Writing Modules for mod_perl

Now that we have seen how easy it is to write a PerlHandler module, let's look at how to install this module on our web server. We do this in the configuration file, typically named httpd.conf. If your copy of Apache uses three .conf files, understand that the division between them is artificial and based on the server's history, rather than any real need for three files. Apache developers recognized this increasingly artificial division and recently decided that future versions of the server will have a single file, httpd.conf, rather than three.

Apache configuration files depend on directives, which are variable assignments in disguise. That is, the statement

ServerName lerner.co.il

sets the “ServerName” variable to the value “lerner.co.il”.

If you want a directive to affect a subset of the files or directories on the server, you can use a “section”. For instance, if we say:

<Directory /usr/local/apache/share/cgi-bin>
AllowOverride None
Options ExecCGI
</Directory>

then the AllowOverride and Options directives apply only to the directory /usr/local/apache/share/cgi-bin. In this way, we can apply different directives to different files.

“Directory” sections allow us to modify the behavior of particular files and directories. We can also use “Location” sections to modify the behavior of URLs not connected to directories. Location sections work in the same way as Directory sections, except that Location takes its argument relative to URLs, while Directory takes its argument relative to the server's file system.

For example, we could rewrite the above Directory section as the following Location section:

<Location /cgi-bin>
AllowOverride None
Options ExecCGI
</Location>

Of course, this assumes that URLs beginning with /cgi-bin point to /usr/local/apache/share/cgi-bin on the server file system.

All this background is necessary to understand how we will install our PerlHandler module. After all, our PerlHandler will influence the way in which one or more URLs will be affected. If we (unwisely) want our PerlHandler module to affect all the files in /cgi-bin, then we use

<Location /cgi-bin>
SetHandler perl-script
PerlHandler Apache::TestModule
</Location>

This tells Apache we will be handling all URLs under /cgi-bin with a Perl handler. We then tell Apache which PerlHandler to use, naming Apache::TestModule. If we did not install Apache::TestModule in the appropriate place on the server file system and if the package was not named correctly, this will cause an error.

The above example is unwise for a number of reasons, including the fact that it masks all the CGI programs on our server. Let's try a slightly more useful Location section:

<Location /hello>
SetHandler perl-script
PerlHandler Apache::TestModule
</Location>

The above Location section means that every time someone requests the URL “/hello” from our server, Apache will run the “handler” routine in Apache::TestModule. Because we used a Location section, we need not worry whether /hello corresponds to a directory on our server's file system.

This is how mod_perl creates a status monitor:

<Location /perl-status>
SetHandler perl-script
PerlHandler Apache::Status
</Location>

Each time someone requests the /perl-status URL from our server, the Apache::Status module is invoked. This module, which comes with mod_perl, provides us with status information about our mod_perl subsystem. Again, because we use a Location section, we need not worry about whether /perl-status corresponds to a directory on disk. In this way, we can create applications that exist independent of the file system.

Once we have created this Location section in httpd.conf, we must restart Apache. We can send it an HUP signal with

killall -HUP -v httpd

or we can even restart Apache altogether, with the program apachectl that comes with modern versions of the server:

apachectl restart

We can test to see if things work by going to the URL /hello. On my home machine, I pointed my browser to http://localhost/hello and received the “testing” message soon after. If you don't see this message, check the Apache error log on your system. If there was a syntax error in the module, you will need to modify the module and restart the server as described above.

The first time you invoke a PerlHandler module, it may take some time for Apache to respond. This is because the first time a PerlHandler is invoked on a given Apache process, the Perl system must be invoked and the module loaded. You can avoid this problem to a certain degree with the PerlModule directive, described later in this article.