Date Published: 2002-08-26
In this article I'll give an initial introduction to mod_perl, make you want to give it a try and present a few examples of the well known sites that are powered by mod_perl enabled Apache.
Stas Beckman <firstname.lastname@example.org>
mod_perl is at the heart of the Apache/Perl integration project, which
brings together the full power of the Perl programming language and
the Apache Web server.
From the outset Apache was designed so that you can extend it by the
addition of `modules'. Modules can do anything you need to do, such
as rewrite HTTP requests, restrict access to certain pages, perform
database lookups and so on. Modules are normally written in C, which
can be hard work. mod_perl is a module which lets you do all of these
things, and more, using Perl -- which makes the development much
quicker than C. Apache is the most popular Web server on the Internet
and mod_perl is one of the most popular modules for extending it.
If you love Perl and your favorite web-server is Apache you will love
mod_perl at first sight. Once you try it in action you will never look
back, you will find mod_perl has everything you need. But even if you
do find that there is something missing, just speak up. Before you
can count to three, someone will have made it for you. Which of course
will make you want to give something in return. Eventually you will
contribute something on your own, and that will save time for a huge
mod_perl community so that they can create even more things for others
You get the picture -- mod_perl empowers its users, who in turn
empower mod_perl, which in turn empowers its users, who in
turn... It's as simple as the nuclear reaction you learned about at
school, or will learn at some point :)
With mod_perl it is possible to write Apache modules entirely in Perl,
this lets you easily do things that are more difficult or impossible
in regular CGI programs, such as running sub-requests, writing your
authentication and logging handlers and much more.
The primary advantages of mod_perl are power and speed. You have full
access to the inner workings of the Web server and you can intervene
at any stage of HTTP request processing. This allows for customized
processing of the various phases, for example URI to filename
translation, authorization, response generation and logging.
There are big savings in startup and compilation times. Having the
Perl interpreter embedded in the server saves the very considerable
overhead of starting an external interpreter for any HTTP request
which needs to run Perl code. At least as important is code caching:
the modules and scripts are loaded and compiled only once, when the
server is first started. Then for the rest of the server's life the
scripts are served from the cache, so the server only has to run the
pre-compiled code. In many cases this is as fast as running compiled C
There is very little run-time overhead. In particular, under mod_perl
there is no need to start a separate process per request, as is often
done with other web-server extensions. The most wide-spread such
extension mechanism, the Common Gateway Interface (CGI), is replaced
entirely with Perl code that handles the response generation phase of
request processing. Bundled with mod_perl are two general purpose
modules for this purpose:
Apache::Registry, which can transparently
run existing unmodified Perl CGI scripts and
does a similar job but allows you to run scripts which are to some
mod_perl allows you to configure your Apache server and handlers in
Perl (using the
PerlSetVar directive and the <Perl>
sections). This makes the administration of servers with many virtual
hosts and complex configuration a piece of cake. Hey, you can even
define your own configuration directives!
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 whom you ask. Developers report speed boosts from 200% to
2000%. The best way to measure is to try it and see for yourself! (see
http://perl.apache.org/stories/ for the facts).
Every second of every day, thousands of Web sites all over the world
are using mod_perl to serve hundreds of thousands of Web pages.
Apache and mod_perl are some of the best-tested programs ever written.
Of course they are continually being developed and improved, but you
do not have to work on the ``bleeding edge'' of development -- you can
use the stable products for your sites and let others do the testing
of the new versions for you.
I want to show you just a few of the many very busy and popular sites
that are driven by mod_perl. A thousand words can't substitute the
experience. Visit the sites and feel the difference. They will
persuade you that mod_perl rules!
ValueClick -- http://www.valueclick.com/ serves more than 70M
requests per day from about 20 machines. Every response is dynamic,
with all sorts of calculation, storing, logging, counting, you name
it. All of their ``application'' programming is done in Perl.
Singles Heaven -- http://singlesheaven.com is a Match Maker site
with 35,000+ members and growing. The site is driven by mod_perl, DBI,
Apache::DBI (which provides a persistence to DB connections) and
MySQL. The speed is enormous, chatting with mod_perl is a
pleasure. Every page is generated by about 10 SQL queries, for it does
many dynamic checks on every page -- like checking for new emails,
users who are watched by various watchdogs and many more. You don't
feel these queries are actually happening, the speed is as fast as the
``Hello World'' script.
