NAME
Apache::Filter - Alter the output of previous handlers
SYNOPSIS
#### In httpd.conf:
PerlModule Apache::Filter
# That's it - this isn't a handler.
SetHandler perl-script
PerlSetVar Filter On
PerlHandler Filter1 Filter2 Filter3
#### In Filter1, Filter2, and Filter3:
my $fh = $r->filter_input();
while (<$fh>) {
s/ something / something else /;
print;
}
#### or, alternatively:
my ($fh, $status) = $r->filter_input();
return $status unless $status == OK; # The Apache::Constants OK
while (<$fh>) {
s/ something / something else /;
print;
}
DESCRIPTION
Each of the handlers Filter1, Filter2, and Filter3 will make a call to
$r->filter_input(), which will return a filehandle. For Filter1, the
filehandle points to the requested file. For Filter2, the filehandle
contains whatever Filter1 wrote to STDOUT. For Filter3, it contains
whatever Filter3 wrote to STDOUT. The output of Filter3 goes directly to
the browser.
Note that the modules Filter1, Filter2, and Filter3 are listed in
forward order, in contrast to the reverse-order listing of
Apache::OutputChain.
When you've got this module, you can use the same handler both as a
stand-alone handler, and as an element in a chain. Just make sure that
whenever you're chaining, all the handlers in the chain are "Filter-
aware," i.e. they each call $r->filter_input() exactly once, before they
start printing to STDOUT. There should be almost no overhead for doing
this when there's only one element in the chain.
Currently the following public modules are Filter-aware. Please tell me
of others you know about.
Apache::Registry (using Apache::RegistryFilter, included here)
Apache::SSI
Apache::ASP
HTML::Mason
Apache::SimpleReplace
METHODS
This module doesn't create an Apache handler class of its own - rather,
it adds some methods to the Apache:: class. Thus, it's really a mix-in
package that just adds functionality to the $r request object.
* $r->filter_input()
This method will give you a filehandle that contains either the file
requested by the user ($r->filename), or the output of a previous
filter. If called in a scalar context, that filehandle is all you'll
get back. If called in a list context, you'll also get an Apache
status code (OK, NOT_FOUND, or FORBIDDEN) that tells you whether $r-
>filename was successfully found and opened. If it was not, the
filehandle returned will be undef.
If for some reason you have already opened the filehandle you'll
want to read from, call `$r->filter_input(handle='$handle)>, and
`filter_input()' won't try to open any files. It will pass your
handle back to you.
* $r->changed_since($time)
Returns true or false based on whether the current input seems like
it has changed since `$time'. Currently the criteria to figure this
out is this: if the file pointed to by `$r->finfo' hasn't changed
since the time given, and if all previous filters in the chain are
deterministic (see below), then we return false. Otherwise we return
true.
A caution: always call the `changed_since()' and `deterministic()'
methods AFTER the `filter_input()' method. This is because
Apache::Filter uses a crude counting method to figure out which
handler in the chain is currently executing, and calling these
routines out of order messes up the counting.
* $r->deterministic(1|0);
As of version 0.07, the concept of a "deterministic" filter is
supported. A deterministic filter is one whose output is entirely
determined by the contents of its input file (whether the $r-
>filename file or the output of another filter), and doesn't depend
at all on outside factors. For example, a filter that translates all
its output to upper-case is deterministic, but a filter that adds a
date stamp to a page, or looks things up in a database which may
vary over time, is not.
Why is this a big deal? Let's say you have the following setup:
SetHandler perl-script
PerlSetVar Filter On
PerlHandler Apache::FormatNumbers Apache::DoBigCalculation
# The above are fake modules, you get the idea
Suppose the FormatNumbers module is deterministic, and the
DoBigCalculation module takes a long time to run. The
DoBigCalculation module can now cache its results, so that when an
input file is unchanged on disk, its results will remain known when
passed through the FormatNumbers module, and the DoBigCalculation
module will be able to used cached results from a previous run.
