children. The recommended procedure is to PerlRequire a Perl script, inside which we will specify which modules and such to build into the parent server:

Alias /cgi-perl/ "/usr/local/apache/cgi-perl/" 

PerlRequire /usr/local/apache/cgi-perl/startup.pl
<Location /cgi-perl>
 SetHandler perl-script
 PerlHandler Apache::Registry
 Options ExecCGI
 PerlSendHeader On
</Location>

Let's consider a sample startup.pl script, which fills the Apache parent server with knowledge:

#! /usr/bin/perl
use strict;
 
use Apache::Registry ();
 
# pull in the Perl modules and packages we want to compile once,
# each spawned Apache child will inherit this knowledge
use CGI (); CGI->compile(':all');
use DBI ();
use DBD::mysql ();

require "/usr/local/apache/cgi-perl/fastdb.pl";
 
1;

Our startup.pl is typical for a server which will execute Perl scripts which perform database queries -- a common scenario. The thinking is that all of our Perl scripts which run under this mod_perl server will want the CGI module for easy access to environment variables and form parameters, as well as the database modules DBI and DBD::mysql since we happen to be using the MySQL database.

Notice the inclusion of another Perl script, fastdb.pl. In fact, we'll soon see the value of this script, which we use to optimize database queries in several fun and fascinating ways.

To summarize, we've seen that it is wise to prepare the Apache parent server process with certain bits of "knowledge", or Perl code, which will be compiled only once and passed on to each spawned child process.

The Perl You Need to Know Special: Introduction to mod_perl Part 3
The Perl You Need to Know
More Who's Your Daddy