11.3 mod_perl Handlers

To understand mod_perl, you should understand how the Apache server works. When Apache receives a request, it processes it in several stages. First, it translates the URL to the associated resource (i.e., filename, CGI script, etc.) on the server machine. Then it checks to see if the user is authorized to access that resource, perhaps by requesting and checking an ID and password. Once the user has passed inspection, the server figures out what kind of data it's sending back (e.g., it decides a file ending in .html is probably a text/html file), creates some headers, and sends those headers back to the client with the resource itself. When all is said and done, the server makes a log entry.

At each stage of this process, Apache looks for routines to "handle" the request. Apache supplies its own handlers; for example, one of the default handlers is cgi-script, often seen applied to /cgi-bin:

<Location /cgi-bin>
  ...
SetHandler cgi-script
  ...
</Location\>

mod_perl allows you to write your own handlers in Perl, by embedding the Perl runtime library directly into the Apache httpd server executable. To use mod_perl for CGI (which is all that most people want to do with it), you assign the SetHandler directive to perl-script, and then assign the mod_perl-specific PerlHandler directive to a special Perl module called Apache::Registry.

SetHandler perl-script
PerlHandler Apache::Registry

PerlHandler is the mod_perl handler for the content retrieval stage of the transaction.

To use other handlers, you don't need to reassign SetHandler. For example, to identify a handler for the logging stage of the request:

<Location /snoop/>
PerlLogHandler Apache::DumpHeaders
</Location\>

In order for this to work, mod_perl must have been built with the logging hooks enabled (as described in the previous section), and the Apache::DumpHeaders module must have been installed. mod_perl looks in Apache::DumpHeaders for a routine called handler() and executes it as the logging handler for that resource.

The following is a list of each of the handler directives that can be enabled by mod_perl, and the stages that each is used for. Only PerlHandler is enabled by default.

Handler	Purpose
PerlAccessHandler	Access stage
PerlAuthenHandler	Authentication stage
PerlAuthzHandler	Authorization stage
PerlChildInitHandler	Child initialization stage
PerlChildExitHandler	Child termination stage
PerlCleanupHandler	Cleanup stage
PerlFixupHandler	Fixup stage
PerlHandler	Response stage
PerlHeaderParserHandler	Header-parsing stage
PerlInitHandler	Initialization
PerlLogHandler	Logging stage
PerlPostReadRequestHandler	Post-request stage
PerlTransHandler	Translation stage
PerlTypeHandler	Type-handling stage

You can write your own handlers for each of these stages. But there are also dozens of modules that you can download from CPAN, some of which are listed at the end of this chapter.