NAME
    CHI - Unified cache handling interface

VERSION
    version 0.47

SYNOPSIS
        use CHI;

        # Choose a standard driver
        #
        my $cache = CHI->new( driver => 'Memory', global => 1 );
        my $cache = CHI->new( driver => 'RawMemory', global => 1 );
        my $cache = CHI->new( driver => 'File',
            root_dir => '/path/to/root'
        );
        my $cache = CHI->new( driver => 'FastMmap',
            root_dir   => '/path/to/root',
            cache_size => '1k'
        );
        my $cache = CHI->new( driver  => 'Memcached::libmemcached',
            servers => [ "10.0.0.15:11211", "10.0.0.15:11212" ],
            l1_cache => { driver => 'FastMmap', root_dir => '/path/to/root' }
        );
        my $cache = CHI->new( driver => 'DBI',
            dbh => $dbh
        );
        my $cache = CHI->new( driver => 'BerkeleyDB',
            root_dir => '/path/to/root'
        );

        # Create your own driver
        # 
        my $cache = CHI->new( driver_class => 'My::Special::Driver', ... );

        # Cache operations
        #
        my $customer = $cache->get($name);
        if ( !defined $customer ) {
            $customer = get_customer_from_db($name);
            $cache->set( $name, $customer, "10 minutes" );
        }
        my $customer2 = $cache->compute($name2, "10 minutes", sub {
            get_customer_from_db($name2)
        });
        $cache->remove($name);

DESCRIPTION
    CHI provides a unified caching API, designed to assist a developer in
    persisting data for a specified period of time.

    The CHI interface is implemented by driver classes that support
    fetching, storing and clearing of data. Driver classes exist or will
    exist for the gamut of storage backends available to Perl, such as
    memory, plain files, memory mapped files, memcached, and DBI.

    CHI is intended as an evolution of DeWitt Clinton's Cache::Cache
    package, adhering to the basic Cache API but adding new features and
    addressing limitations in the Cache::Cache implementation.

FEATURES
    *   Easy to create new drivers

    *   Uniform support for namespaces

    *   Automatic serialization of keys and values

    *   Multilevel caches

    *   Probabilistic expiration and busy locks, to reduce cache miss
        stampedes

    *   Optional logging and statistics collection of cache activity

AVAILABILITY OF DRIVERS
    The following drivers are currently available as part of this
    distribution:

    *   CHI::Driver::Memory - In-process memory based cache

    *   CHI::Driver::RawMemory - In-process memory based cache that stores
        references directly instead of deep-copying

    *   CHI::Driver::File - File-based cache using one file per entry in a
        multi-level directory structure

    *   CHI::Driver::FastMmap - Shared memory interprocess cache via mmap'ed
        files

    *   CHI::Driver::Null - Dummy cache in which nothing is stored

    *   CHI::Driver::CacheCache - CHI wrapper for Cache::Cache

    The following drivers are currently available as separate CPAN
    distributions:

    *   CHI::Driver::Memcached - Distributed memory-based cache (works with
        Cache::Memcached, Cache::Memcached::Fast, and
        Cache::Memcached::libmemcached)

    *   CHI::Driver::DBI - Cache in any DBI-supported database

    *   CHI::Driver::BerkeleyDB - Cache in BerkeleyDB files

    *   CHI::Driver::Redis - Cache in Redis

    This list is likely incomplete. A complete set of drivers can be found
    on CPAN by searching for "CHI::Driver".

RELATION TO OTHER MODULES
  Cache::Cache
    CHI is intended as an evolution of DeWitt Clinton's Cache::Cache
    package. It starts with the same basic API (which has proven durable
    over time) but addresses some implementation shortcomings that cannot be
    fixed in Cache::Cache due to backward compatibility concerns. In
    particular:

    Performance
        Some of Cache::Cache's subclasses (e.g. Cache::FileCache) have been
        justifiably criticized as inefficient. CHI has been designed from
        the ground up with performance in mind, both in terms of general
        overhead and in the built-in driver classes. Method calls are kept
        to a minimum, data is only serialized when necessary, and metadata
        such as expiration time is stored in packed binary format alongside
        the data.

        As an example, using Rob Mueller's cacheperl benchmarks, CHI's file
        driver runs 3 to 4 times faster than Cache::FileCache.