The guts of the modules would look something like this:
sub Apache::FormatNumbers::handler {
my $r = shift;
$r->content_type("text/html");
my ($fh, $status) = $r->filter_input();
return $status unless $status == OK;
$r->deterministic(1); # Set to true; default is false
# ... do some formatting, print to STDOUT
return OK;
}
sub Apache::DoBigCalculation::handler {
my $r = shift;
$r->content_type("text/html");
my ($fh, $status) = $r->filter_input();
return $status unless $status == OK;
# This module implements a caching scheme by using the
# %cache_time and %cache_content hashes.
my $time = $cache_time{$r->filename};
my $output;
if ($r->changed_since($time)) {
# Read from <$fh>, perform a big calculation on it, and print to STDOUT
} else {
print $cache_content{$r->filename};
}
return OK;
}
A caution: always call the `changed_since()' and `deterministic()'
methods AFTER the `filter_input()' method. This is because
Apache::Filter uses a crude counting method to figure out which
handler in the chain is currently executing, and calling these
routines out of order messes up the counting.
HEADERS
In order to make a decent web page, each of the filters shouldn't call
$r->send_http_header() or you'll get lots of headers all over your page.
This is so obvious that the previous sentence should be a lot shorter.
So the current solution is to have _none_ of the filters send the
headers, and this module will send them for you when the last filter
calls $r->filter_input(). You should still set up the content-type
(using $r->content_type), and any other headers you want to send, before
calling $r->filter_input(). filter_input will simply call $r-
>send_http_header() with no arguments to send whatever headers you have
set.
One downside of this is that all the filters in the stack will probably
call $r->content_type, most of them for no reason, but say la vee. If
anyone's got better ideas, don't hold them back.
NOTES
You'll notice in the SYNOPSIS that I say `"PerlSetVar Filter On"'. That
information isn't actually used by this module, it's used by modules
which are themselves filters (like Apache::SSI). I hereby suggest that
filtering modules use this parameter, using it as the switch to detect
whether they should call $r->filter_input.
VERY IMPORTANT: if one handler in a stacked handler chain uses
`Apache::Filter', then THEY ALL MUST USE IT. This means they all must
call $r->filter_input exactly once. Otherwise `Apache::Filter' couldn't
capture the output of the handlers properly, and it wouldn't know when
to release the output to the browser.
The output of each filter (except the last) is accumulated in memory
before it's passed to the next filter, so memory requirements are large
for large pages. Apache::OutputChain only needs to keep one item from
print()'s argument list in memory at a time, so it doesn't have this
problem, but there are others (each chunk is filtered independently, so
content spanning several chunks won't be properly parsed). In future
versions I might find a way around this, or cache large pages to disk so
memory requirements don't get out of hand. We'll see whether it's a
problem.
A couple examples of filters are provided with this distribution in the
t/ subdirectory: UC.pm converts all its input to upper-case, and
Reverse.pm prints the lines of its input reversed.
Finally, a caveat: in version 0.09 I started explicitly setting the
Content-Length to undef inside $r->filter_input. This prevents early
filters from incorrectly setting the content length, which will almost
certainly be wrong if there are any filters after it. This means that if
you write any filters which set the content length, they should do it
after the $r->filter_input call.
TO DO
Add a buffered mode to the final output, so that we can send a proper
Content-Length header. [gozer@hbesoftware.com (Philippe M. Chiasson)]
BUGS
This uses some funny stuff to figure out when the currently executing
handler is the last handler in the chain. As a result, code that
manipulates the handler list at runtime (using push_handlers and the
like) might produce mayhem. Poke around a bit in the code before you try
anything. Let me know if you have a better idea.
As of 0.07, Apache::Filter will automatically return DECLINED when $r-
>filename points to a directory. This is just because in most cases this
is what you want to do (so that mod_dir can take care of the request),
and because figuring out the "right" way to handle directories seems
pretty tough - the right way would allow a directory indexing handler to
be a filter, which isn't possible now. Also, you can't properly pass
control to a non-mod_perl indexer like mod_autoindex. Suggestions are
welcome.
I haven't considered what will happen if you use this and you haven't
turned on PERL_STACKED_HANDLERS.
AUTHOR
Ken Williams (ken@forum.swarthmore.edu)
COPYRIGHT
Copyright 1998 Ken Williams. All rights reserved.
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
SEE ALSO
perl(1).