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.

SEE ALSO

       Cache

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 $