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=over 4

=item Notes:

The lifetime defined by C<Expires> depends on accuracy of time settings on client machine.
If the client's local clock is running 1 hour back, the cached copy of
the page will be alive 60 minutes longer on that machine.

C<Apache::Dynagzip> never overwrites C<Expires> header set by earlier handler inside the filter-chain.

=back

=head2 Support for the Server-Side Cache

In order to support the Server-Side Cache
I place a reference to the dynamically generated document to the C<notes()>
when the Server-Side Cache Support is ordered.
The referenced document could be already compressed with
an C<extra light> compression (if an C<extra light> compression is in effect for the current request).

In this case the regular dynamic C<gzip> compression takes place as usual
and the effective C<gzip> compression is supposed to take place within the C<log> stage
of the request processing flow.

You usually should not care about this feature of C<Apache::Dynagzip>
unless you use it in your own chain of handlers for the various phases of the request processing.

=head2 Control over the Proxy Cache.

Control over the (possible) proxy cache is provided through the implementation of C<Vary>
HTTP header.
Within C<Apache::Dynagzip> this header is under the control of few simple rules:

=over 4

=item *

C<Apache::Dynagzip> does never generate this header unless C<gzip> compression is provided.

=item *

The value of C<Accept-Encoding> is always provided for this header, accompanying C<gzip> compression.

=item *

Advanced control over the proxy cache is provided since the version 0.07
with optional extension of Vary HTTP header.
This extension could be placed into your configuration file, using directive

C<PerlSetVar Vary E<lt>valueE<gt>>

Particularly, it might be helpful to indicate the content, which depends on some conditions,
other than just compression features.
For example, when the content is personalized, someone might wish to use
the "*" C<Vary> extension in order to prevent any proxy caching.

When the outgoing content is gzipped, this extension will be appended to the regular C<Vary> header,
like in the following example:

Using the following fragment within the C<httpd.conf>:

  PerlModule Apache::Dynagzip
  <Files ~ "*\.html">
    SetHandler perl-script
    PerlHandler Apache::Dynagzip
    PerlSetVar LightCompression On
    PerlSetVar Vary *
  </Files>

We can observe the client-side log in the form of:

  C05 --> S06 GET /devdoc/Dynagzip/Dynagzip.html HTTP/1.1
  C05 --> S06 Accept: */*
  C05 --> S06 Referer: http://devl4.outlook.net/devdoc/Dynagzip/
  C05 --> S06 Accept-Language: en-us
  C05 --> S06 Accept-Encoding: gzip, deflate
  C05 --> S06 User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows 98)
  C05 --> S06 Host: devl4.outlook.net
  C05 --> S06 Pragma: no-cache
  C05 --> S06 Accept-Charset: ISO-8859-1
  == Body was 0 bytes ==
  
  C05 <-- S06 HTTP/1.1 200 OK
  C05 <-- S06 Date: Sun, 11 Aug 2002 21:28:43 GMT
  C05 <-- S06 Server: Apache/1.3.22 (Unix) Debian GNU/Linux mod_perl/1.26
  C05 <-- S06 X-Module-Sender: Apache::Dynagzip
  C05 <-- S06 Expires: Sunday, 11-August-2002 21:33:43 GMT
  C05 <-- S06 Vary: Accept-Encoding,*
  C05 <-- S06 Transfer-Encoding: chunked
  C05 <-- S06 Content-Type: text/html; charset=iso-8859-1
  C05 <-- S06 Content-Encoding: gzip
  C05 <-- S06 == Incoming Body was 11311 bytes ==
  == Transmission: text gzip chunked ==
  == Chunk Log ==
  a (hex) = 10 (dec)
  1c78 (hex) = 7288 (dec)
  f94 (hex) = 3988 (dec)
  0 (hex) = 0 (dec)
  == Latency = 0.160 seconds, Extra Delay = 0.170 seconds
  == Restored Body was 47510 bytes ==

=item *

Simple form

C<Vary: Accept-Encoding>

is provided as a default for the gzipped content.

=back

=head1 CUSTOMIZATION

C<Apache::Dynagzip> can be used in order

=over 4

=item *

to compress dynamic web content generated in C<Apache::Filter> chain;