man CGI::Compress::Gzip (3): CGI with automatically compressed output

LICENSE

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

SYNOPSIS

use CGI::Compress::Gzip;

my $cgi = new CGI::Compress::Gzip;
print $cgi->header();
print "<html> ...";

See the CAVEATS section below!

DESCRIPTION

CGI::Compress::Gzip extends the CGI module to auto-detect whether the client browser wants compressed output and, if so and if the script chooses HTML output, apply gzip compression on any content header for STDOUT. This module is intended to be a drop-in replacement for CGI.pm.

Apache mod_perl users may wish to consider the Apache::Compress or Apache::GzipChain modules, which allow more transparent output compression than this module can provide. However, as of this writing those modules are more aggressive about compressing, regardless of Content-Type.

Headers

At the time that a header is requested, CGI::Compress::Gzip checks the HTTP_ACCEPT_ENCODING environment variable (passed by Apache). If this variable includes the flag ``gzip'' and the outgoing mime-type is ``text/*'', then gzipped output is preferred. [the default mime-type selection of text/* can be changed by subclassing --- see below] The header is altered to add the ``Content-Encoding: gzip'' flag which indicates that compression is turned on.

Naturally, it is crucial that the CGI application output nothing before the header is printed. If this is violated, things will go badly.

Compression

When the header is created, this module sets up a new filehandle to accept data. STDOUT is redirected through that filehandle. The new filehandle passes data verbatim until it detects the end of the CGI header. At that time, it switches over to Gzip output for the remainder of the CGI run.

Note that the Zlib library on which this code is ultimately based requires a fileno for the output filehandle. Where the output filehandle is faked (i.e. in mod_perl), we instead use in-memory compression. This is more wasteful of RAM, but it is the only solution I've found (and it is one shared by the Apache::* compression modules).

Debugging note: if you set $CGI::Compress::Gzip::global_give_reason to a true value, then this module will add an HTTP header entry called X-non-gzip-reason with an explanation of why it chose not to gzip the output stream.

Buffering

The Zlib library introduces latencies. In some cases, this module may delay output until the CGI object is garbage collected, presumably at the end of the program. This buffering can be detrimental to long-lived programs which are supposed to have incremental output, causing browser timeouts. To compensate, compression is automatically disabled when autoflush (i.e. the $| variable) is set to true. Future versions may try to enable autoflushing on the Zlib filehandles, if possible [Help wanted].

CLASS METHODS

$pkg->new([CGI ARGS])

Create a new object. This resets the environment before creating a CGI.pm object. This should not be called more than once per script run! All arguments are passed to the parent class.

$pkg->useCompression($boolean)

$self->useCompression($boolean)

This can be used as a class method or an instance method. The former is included for backward compatibility, and is NOT recommended. As a class method, this changes the default value. As an instance method it affects only the specified instance.

Turn compression on/off for the target. If turned on, compression will be used only if the prerequisite compression libraries are available and if the client browser requests compression.

Defaults to on.

INSTANCE METHODS

$self->useFileHandle($filehandle)

Manually set the output filehandle. Because of limitations of Zlib, this MUST be a real filehandle (with valid results from fileno()) and not a pseudo filehandle like IO::String.

If this is not set, STDOUT is used.

$self->isCompressibleType($content_type)

Given a MIME type (with possible charset attached), return a boolean indicating if this media type is a good candidate for compression. This implementation is simply:

    return $type =~ /^text\//;

Subclasses may wish to override this method to apply different criteria.

$self->header([HEADER ARGS])

Overrides the "header()" method in CGI.

Return a CGI header with the compression flags set properly. Returns an empty string is a header has already been printed.

This method engages the Gzip output by fiddling with the default output filehandle. All subsequent output via usual Perl print() will be automatically gzipped except for this header (which must go out as plain text).

Any arguments will be passed on to CGI::header. This method should NOT be called if you don't want your header or STDOUT to be fiddled with.

$self->DESTROY()

Override the CGI destructor so we can close the Gzip output stream, if there is one open.

CAVEATS

Apache::Registry

Under Apache::Registry, global variables may not go out of scope in time. This may causes timing bugs, since this module makes use of the DESTROY() method. To avoid this issue, make sure your CGI object is stored in a scoped variable.

   # BROKEN CODE
   use CGI::Compress::Gzip;
   $q = CGI::Compress::Gzip->new;
   print $q->header;
   print "Hello, world\n";
   
   # WORKAROUND CODE
   use CGI::Compress::Gzip;
   do {
     my $q = CGI::Compress::Gzip->new;
     print $q->header;
     print "Hello, world\n";
   }

Filehandles

This module works by changing the default filehandle. It does not change STDOUT at all. As a consequence, your programs should call "print" without a filehandle argument.

   # BROKEN CODE
   use CGI::Compress::Gzip;
   my $q = CGI::Compress::Gzip->new;
   print STDOUT $q->header;
   print STDOUT "Hello, world\n";
   
   # WORKAROUND CODE
   use CGI::Compress::Gzip;
   my $q = CGI::Compress::Gzip->new;
   print $q->header;
   print "Hello, world\n";

Future versions may steal away STDOUT and replace it with the compression filehandle, but that seemed too risky for this version.

Header Munging

When sending compressed output, the HTTP headers must remain uncompressed. So, this module goes to great effort to keep the headers and body separate. That has led to CGI::header() emulation code that is a little brittle. Most potential problems arise because STDOUT gets tweaked as soon as header() is called.

If you use the CGI.pm header() API as specified in CGI.pm, then all should go well. But if you do anything unusual, this module may break. For example:

   # BROKEN CODE
   use CGI::Compress::Gzip;
   my $q = CGI::Compress::Gzip->new;
   print "Set-Cookie: foo=bar\n" . $q->header;
   print "Hello, world\n";
   # WORKAROUND 1 (preferred)
   use CGI::Compress::Gzip;
   my $q = CGI::Compress::Gzip->new;
   print $q->header("-Set_Cookie" => "foo=bar");
   print "Hello, world\n";
   # WORKAROUND 2
   use CGI::Compress::Gzip;
   my $q = CGI::Compress::Gzip->new;
   print "Set-Cookie: foo=bar\n";
   print $q->header;
   print "Hello, world\n";

Future versions could try to parse the header to look for its end rather than insisting that the printed version match the version returned by header(). Patches would be very welcome.

AUTHOR

Chris Dolan

This module was originally developed by me at Clotho Advanced Media Inc. Now I maintain it in my spare time.

ACKNOWLEDGMENTS

Clotho greatly appreciates the assistance and feedback the community has extended to help refine this module.

Thanks to Rhesa Rozendaal who noticed the -Type omission in v0.17.

Thanks to Laga Mahesa who did some Windows testing and experimentation.

Thanks to Slaven Rezic who 1) found several header handling bugs, 2) discovered the Apache::Registry and Filehandle caveats, 3) provided a patch incorporated into v0.17, and 4) persisted with smoke tests that reproduced the envvar problem fixed in v0.23.

Thanks to Jan Willamowius who found a header handling bug.

Thanks to Andreas J. Koenig and brian d foy for module naming advice.

HELP WANTED

If you like this module, please help by testing on Windows or in a "FastCGI" environment, since I have neither available for easy testing.

Personally, I don't use this module much anymore as all of my work is on Catalyst and mod_perl now.