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NAME ^

Introducing mod_perl Handlers

Description ^

This chapter provides an introduction into mod_perl handlers.

What are Handlers? ^

Apache distinguishes between numerous phases for which it provides hooks (because the C functions are called ap_hook_<phase_name>) where modules can plug various callbacks to extend and alter the default behavior of the webserver. mod_perl provides a Perl interface for most of the available hooks, so mod_perl modules writers can change the Apache behavior in Perl. These callbacks are usually referred to as handlers and therefore the configuration directives for the mod_perl handlers look like: PerlFooHandler, where Foo is one of the handler names. For example PerlResponseHandler configures the response callback.

A typical handler is simply a perl package with a handler subroutine. For example:

  file:MyApache2/CurrentTime.pm
  ----------------------------
  package MyApache2::CurrentTime;
  
  use strict;
  use warnings;
  
  use Apache2::RequestRec ();
  use Apache2::RequestIO ();
  
  use Apache2::Const -compile => qw(OK);
  
  sub handler {
      my $r = shift;
  
      $r->content_type('text/plain');
      $r->print("Now is: " . scalar(localtime) . "\n");
  
      return Apache2::Const::OK;
  }
  1;

This handler simply returns the current date and time as a response.

Since this is a response handler, we configure it as a such in httpd.conf:

  PerlResponseHandler MyApache2::CurrentTime

Since the response handler should be configured for a specific location, let's write a complete configuration section:

  PerlModule MyApache2::CurrentTime
  <Location /time>
      SetHandler modperl
      PerlResponseHandler MyApache2::CurrentTime
  </Location>

Now when a request is issued to http://localhost/time this response handler is executed and a response that includes the current time is returned to the client.

Handler Return Values ^

Different handler groups are supposed to return different values.

Make sure that you always explicitly return a wanted value and don't rely on the result of last expression to be used as the return value -- things will change in the future and you won't know why things aren't working anymore.

The only value that can be returned by all handlers is Apache2::Const::OK, which tells Apache that the handler has successfully finished its execution.

Apache2::Const::DECLINED is another return value that indicates success, but it's only relevant for phases of type RUN_FIRST.

HTTP handlers may also return Apache2::Const::DONE which tells Apache to stop the normal HTTP request cycle and fast forward to the PerlLogHandler, followed by PerlCleanupHandler. HTTP handlers may return any HTTP status, which similarly to Apache2::Const::DONE will cause an abort of the request cycle, by also will be interpreted as an error. Therefore you don't want to return Apache2::Const::HTTP_OK from your HTTP response handler, but Apache2::Const::OK and Apache will send the 200 OK status by itself.

Filter handlers return Apache2::Const::OK to indicate that the filter has successfully finished. If the return value is Apache2::Const::DECLINED, mod_perl will read and forward the data on behalf of the filter. Please notice that this feature is specific to mod_perl. If there is some problem with obtaining or sending the bucket brigades, or the buckets in it, filters need to return the error returned by the method that tried to manipulate the bucket brigade or the bucket. Normally it'd be an APR:: constant.

Protocol handler return values aren't really handled by Apache, the handler is supposed to take care of any errors by itself. The only special case is the PerlPreConnectionHandler handler, which, if returning anything but Apache2::Const::OK or Apache2::Const::DONE, will prevent from PerlConnectionHandler to be run. PerlPreConnectionHandler handlers should always return Apache2::Const::OK.

mod_perl Handlers Categories ^

The mod_perl handlers can be divided by their application scope in several categories:

Stacked Handlers ^

For each phase there can be more than one handler assigned (also known as hooks, because the C functions are called ap_hook_<phase_name>). Phases' behavior varies when there is more then one handler registered to run for the same phase. The following table specifies each handler's behavior in this situation:

    Directive                   Type
  --------------------------------------
  PerlOpenLogsHandler          RUN_ALL
  PerlPostConfigHandler        RUN_ALL
  PerlChildInitHandler         VOID
  PerlChildExitHandler         VOID
  
  PerlPreConnectionHandler     RUN_ALL
  PerlProcessConnectionHandler RUN_FIRST
  
  PerlPostReadRequestHandler   RUN_ALL
  PerlTransHandler             RUN_FIRST
  PerlMapToStorageHandler      RUN_FIRST
  PerlInitHandler              RUN_ALL
  PerlHeaderParserHandler      RUN_ALL
  PerlAccessHandler            RUN_ALL
  PerlAuthenHandler            RUN_FIRST
  PerlAuthzHandler             RUN_FIRST
  PerlTypeHandler              RUN_FIRST
  PerlFixupHandler             RUN_ALL
  PerlResponseHandler          RUN_FIRST
  PerlLogHandler               RUN_ALL
  PerlCleanupHandler           RUN_ALL
  
  PerlInputFilterHandler       VOID
  PerlOutputFilterHandler      VOID

Note: PerlChildExitHandler and PerlCleanupHandler are not real Apache hooks, but to mod_perl users they behave as all other hooks.

And here is the description of the possible types:

VOID

Handlers of the type VOID will be all executed in the order they have been registered disregarding their return values. Though in mod_perl they are expected to return Apache2::Const::OK.

RUN_FIRST

Handlers of the type RUN_FIRST will be executed in the order they have been registered until the first handler that returns something other than Apache2::Const::DECLINED. If the return value is Apache2::Const::DECLINED, the next handler in the chain will be run. If the return value is Apache2::Const::OK the next phase will start. In all other cases the execution will be aborted.

RUN_ALL

Handlers of the type RUN_ALL will be executed in the order they have been registered until the first handler that returns something other than Apache2::Const::OK or Apache2::Const::DECLINED.

For C API declarations see include/ap_config.h, which includes other types which aren't exposed by mod_perl handlers.

Also see mod_perl Directives Argument Types and Allowed Location

Hook Ordering (Position) ^

The following constants specify how the new hooks (handlers) are inserted into the list of hooks when there is at least one hook already registered for the same phase.

META: Not working yet.

META: need to verify the following:

META: more information in mod_example.c talking about position/predecessors, etc.

Bucket Brigades ^

Apache 2.0 allows multiple modules to filter both the request and the response. Now one module can pipe its output as an input to another module as if another module was receiving the data directly from the TCP stream. The same mechanism works with the generated response.

With I/O filtering in place, simple filters, like data compression and decompression, can be easily implemented and complex filters, like SSL, are now possible without needing to modify the the server code which was the case with Apache 1.3.

In order to make the filtering mechanism efficient and avoid unnecessary copying, while keeping the data abstracted, the Bucket Brigades technology was introduced. It's also used in protocol handlers.

A bucket represents a chunk of data. Buckets linked together comprise a brigade. Each bucket in a brigade can be modified, removed and replaced with another bucket. The goal is to minimize the data copying where possible. Buckets come in different types, such as files, data blocks, end of stream indicators, pools, etc. To manipulate a bucket one doesn't need to know its internal representation.

The stream of data is represented by bucket brigades. When a filter is called it gets passed the brigade that was the output of the previous filter. This brigade is then manipulated by the filter (e.g., by modifying some buckets) and passed to the next filter in the stack.

The following figure depicts an imaginary bucket brigade:

The figure tries to show that after the presented bucket brigade has passed through several filters some buckets were removed, some modified and some added. Of course the handler that gets the brigade cannot tell the history of the brigade, it can only see the existing buckets in the brigade.

Bucket brigades are discussed in detail in the protocol handlers and I/O filtering chapters.

Maintainers ^

Maintainer is the person(s) you should contact with updates, corrections and patches.

Authors ^

Only the major authors are listed above. For contributors see the Changes file.

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