=head1 NAME

Introducing mod_perl Handlers

=head1 Description

This chapter provides an introduction into mod_perl handlers.


=head1 What are Handlers?

Apache distinguishes between numerous phases for which it provides
hooks (because the C functions are called
I<ap_hook_E<lt>phase_nameE<gt>>) 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 I<handlers> and therefore the
configuration directives for the mod_perl handlers look like:
C<PerlFooHandler>, where C<Foo> is one of the handler names. For
example C<PerlResponseHandler> configures the response callback.

A typical handler is simply a perl package with a I<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
I<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 I<http://localhost/time> this response
handler is executed and a response that includes the current time is
returned to the client.





=head1 Handler Return Values

Different handler groups are supposed to return different values.

Make sure that you B<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
C<Apache2::Const::OK>, which tells Apache that the handler has
successfully finished its execution.

C<Apache2::Const::DECLINED> is another return value that indicates
success, but it's only relevant for
L<phases|docs::2.0::user::handlers::intro/Stacked_Handlers> of type
C<L<RUN_FIRST|/RUN_FIRST>>.

L<HTTP handlers|docs::2.0::user::handlers::http> may also return
C<Apache2::Const::DONE> which tells Apache to stop the normal L<HTTP
request
cycle|docs::2.0::user::handlers::http/HTTP_Request_Cycle_Phases> and
fast forward to the
C<L<PerlLogHandler|docs::2.0::user::handlers::http/PerlLogHandler>>,
followed by
C<L<PerlCleanupHandler|docs::2.0::user::handlers::http/PerlCleanupHandler>>.
L<HTTP handlers|docs::2.0::user::handlers::http> may return any HTTP
status, which similarly to C<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 C<Apache2::Const::HTTP_OK>
from your HTTP response handler, but C<Apache2::Const::OK> and Apache
will send the C<200 OK> status by itself.

L<Filter handlers|docs::2.0::user::handlers::filters> return
C<Apache2::Const::OK> to indicate that the filter has successfully
finished. If the return value is C<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
C<L<APR::|docs::2.0::api::APR::Const>> constant.

L<Protocol handler|docs::2.0::user::handlers::protocols> 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
C<L<PerlPreConnectionHandler|docs::2.0::user::handlers::protocols/PerlPreConnectionHandler>>
handler, which, if returning anything but C<Apache2::Const::OK> or
C<Apache2::Const::DONE>, will prevent from
C<L<PerlConnectionHandler|docs::2.0::user::handlers::protocols/PerlConnectionHandler>>
to be run.
C<L<PerlPreConnectionHandler|docs::2.0::user::handlers::protocols/PerlPreConnectionHandler>>
handlers should always return C<Apache2::Const::OK>.





=head1 mod_perl Handlers Categories

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

=over

=item * L<Server life cycle|docs::2.0::user::handlers::server/>

=over

=item * C<L<PerlOpenLogsHandler|docs::2.0::user::handlers::server/C_PerlOpenLogsHandler_>>

=item * C<L<PerlPostConfigHandler|docs::2.0::user::handlers::server/C_PerlPostConfigHandler_>>

=item * C<L<PerlChildInitHandler|docs::2.0::user::handlers::server/C_PerlChildInitHandler_>>

=item * C<L<PerlChildExitHandler|docs::2.0::user::handlers::server/C_PerlChildExitHandler_>>

=back



=item * L<Protocols|docs::2.0::user::handlers::protocols/>

=over

=item * C<L<PerlPreConnectionHandler|docs::2.0::user::handlers::protocols/PerlPreConnectionHandler>>

=item * C<L<PerlProcessConnectionHandler|docs::2.0::user::handlers::protocols/PerlProcessConnectionHandler>>

=back




=item * L<Filters|docs::2.0::user::handlers::filters/>

=over

=item * C<L<PerlInputFilterHandler|docs::2.0::user::handlers::filters/C_PerlInputFilterHandler_>>

=item * C<L<PerlOutputFilterHandler|docs::2.0::user::handlers::filters/C_PerlOutputFilterHandler_>>

=back




=item * L<HTTP Protocol|docs::2.0::user::handlers::http/>

=over

=item * C<L<PerlPostReadRequestHandler|docs::2.0::user::handlers::http/PerlPostReadRequestHandler>>

=item * C<L<PerlTransHandler|docs::2.0::user::handlers::http/PerlTransHandler>>

=item * C<L<PerlMapToStorageHandler|docs::2.0::user::handlers::http/PerlMapToStorageHandler>>

=item * C<L<PerlInitHandler|docs::2.0::user::handlers::http/PerlInitHandler>>

=item * C<L<PerlHeaderParserHandler|docs::2.0::user::handlers::http/PerlHeaderParserHandler>>

=item * C<L<PerlAccessHandler|docs::2.0::user::handlers::http/PerlAccessHandler>>

=item * C<L<PerlAuthenHandler|docs::2.0::user::handlers::http/PerlAuthenHandler>>

=item * C<L<PerlAuthzHandler|docs::2.0::user::handlers::http/PerlAuthzHandler>>

=item * C<L<PerlTypeHandler|docs::2.0::user::handlers::http/PerlTypeHandler>>

=item * C<L<PerlFixupHandler|docs::2.0::user::handlers::http/PerlFixupHandler>>

=item * C<L<PerlResponseHandler|docs::2.0::user::handlers::http/PerlResponseHandler>>

=item * C<L<PerlLogHandler|docs::2.0::user::handlers::http/PerlLogHandler>>

=item * C<L<PerlCleanupHandler|docs::2.0::user::handlers::http/PerlCleanupHandler>>

=back


=back













=head1 Stacked Handlers

For each phase there can be more than one handler assigned (also known
as I<hooks>, because the C functions are called
I<ap_hook_E<lt>phase_nameE<gt>>). 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:
C<L<PerlChildExitHandler|docs::2.0::user::handlers::http/C_PerlChildExitHandler_>>
and
C<L<PerlCleanupHandler|docs::2.0::user::handlers::http/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:

=head2 C<VOID>

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

=head2 C<RUN_FIRST>

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

=head2 C<RUN_ALL>

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

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

Also see L<mod_perl Directives Argument Types and Allowed
Location|docs::2.0::user::config::config/mod_perl_Directives_Argument_Types_and_Allowed_Location>











=head1 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:

=over

=item *  C<APR::Const::HOOK_REALLY_FIRST>

run this hook first, before ANYTHING.

=item *  C<APR::Const::HOOK_FIRST>

run this hook first.

=item *  C<APR::Const::HOOK_MIDDLE>

run this hook somewhere.

=item *  C<APR::Const::HOOK_LAST>

run this hook after every other hook which is defined.

=item *  C<APR::Const::HOOK_REALLY_LAST>

run this hook last, after EVERYTHING.

=back

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













=head1 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 I<Bucket
Brigades> technology was introduced. It's also used in L<protocol
handlers|docs::2.0::user::handlers::protocols>.

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:

=for html
<img src="bucket_brigades.gif" width="590" height="400" 
 align="middle" alt="bucket brigades"><br><br>

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 L<protocol
handlers|docs::2.0::user::handlers::protocols> and L<I/O
filtering|docs::2.0::user::handlers::filters> chapters.






=head1 Maintainers

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

=over

=item *

Stas Bekman [http://stason.org/]

=back


=head1 Authors

=over

=item *

=back

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



=cut
