Cache::File

NAME

Cache::File - Filesystem based implementation of the Cache interface

SYNOPSIS

  use Cache::File;

  my $cache = Cache::File->new( cache_root => '/tmp/mycache',
                                default_expires => '600 sec' );

See Cache for the usage synopsis.

DESCRIPTION

The Cache::File class implements the Cache interface. This cache stores data in the filesystem so that it can be shared between processes and persists between process invocations.

CONSTRUCTOR

  my $cache = Cache::File->new( %options )

The constructor takes cache properties as named arguments, for example:

  my $cache = Cache::File->new( cache_root => '/tmp/mycache',
                                lock_level => Cache::File::LOCK_LOCAL(),
                                default_expires => '600 sec' );

Note that you MUST provide a cache_root property.

See 'PROPERTIES' below and in the Cache documentation for a list of all available properties that can be set.

METHODS

See 'Cache' for the API documentation.

PROPERTIES

Cache::File adds the following properties in addition to those discussed in the 'Cache' documentation.

cache_root

Used to specify the location of the cache store directory. All methods will work ONLY data stored within this directory. This parameter is REQUIRED when creating a Cache::File instance.

 my $ns = $c->cache_root();

cache_depth

The number of subdirectories deep to store cache entires. This should be large enough that no cache directory has more than a few hundred object. Defaults to 2 unless explicitly set.

 my $depth = $c->cache_depth();

cache_umask

Specifies the umask to use when creating entries in the cache directory. By default the umask is '077', indicating that only the same user may access the cache files.

 my $umask = $c->cache_umask();

lock_level

Specify the level of locking to be used. There are three different levels available:

Cache::File::LOCK_NONE(): No locking is performed. Useful when you can guarantee only one process will be accessing the cache at a time.
Cache::File::LOCK_LOCAL(): Locking is performed, but it is not suitable for use over NFS filesystems. However it is more efficient.
Cache::File::LOCK_NFS(): Locking is performed in a way that is suitable for use on NFS filesystems.

 my $level = $c->cache_lock_level();

CAVEATS

There are a couple of caveats in the current implementation of Cache::File. None of these will present a problem in using the class, it's more of a TODO list of things that could be done better.

external cache modification (and re-syncronization)

Cache::File maintains indexes of entries in the cache, including the number of entries and the total size. Currently there is no process of checking that the count or size are in syncronization with the actual data on disk, and thus any modifications to the cache store by another program (eg. a user shell) will result in an inconsitency in the index. A better process would be for Cache::File to resyncronize at an appropriate time (eg whenever the size or count is initially requested - this would only need happen once per instance). This resyncronization would involve calculating the total size and count as well as checking that entries in the index accurately reflect what is on the disk (and removing any entries that have dissapeared or adding any new ones).

index efficiency

Currently Berkeley DB's are used for indexes of expiry time, last use and entry age. They use the BTREE variant in order to implement a heap (see Cache::File::Heap). This is probably not the most efficient format and having 3 separate index files adds overhead. These are also cross-referenced with a fourth index file that uses a normal hash db and contains all these time stamps (frozen together with the validity object to a single scalar via Storable) indexed by key. Needless to say, all this could be done more efficiently - probably by using a single index in a custom format.

locking efficiency

Currently LOCK_LOCAL is not implemented (if uses the same code as LOCK_NFS).

There are two points of locking in Cache::File, index locking and entry locking. The index locking is always exclusive and the lock is required briefly during most operations. The entry locking is either shared or exclusive and is also required during most operations. When locking is enabled, File::NFSLock is used to provide the locking for both situations. This is not overly efficient, especially as the entry lock is only ever grabbed whilst the index lock is held.

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