NAME

       HTTP::Cache::Transparent - Cache the result of http get-requests persistently.

SYNOPSIS

         use LWP::Simple;
         use HTTP::Cache::Transparent;

         HTTP::Cache::Transparent::init( {
           BasePath => '/tmp/cache',
         } );

         my $data = get( 'http://www.sn.no' );

DESCRIPTION

       An implementation of http get that keeps a local cache of fetched pages to avoid fetching the same data
       from the server if it hasn't been updated. The cache is stored on disk and is thus persistent between
       invocations.

       Uses the http-headers If-Modified-Since and ETag to let the server decide if the version in the cache is
       up-to-date or not.

       The cache is implemented by modifying the LWP::UserAgent class to seamlessly cache the result of all
       requests that can be cached.

INITIALIZING THE CACHE

       HTTP::Cache::Transparent provides an init-method that sets the parameters for the cache and overloads a
       method in LWP::UserAgent to activate the cache.After init has been called, the normal LWP-methods
       (LWP::Simple as well as the more full-fledged LWP::Request methods) should be used as usual.

       init
           Initialize the HTTP cache. Takes a single parameter which is a hashref containing named arguments to
           the object.

             HTTP::Cache::Transparent::init( {

               # Directory to store the cache in.
               BasePath  => "/tmp/cache",

               # How many hours should items be kept in the cache
               # after they were last requested?
               # Default is 8*24.
               MaxAge    => 8*24,

               # Print progress-messages to STDERR.
               # Default is 0.
               Verbose   => 1,

               # If a request is made for a url that has been requested
               # from the server less than NoUpdate seconds ago, the
               # response will be generated from the cache without
               # contacting the server.
               # Default is 0.
               NoUpdate  => 15*60,

               # When a url has been downloaded and the response indicates that
               # has been modified compared to the content in the cache,
               # the ApproveContent callback is called with the HTTP::Response.
               # The callback shall return true if the response shall be used and
               # stored in the cache or false if the response shall be discarded
               # and the response in the cache used instead.
               # This mechanism can be used to work around servers that return errors
               # intermittently. The default is to accept all responses.
               ApproveContent => sub { return $_[0]->is_success },
            } );

           The directory where the cache is stored must be writable. It must also only contain files created by
           HTTP::Cache::Transparent.

       Initializing from use-line
           An alternative way of initializing HTTP::Cache::Transparent is to supply parameters in the use-line.
           This allows you to write

             use HTTP::Cache::Transparent ( BasePath => '/tmp/cache' );

           which is exactly equivalent to

             use HTTP::Cache::Transparent;
             HTTP::Cache::Transparent::init( BasePath => '/tmp/cache' );

           The advantage to using this method is that you can do

             perl -MHTTP::Cache::Transparent=BasePath,/tmp/cache myscript.pl

           or even set the environment variable PERL5OPT

             PERL5OPT=-MHTTP::Cache::Transparent=BasePath,/tmp/cache
             myscript.pl

           and have all the http-requests performed by myscript.pl go through the cache without changing
           myscript.pl

INSPECTING CACHE BEHAVIOR

       The HTTP::Cache::Transparent inserts two special headers in the HTTP::Response object. These can be
       accessed via the HTTP::Response::header()-method.

       X-Cached
           This header is inserted and set to 1 if the response is delivered from the cache instead of from the
           server.

       X-Content-Unchanged
           This header is inserted and set to 1 if the content returned is the same as the content returned the
           last time this url was fetched. This header is always inserted and set to 1 when the response is
           delivered from the cache.

LIMITATIONS

       This module has a number of limitations that you should be aware of before using it.

       -   There is no upper limit to how much diskspace the cache requires. The only limiting mechanism is that
           data for urls that haven't been requested in the last MaxAge hours will be removed from the cache the
           next time the program exits.

       -   Currently, only get-requests that store the result in memory (i.e. do not use the option to have the
           result stored directly in a file or delivered via a callback) is cached. I intend to remove this
           limitation in a future version.

       -   The support for Ranges is a bit primitive. It creates a new object in the cache for each unique
           combination of url and range. This will work ok as long as you always request the same range(s) for a
           url.

       -   The cache doesn't properly check and store all headers in the HTTP request and response. Therefore,
           if you request the same url repeatedly with different sets of headers (cookies, accept-encoding etc),
           and these headers affect the response from the server, the cache may return the wrong response.

CACHE FORMAT

       The cache is stored on disk as one file per cached object. The filename is equal to the md5sum of the url
       and the Range-header if it exists.  The file contains a set of key/value-pairs with metadata (one entry
       per line) followed by a blank line and then the actual data returned by the server.

       The last modified date of the cache file is set to the time when the cache object was last requested by a
       user.

AUTHOR

       Mattias Holmlund, <$firstname -at- $lastname -dot- se> <http://www.holmlund.se/mattias/>

GIT REPOSITORY

       A git repository containing the source for this module can be found via http://git.holmlund.se/

COPYRIGHT AND LICENSE

       Copyright (C) 2004-2007 by Mattias Holmlund

       This library is free software; you can redistribute it and/or modify it under the same terms as Perl
       itself, either Perl version 5.8.4 or, at your option, any later version of Perl 5 you may have available.