NAME

       Cache - the Cache interface

DESCRIPTION

       The Cache modules are designed to assist a developer in persisting data for a specified
       period of time.  Often these modules are used in web applications to store data locally to
       save repeated and redundant expensive calls to remote machines or databases.

       The Cache interface is implemented by derived classes that store cached data in different
       manners (such as as files on a filesystem, or in memory).

USAGE

       To use the Cache system, a cache implementation must be chosen to suit your needs.  The
       most common is Cache::File, which is suitable for sharing data between multiple
       invocations and even between concurrent processes.

       Using a cache is simple.  Here is some very simple sample code for instantiating and using
       a file system based cache.

         use Cache::File;

         my $cache = Cache::File->new( cache_root => '/tmp/cacheroot' );
         my $customer = $cache->get( $name );

         unless ($customer) {
             $customer = get_customer_from_db( $name );
             $cache->set( $name, $customer, '10 minutes' );
         }

         return $customer;

       Of course, far more powerful methods are available for accessing cached data.  Also see
       the TIE INTERFACE below.

METHODS

       my $cache_entry = $c->entry( $key )
           Return a 'Cache::Entry' object for the given key.  This object can then be used to
           manipulate the cache entry in various ways.  The key can be any scalar string that
           will uniquely identify an entry in the cache.

       $c->purge()
           Remove all expired data from the cache.

       $c->clear()
           Remove all entries from the cache - regardless of their expiry time.

       my $num = $c->count()
           Returns the number of entries in the cache.

       my $size = $c->size()
           Returns the size (in bytes) of the cache.

PROPERTIES

       When a cache is constructed these properties can be supplied as options to the new()
       method.

       default_expires
           The current default expiry time for new entries into the cache.  This property can
           also be reset at any time.

            my $time = $c->default_expires();
            $c->set_default_expires( $expiry );

       removal_strategy
           The removal strategy object for the cache.  This is used to remove object from the
           cache in order to maintain the cache size limit.

           When setting the removal strategy in new(), the name of a strategy package or a
           blessed strategy object reference should be provided  (in the former case an object is
           constructed by calling the new() method of the named package).

           The strategies 'Cache::RemovalStrategy::LRU' and 'Cache::RemovalStrategy::FIFO' are
           available by default.

            my $strategy = $c->removal_strategy();

       size_limit
           The size limit for the cache.

            my $limit = $c->size_limit();

       load_callback
           The load callback for the cache.  This may be set to a function that will get called
           anytime a 'get' is issued for data that does not exist in the cache.

            my $limit = $c->load_callback();
            $c->set_load_callback($callback_func);

       validate_callback
           The validate callback for the cache.  This may be set to a function that will get
           called anytime a 'get' is issued for data that does not exist in the cache.

            my $limit = $c->validate_callback();
            $c->set_validate_callback($callback_func);

SHORTCUT METHODS

       These methods all have counterparts in the Cache::Entry package, but are provided here as
       shortcuts.  They all default to just wrappers that do '$c->entry($key)->method_name()'.
       For documentation, please refer to Cache::Entry.

       my $bool = $c->exists( $key )
       $c->set( $key, $data, [ $expiry ] )
       my $data = $c->get( $key )
       my $data = $c->size( $key )
       $c->remove( $key )
       $c->expiry( $key )
       $c->set_expiry( $key, $time )
       $c->handle( $key, [$mode, [$expiry] ] )
       $c->validity( $key )
       $c->set_validity( $key, $data )
       $c->freeze( $key, $data, [ $expiry ] )
       $c->thaw( $key )

TIE INTERFACE

         tie %hash, 'Cache::File', { cache_root => $tempdir };

         $hash{'key'} = 'some data';
         $data = $hash{'key'};

       The Cache classes can be used via the tie interface, as shown in the synopsis.  This
       allows the cache to be accessed via a hash.  All the standard methods for accessing the
       hash are supported , with the exception of the 'keys' or 'each' call.

       The tie interface is especially useful with the load_callback to automatically populate
       the hash.

REMOVAL STRATEGY METHODS

       These methods are only for use internally (by concrete Cache implementations).

       These methods define the interface by which the removal strategy object can manipulate the
       cache (the Cache is the 'context' of the strategy).  By default, methods need to be
       provided to remove the oldest or stalest objects in the cache - thus allowing support for
       the default FIFO and LRU removal strategies.  All derived Cache implementations should
       support these methods and may also introduce additional methods (and additional removal
       strategies to match).

       my $size = $c->remove_oldest()
           Removes the oldest entry in the cache and returns its size.

       my $size = $c->remove_stalest()
           Removes the 'stalest' (least used) object in the cache and returns its size.

       $c->check_size( $size )
           This method isn't actually part of the strategy interface, nor does it need to be
           defined by Cache implementations.  Instead it should be called by implementations
           whenever the size of the cache increases.  It will take care of checking the size
           limit and invoking the removal strategy if required.  The size argument should be the
           new size of the cache.

UTILITY METHODS

       These methods are only for use internally (by concrete Cache implementations).

       my $time = Cache::Canonicalize_Expiration_Time($timespec)
           Converts a timespec as described for Cache::Entry::set_expiry() into a unix time.

SEE ALSO

       Cache::Entry, Cache::File, Cache::RemovalStrategy

DIFFERENCES FROM CACHE::CACHE

       The Cache modules are a total redesign and reimplementation of Cache::Cache and thus not
       directly compatible.  It would be, however, quite possible to write a wrapper module that
       provides an identical interface to Cache::Cache.

       The semantics of use are very similar to Cache::Cache, with the following exceptions:

       The get/set methods DO NOT serialize complex data types.  Use freeze/thaw instead (but
       read the notes in Cache::Entry).
       The get_object / set_object methods are not available, but have been superseded by the
       more flexible entry method and Cache::Entry class.
       There is no concept of 'namespace' in the basic cache interface, although implementations
       (eg. Cache::Memory) may choose to provide them.  For instance, File::Cache does not
       provide this - but different namespaces can be created by varying cache_root.
       In the current Cache implementations purging is done automatically - there is no need to
       explicitly enable auto purge on get/set.  The purging algorithm is no longer implemented
       in the base Cache class, but is left up to the implementations and may thus be implemented
       in the most efficient way for the storage medium.
       Cache::SharedMemory is not yet available.
       Cache::File no longer supports separate masks for entries and directories.  It is not a
       very secure configuration and presents numerous issues for cache consistency and is hence
       depricated.  There is still some work to be done to ensure cache consistency between
       accesses by different users.

AUTHOR

        Chris Leishman <chris@leishman.org>
        Based on work by DeWitt Clinton <dewitt@unto.net>

COPYRIGHT

        Copyright (C) 2003-2006 Chris Leishman.  All Rights Reserved.

       This module is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, either
       expressed or implied. This program is free software; you can redistribute or modify it
       under the same terms as Perl itself.

       $Id: Cache.pm,v 1.7 2006/01/31 15:23:58 caleishm Exp $