    Ease of subclassing
        New Cache::Cache subclasses can be tedious to create, due to a lack
        of code refactoring, the use of non-OO package subroutines, and the
        separation of "cache" and "backend" classes. With CHI, the goal is
        to make the creation of new drivers as easy as possible, roughly the
        same as writing a TIE interface to your data store. Concerns like
        serialization and expiration options are handled by the driver base
        class so that individual drivers don't have to worry about them.

    Increased compatibility with cache implementations
        Probably because of the reasons above, Cache::Cache subclasses were
        never created for some of the most popular caches available on CPAN,
        e.g. Cache::FastMmap and Cache::Memcached. CHI's goal is to be able
        to support these and other caches with a minimum performance
        overhead and minimum of glue code required.

  Cache
    The Cache distribution is another redesign and implementation of Cache,
    created by Chris Leishman in 2003. Like CHI, it improves performance and
    reduces the barrier to implementing new cache drivers. It breaks with
    the Cache::Cache interface in a few ways that I considered
    non-negotiable - for example, get/set do not serialize data, and
    namespaces are an optional feature that drivers may decide not to
    implement.

  Cache::Memcached, Cache::FastMmap, etc.
    CPAN sports a variety of full-featured standalone cache modules
    representing particular backends. CHI does not reinvent these but simply
    wraps them with an appropriate driver. For example,
    CHI::Driver::Memcached and CHI::Driver::FastMmap are thin layers around
    Cache::Memcached and Cache::FastMmap.

    Of course, because these modules already work on their own, there will
    be some overlap. Cache::FastMmap, for example, already has code to
    serialize data and handle expiration times. Here's how CHI resolves
    these overlaps.

    Serialization
        CHI handles its own serialization, passing a flat binary string to
        the underlying cache backend.

    Expiration
        CHI packs expiration times (as well as other metadata) inside the
        binary string passed to the underlying cache backend. The backend is
        unaware of these values; from its point of view the item has no
        expiration time. Among other things, this means that you can use CHI
        to examine expired items (e.g. with $cache->get_object) even if this
        is not supported natively by the backend.

        At some point CHI will provide the option of explicitly notifying
        the backend of the expiration time as well. This might allow the
        backend to do better storage management, etc., but would prevent CHI
        from examining expired items.

    Naturally, using CHI's FastMmap or Memcached driver will never be as
    time or storage efficient as simply using Cache::FastMmap or
    Cache::Memcached. In terms of performance, we've attempted to make the
    overhead as small as possible, on the order of 5% per get or set
    (benchmarks coming soon). In terms of storage size, CHI adds about 16
    bytes of metadata overhead to each item. How much this matters obviously
    depends on the typical size of items in your cache.

SUPPORT AND DOCUMENTATION
    Questions and feedback are welcome, and should be directed to the
    perl-cache mailing list:

        http://groups.google.com/group/perl-cache-discuss

    Bugs and feature requests will be tracked at RT:

        http://rt.cpan.org/NoAuth/Bugs.html?Dist=CHI
        bug-chi@rt.cpan.org

    The latest source code can be browsed and fetched at:

        http://github.com/jonswar/perl-chi/tree/master
        git clone git://github.com/jonswar/perl-chi.git

TODO
    *   Perform cache benchmarks comparing both CHI and non-CHI cache
        implementations

    *   Release BerkeleyDB drivers as separate CPAN distributions

    *   Add docs comparing various strategies for reducing miss stampedes
        and cost of recomputes

    *   Add expires_next syntax (e.g. expires_next => 'hour')

    *   Support automatic serialization and escaping of keys

    *   Create XS versions of main functions in Driver.pm (e.g. get, set)

ACKNOWLEDGMENTS
    Thanks to Dewitt Clinton for the original Cache::Cache, to Rob Mueller
    for the Perl cache benchmarks, and to Perrin Harkins for the discussions
    that got this going.

    CHI was originally designed and developed for the Digital Media group of
    the Hearst Corporation, a diversified media company based in New York
    City. Many thanks to Hearst management for agreeing to this open source
    release.

SEE ALSO
    Cache::Cache

AUTHOR
    Jonathan Swartz <swartz@pobox.com>

COPYRIGHT AND LICENSE
    This software is copyright (c) 2011 by Jonathan Swartz.

    This is free software; you can redistribute it and/or modify it under
    the same terms as the Perl 5 programming language system itself.

