NAME

Tie::Cache - LRU Cache in Memory

SYNOPSIS

use Tie::Cache;
tie %cache, 'Tie::Cache', 100, {Debug => 1};   

# 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 PII 300, writes to the hash were done at a rate of at least 3000 per second.

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

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 old data is removed from the cache, often called write through.

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"; }

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

NOTES

Many thanks to Tom Hukins who provided me insight and motivation for finishing this module.

AUTHOR

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

COPYRIGHT

Copyright (c) 1998 Joshua Chamas. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.