NAME
    Tie::Cache - LRU Cache in Memory

SYNOPSIS
     use Tie::Cache;
     tie %cache, 'Tie::Cache', 100, { Debug => 1 };   
     tie %cache2, 'Tie::Cache', { MaxCount => 100, MaxBytes => 50000 };
     tie %cache3, 'Tie::Cache', 100, { Debug => 1 , WriteSync => 0};   

     # Options ##################################################################
     #
     # Debug =>      0 - DEFAULT, no debugging output
     #               1 - prints cache statistics upon destroying
     #               2 - prints detailed debugging info
     #
     # MaxCount =>   Maximum entries in cache.
     #
     # MaxBytes =>   Maximum bytes taken in memory for cache based on approximate 
     #               size of total cache structure in memory
     #
     #               There is approximately 240 bytes used per key/value pair in the cache for 
     #               the cache data structures, so a cache of 5000 entries would take
     #               at approximately 1.2M plus the size of the data being cached.
     #
     # MaxSize  =>   Maximum size of each cache entry. Larger entries are not cached.
     #                   This helps prevent much of the cache being flushed when 
     #                   you set an exceptionally large entry.  Defaults to MaxBytes/10
     #
     # WriteSync =>  1 - DEFAULT, write() when data is dirtied for 
     #                   TRUE CACHE (see below)
     #               0 - write() dirty data as late as possible, when leaving 
     #                   cache, or when cache is being DESTROY'd
     #
     ############################################################################

     # cache supports normal tied hash functions
     $cache{1} = 2;       # STORE
     print "$cache{1}\n"; # FETCH

     # FIRSTKEY, NEXTKEY
     while(($k, $v) = each %cache) { print "$k: $v\n"; } 
 
     delete $cache{1};    # DELETE
     %cache = ();         # CLEAR

DESCRIPTION
    This module implements a least recently used (LRU) cache in memory
    through a tie interface. Any time data is stored in the tied hash, that
    key/value pair has an entry time associated with it, and as the cache
    fills up, those members of the cache that are the oldest are removed to
    make room for new entries.

    So, the cache only "remembers" the last written entries, up to the size
    of the cache. This can be especially useful if you access great amounts
    of data, but only access a minority of the data a majority of the time.

    The implementation is a hash, for quick lookups, overlaying a doubly
    linked list for quick insertion and deletion. On a WinNT PII 300, writes
    to the hash were done at a rate 3100 per second, and reads from the hash
    at 6300 per second. Work has been done to optimize refreshing cache
    entries that are frequently read from, code like $cache{entry}, which
    moves the entry to the end of the linked list internally.

INSTALLATION
    Tie::Cache installs easily using the make or nmake commands as shown
    below. Otherwise, just copy Cache.pm to $PERLLIB/site/Tie

            > perl Makefile.PL
            > make
            > make test 
            > make install

            * use nmake for win32
            ** you can also just copy Cache.pm to $perllib/Tie

BENCMARKS
    There is another simpler LRU cache implementation in CPAN,
    Tie::Cache::LRU, which has the same basic size limiting functionality,
    and for this functionality, the exact same interface.

    Through healthy competition, Michael G Schwern got Tie::Cache::LRU
    mostly faster than Tie::Cache on reads & writes:

     Cache Size 5000       Tie::Cache 0.17  Tie::Cache::LRU 0.21
     10000 Writes             1.55 CPU sec          1.10 CPU sec
     40000 Reads              1.82 CPU sec          1.58 CPU sec
     10000 Deletes            0.55 CPU sec          0.59 CPU sec

    Unless you are using TRUE CACHE or MaxBytes functionality, using
    Tie::Cache::LRU should be an easy replacement for Tie::Cache.

TRUE CACHE
    To use class as a true cache, which acts as the sole interface for some
    data set, subclass the real cache off Tie::Cache, with @ISA = qw(
    'Tie::Cache' ) notation. Then override the read() method for behavior
    when there is a cache miss, and the write() method for behavior when the
    cache's data changes.

    When WriteSync is 1 or TRUE (DEFAULT), write() is called immediately
    when data in the cache is modified. If set to 0, data that has been
    modified in the cache gets written out when the entries are deleted or
    during the DESTROY phase of the cache object, usually at the end of a
    script.

    To have the dirty data write() periodically while WriteSync is set to 0,
    there is a flush() cache API call that will flush the dirty writes in
    this way. Just call the flush() API like:

      my $write_flush_count = tied(%cache)->flush();

    The flush() API was added in the .17 release thanks to Rob Bloodgood.

TRUE CACHE EXAMPLE
     use Tie::Cache;

     # personalize the Tie::Cache object, by inheriting from it
     package My::Cache;
     @ISA = qw(Tie::Cache);

     # override the read() and write() member functions
     # these tell the cache what to do with a cache miss or flush
     sub read { 
        my($self, $key) = @_; 
        print "cache miss for $key, read() data\n";
        rand() * $key; 
     }
     sub write { 
        my($self, $key, $value) = @_;
        print "flushing [$key, $value] from cache, write() data\n";
     }

     my $cache_size   = $ARGV[0] || 2;
     my $num_to_cache = $ARGV[1] || 4;   
     my $Debug = $ARGV[2] || 1;

     tie %cache, 'My::Cache', $cache_size, {Debug => $Debug};   

     # load the cache with new data, each through its contents,
     # and then reload in reverse order.
     for(1..$num_to_cache) { print "read data $_: $cache{$_}\n" }
     while(my($k, $v) = each %cache) { print "each data $k: $v\n"; }
     for(my $i=$num_to_cache; $i>0; $i--) { print "read data $i: $cache{$i}\n"; }

     # flush writes now, trivial use since will happen in DESTROY() anyway
     tied(%cache)->flush(); 

     # clear cache in 2 ways, write will flush out to disk
     %cache = ();
     undef %cache;

NOTES
    Many thanks to all those who helped me make this module a reality,
    including:

            :) Tom Hukins who provided me insight and motivation for
               finishing this module.
            :) Jamie McCarthy, for trying to make Tie::Cache be all
               that it can be.
            :) Rob Fugina who knows how to "TRULY CACHE".
            :) Rob Bloodgood, for the TRUE CACHE flush() API

AUTHOR
    Please send any questions or comments to Joshua Chamas at
    chamas@alumni.stanford.org

COPYRIGHT
    Copyright (c) 1999-2002 Joshua Chamas, Chamas Enterprises Inc. Sponsored
    by development on NodeWorks http://www.nodeworks.com

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

