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Introducing mod_perl Handlers | ||||
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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:MyApache/CurrentTime.pm ---------------------------- package MyApache::CurrentTime; use strict; use warnings; use Apache::RequestRec (); use Apache::RequestIO (); use Apache::Const -compile => qw(OK); sub handler { my $r = shift; $r->content_type('text/plain'); $r->print("Now is: " . scalar(localtime) . "\n"); return Apache::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 MyApache::CurrentTime
Since the response handler should be configured for a specific location, let's write a complete configuration section:
PerlModule MyApache::CurrentTime <Location /time> SetHandler modperl PerlResponseHandler MyApache::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.
Different handler groups are supposed to return different values. The
only value that can be returned by all handlers is Apache::OK
,
which tells Apache that the handler has successfully finished its
execution.
Apache::DECLINED
is another return value that indicates success,
but it's only relevant for
phases
of type RUN_FIRST
.
HTTP handlers may also return
Apache::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 Apache::DONE
will cause an abort of the
request cycle, by also will be interpreted as an error. Therefore you
don't want to return Apache::HTTP_OK
from your HTTP response
handler, but Apache::OK
and Apache will send the 200 OK
status
by itself.
Filter handlers return
Apache::OK
to indicate that the filter has successfully
finished. If the return value is Apache::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 Apache::OK
or
Apache::DONE
, will prevent from
PerlConnectionHandler
to be
run. PerlPreConnectionHandler
handlers should always return Apache::OK
.
The mod_perl handlers can be divided by their application scope in several categories:
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 Apache::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 Apache::DECLINED
. If the return value is
Apache::DECLINED
, the next handler in the chain will be run. If the
return value is Apache::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 Apache::OK
or Apache::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
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:
APR::HOOK_REALLY_FIRST
run this hook first, before ANYTHING.
APR::HOOK_FIRST
run this hook first.
APR::HOOK_MIDDLE
run this hook somewhere.
APR::HOOK_LAST
run this hook after every other hook which is defined.
APR::HOOK_REALLY_LAST
run this hook last, after EVERYTHING.
META: more information in mod_example.c talking about position/predecessors, etc.
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.
Maintainer is the person(s) you should contact with updates, corrections and patches.
Stas Bekman <stas (at) stason.org>
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