Perl does some very useful things and provides such huge resources in the CPAN library (http://cpan.org) that it will clearly be with us for a long time yet as a way of writing scripts to run behind Apache. While Perl is powerful, CGI is not a particularly efficient means of connecting Perl to Apache. CGI’s big disadvantage is that each time a script is invoked, Apache has to load the Perl interpreter and then it has to load the script. This is a heavy and pointless overhead on a busy site, and it would obviously be much easier if Perl stayed loaded in memory, together with the scripts, to be invoked each time they were needed. This is what mod_perl does by modifying Apache.
This modification is definitely popular: according to Netcraft surveys in mid-2000, mod_perl was the third most popular add-on to Apache (after FrontPage and PHP), serving more than a million URLs on over 120,000 different IP numbers (http://perl.apache.org/outstanding/stats/netcraft.html).
The reason that this chapter is more than a couple of pages long is
that Perl does not sit easily in a web server. It was originally
designed as a better shell script to run standalone under Unix. It
developed, over time, into a full-blown programming language.
However, because the original Perl was not designed for this kind of
work, various things have to happen. To illustrate them, we will
start with a simple Perl script that runs under
Apache’s mod_cgi and then modify it to run under
mod_perl. (We assume that the reader is familiar enough with Perl to
write a simple script, understands the ideas of Perl modules, use(),
require(), and the BEGIN and
END pragmas.)
On site.mod_perl we have two subdirectories: mod_cgi and mod_perl. In mod_cgi we present a simple script-driven site that runs a home page that has a link to another page.
The Config file is as follows:
User webuser Group webuser ServerName www.butterthlies.com DocumentRoot /usr/www/APACHE3/APACHE3/site.mod_perl/mod_cgi/htdocs TransferLog /usr/www/APACHE3/APACHE3/site.mod_perl/mod_cgi/logs/access_log LogLevel debug ScriptAlias /bin /usr/www/APACHE3/APACHE3/site.mod_perl/cgi-bin ScriptAliasMatch /AA(.*) /usr/www/APACHE3/APACHE3/site.mod_perl/cgi-bin/AA$1 DirectoryIndex /bin/home.pl
When you go to http://www.butterthlies.com, you see the results of running the Perl script home:
#! /usr/local/bin/perl -w use strict; print qq(content-type: text/html\n\n <HTML><HEAD><TITLE>Demo CGI Home Page</TITLE></HEAD> <BODY>Hi: I'm a demo home page <A HREF="/AA_next">Click here to run my mate</A> </BODY></HTML>);
On the browser, this simply says:
Hi: I'm a demo home page. Click here to run my mateAnd when you do, you get:
Hi: I'm a demo next page
Which is printed by the script AA_next:
#! /usr/local/bin/perl -w use strict; print qq(content-type: text/html\n\n <HTML><HEAD><TITLE>NEXT Page</TITLE></HEAD> <BODY>Hi: I'm a demo next page </BODY></HTML>);
Naturally, this is a web site that will run and run and make everyone concerned into e-billionaires. In the process of serving the millions of visitors it will attract, Perl will get loaded and unloaded millions of times, which helps to explain why they are running out of electricity in Silicon Valley. We have to stop this reckless waste of the world’s resources, so we install mod_perl.