Internet Movie Database (Ltd) -- http://www.moviedatabase.com/ -
serves around 2 million page views per day. All database lookups are
handled inside Apache via mod_perl. Each request also goes through
several mod_perl handlers and the output is then reformatted on the
fly with mod_perl SSI to embed advertising banners and give different
views of the site depending on the hostname used.
CMPnet -- http://www.cmpnet.com, a technology information network
serves about 600k page views per day.
CitySearch.com -- http://www.citysearch.com/ is providing online
city guides for more than 100 cities worldwide, citysearch.com helps
people find and plan what they want to do and then lets them take
action, offering local transactions such as buying event tickets and
making hotel and restaurant reservations online. Its traffic exceeds
100,000,000 page views a month.
According to Netcraft ( http://netcraft.com ), as of August, 2001 - 18
million hosts are running the free Apache webserver, which is about
60% of all checked in survey hosts!
Here is the
graph of "Server Share in Internet Web Sites".
What about mod_perl? http://perl.apache.org/netcraft/ reports that
sites running mod_perl account for 2,823,060 host names and 283,180
unique IP addresses. This is actually an underestimate, since when
hosts are scanned for running webservers only well known ports are
checked (80, 81, 8080 and a few others). If a server runs on unusual
port it does not enter the count unless the owner has manually added
it to the Netcraft database. Here is a graph of the growth in mod_perl
For the latest numbers see http://perl.apache.org/netcraft/ .
You probably are all excited about the release of Apache 2.0, the next
generation of the best web-server. The major new features of this new
generation of the webserver are threaded processes, which should make
the server more scalable, and of course the very welcomed filtering
You probably are not less excited about the recent release of Perl
5.6, whose main new feature is (almost) stable support for threads,
something that existed in the previous Perl version but which was
What has all this to do with mod_perl you ask? mod_perl 2.0 is being
developed at this very moment and will benefit enormously from the new
Apache and Perl features. The most important improvement will be a
reduced process size -- a parsed Perl opcodes tree will be almost
completely shared between threads of the same process.
Do you believe in coincidences? Both Perl 5.6 and Apache 2.0 were
released in the same week in March, 2000. Looks very suspicious to
me. If you get the obvious conspiracy uncovered please let me know.
Of course there are lots of bumps ahead of us. It will take time
before all our applications will be able to benefit from the threading
features. The main reason lies in fact that most of the Perl modules
available from CPAN aren't thread safe. But you shouldn't despair. You
can turn off threads for Perl code which is not thread safe or which
uses modules which aren't thread safe.
mod_perl's home is http://perl.apache.org . From the site you will be
able to download the latest mod_perl software and various
documentation; find commercial products and free third party modules;
read the success stories; and learn more about mod_perl.
It's quite important to get yourself subscribed to the mod_perl
list. If you want know what happens with mod_perl, if you want to know
what new features are being developed, if you want to influence and
contribute or if you simply want to get help, then you don't want to
skip this mailing list. To subscribe to the list send an empty email
to email@example.com .
Lincoln Stein and Doug MacEachern wrote Writing Apache Modules with Perl and
C . It can be purchased online from O'Reilly and
You will find a vast list of mod_perl documentation on the mod_perl
home page: http://perl.apache.org/ .
Well, Doug MacEachern is the person to blame :). He is the guy who
gave mod_perl to the mod_perl community. He is the Linus of the
But as you know in a big community there are always people who love to
help, and there is a bunch of developers from all around the world who
patch mod_perl, develop entire Perl modules for it, debug the server
and advocate it. I'm afraid the list of contributing developers is too
long to include here. But you are welcome to join the mod_perl mailing
list and see all these folks in action. I promise you, you won't
regret the show, since you are going to learn much more than just
about mod_perl. See for yourself.
If you are using mod_perl or planning to use it, it's a good idea to
subscribe to the users mod_perl mailing list. You should send an empty
email to firstname.lastname@example.org in order to do that.
If you are interested in helping out with development of mod_perl 2.0,
you are welcome to join us. There are many features that are still
need implementing and a lot of testing has to be done. So there is a
lot of work if you are knowledgeable developer or even if you just a
newbie. And the more help we get the sooner we bring mod_perl 2.0 into
production shape. You can subscribe to the developers mailing list by
sending email to email@example.com .
If you are familiar with mod_perl, you probably know about the big fat
mod_perl guide that I maintain with help of many people
(http://perl.apache.org/guide/). However mod_perl 2.0 has quite a few
things changed so the new documentation project has been started. You
are welcome to check the updates on the http://perl.apache.org/ site
and subscribe to the documentation mailing list to stay up to date by
sending email to firstname.lastname@example.org .