Writing Modules for mod_perl

Discover the flexibility and power of writing mod_perl modules instead of CGI programs.
Writing a Simple PerlHandler

As we know, CGI programs are stand-alone programs that are invoked from an outside process, namely the web server. PerlHandler modules are actually subroutines within the Apache process; Apache invokes our subroutine when a certain set of conditions is fulfilled.

Writing a PerlHandler module is not much different from writing any Perl module. (If you are unfamiliar with writing Perl modules, see the “perlmod” man pages, or any of the books available on the subject.) We create a module with a single subroutine defined, called “handler”, shown in Listing 1. This code has several elements common to many PerlHandler modules.

Listing 1

Listing 1. PerlHandler Module

First of all, the entire module contains a single subroutine, “handler”. We can define additional subroutines if we want, but usually it is easiest to use the established standard and default.

Next, notice the handler is invoked with a single argument, which we call $r. It is an instance of the Apache object, which gives us access to the innards of the Apache web server. $r is our conduit to the outside world of the HTTP server and the user's browser. We invoke certain methods to determine the state of the server and browser and other methods to send output to the user's browser. Without $r we are somewhat lost, so it is natural that our first action upon entering “handler” is to retrieve $r.

We also use the -w and use strict programming aides in our program. While these are normally good ideas for good, clean Perl programs, they are essential when developing under mod_perl. As we will see later, mod_perl's caching and persistence means we need to be extra careful with our use of memory, in order to keep our HTTP server process as slim as possible.

Our handler uses only three methods from $r: content_type, send_http_header and print.

The first method, content_type, allows us to set or retrieve the “Content-type” header that will precede the response. Every HTTP response must be described with such a header, which tells the browser whether the response is an HTML-formatted text file, a GIF image or a zip file.

Once we have set the “Content-type” header to an appropriate value, we send all of the headers to the user's browser with the send_http_header method. Past this point, anything sent to the user's browser will be considered part of the HTTP response body, rather than the headers that describe that body.

The third method, print, is analogous to the built-in “print” function. However, it takes into consideration several factors that “print” might not, such as timeouts. $r->print takes a list of arguments just as the “print” function does. Thus, you can use

$r->print("a", "b", "c");

and expect three characters to be sent to the user's browser.

Constants and Return Codes

Once we have finished writing the response, we exit from our module by returning the OK symbol to the caller. We import OK from Apache::Constants, a module that provides us with a large number of useful symbols. In order not to pollute our name space too much, we explicitly request that only “OK” be imported with no other symbols.

If we were writing a more complicated module, we might use one of the export tags such as :common and :response, which allow us to import a group of symbols without having to name them explicitly. Thus, we could use the statement:

use Apache::Constants qw(:response);

which would import all symbols needed for a response.

Most PerlHandler modules will want their “handler” subroutines to return one of two symbols: either OK, which indicates that the handler successfully dealt with the request and no other PerlHandler needs to do anything, or the DECLINED symbol. If your module's “handler” routine returns DECLINED, it means “I was unable to do anything with the input I was given and would be happy if some other PerlHandler would do something.” Often, returning DECLINED means the default Apache behavior will be applied; if our PerlHandler were to return DECLINED, Apache would try to read the file named in the URL and do something with it. By returning OK, we indicate that our module took care of things, and Apache can move on to the next PerlHandler.