Provided by: libchi-perl_0.60-4_all bug

NAME

       CHI - Unified cache handling interface

VERSION

       version 0.60

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 => '+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

CONSTRUCTOR

       To create a new cache object, call "<CHI->new". It takes the common options listed below.
       driver is required; all others are optional.

       Some drivers will take additional constructor options. For example, the File driver takes
       "root_dir" and "depth" options.

       You can configure default options for each new cache object created - see "SUBCLASSING AND
       CONFIGURING CHI".

       Note that "CHI->new" returns an instance of a subclass of CHI::Driver, not "CHI".

       compress_threshold [INT]
           A value in bytes. Automatically compress values larger than this before storing.
           Requires Compress::Zlib to be installed. Defaults to undef, meaning no automatic
           compression. Inspired by the parameter of the same name in Cache::Memcached.

               # Compress values larger than 1MB
               compress_threshold => 1024*1024

       driver [STRING]
           Required. The name of a cache driver, for example "Memory" or "File".  CHI will prefix
           the string with "CHI::Driver::", unless it begins with '+'. e.g.

               driver => 'File';                   # uses CHI::Driver::File
               driver => '+My::CHI::Driver::File'  # uses My::CHI::Driver::File

       expires_in [DURATION], expires_at [INT], expires_variance [FLOAT]
           Provide default values for the corresponding "set" options.

       expires_on_backend [NUM]
           If set to 0 (the default), CHI alone is aware of the expiration time and does not pass
           it along to the backend driver. This allows you to use "get_object" to retrieve
           expired items.

           If set to 1, pass expiration times to backend driver if the driver supports it -- for
           example, CHI::Driver::Memcached and CHI::Driver::CacheCache. This may allow the driver
           to better manage its space and evict items. Note that only simple expiration time will
           be passed along, e.g. not "expires_variance".

           If set to a number greater than 1 (e.g. 1.25), the time until expiration will be
           multiplied by that number before being passed to the backend driver. This gives you a
           customizable window of opportunity to retrieve expired items.

       key_digester [STRING|HASHREF|OBJECT]
           Digest algorithm to use on keys longer than "max_key_length" - e.g. "MD5", "SHA-1", or
           "SHA-256".

           Can be a Digest object, or a string or hashref which will passed to Digest->new(). You
           will need to ensure Digest is installed to use these options.

           Default is "MD5".

       key_serializer [STRING|HASHREF|OBJECT]
           An object to use for serializing keys that are references. See "serializer" above for
           the different ways this can be passed in. The default is to use the JSON backend in
           canonical mode (sorted hash keys).

       label [STRING]
           A label for the cache as a whole, independent of namespace - e.g.  "web-file-cache".
           Used when referring to the cache in logs, statistics, and error messages. By default,
           set to "short_driver_name".

       l1_cache [HASHREF]
           Add an L1 cache as a subcache. See "SUBCACHES".

       max_key_length [INT]
           Keys over this size will be digested. The default is driver-specific;
           CHI::Driver::File, for example, defaults this to 240 due to file system limits. For
           most drivers there is no maximum.

       mirror_cache [HASHREF]
           Add an mirror cache as a subcache. See "SUBCACHES".

       namespace [STRING]
           Identifies a namespace that all cache entries for this object will be in. This allows
           easy separation of multiple, distinct caches without worrying about key collision.

           Suggestions for easy namespace selection:

           ·   In a class, use the class name:

                   my $cache = CHI->new(namespace => __PACKAGE__, ...);

           ·   In a script, use the script's absolute path name:

                   use Cwd qw(realpath);
                   my $cache = CHI->new(namespace => realpath($0), ...);

           ·   In a web template, use the template name. For example, in Mason, $m->cache will
               set the namespace to the current component path.

           Defaults to 'Default' if not specified.

       on_get_error [STRING|CODEREF]
       on_set_error [STRING|CODEREF]
           How to handle runtime errors occurring during cache gets and cache sets, which may or
           may not be considered fatal in your application. Options are:

           ·   log (the default) - log an error, or ignore if no logger is set - see "LOGGING"

           ·   ignore - do nothing

           ·   warn - call warn() with an appropriate message

           ·   die - call die() with an appropriate message

           ·   coderef - call this code reference with three arguments: an appropriate message,
               the key, and the original raw error message

       serializer [STRING|HASHREF|OBJECT]
           An object to use for serializing data before storing it in the cache, and
           deserializing data after retrieving it from the cache. Only references will be
           serialized; plain scalars will be placed in the cache as-is.

           If this is a string, a Data::Serializer object will be created, with the string passed
           as the 'serializer' option and raw=1. Common options include 'Storable',
           'Data::Dumper', and 'YAML'. If this is a hashref, Data::Serializer will be called with
           the hash. You will need to ensure Data::Serializer is installed to use these options.

           Otherwise, this must be a Data::Serializer object or another object that implements
           serialize() and deserialize().

           e.g.

               # Serialize using raw Data::Dumper
               my $cache = CHI->new(serializer => 'Data::Dumper');

               # Serialize using Data::Dumper, compressed and (per Data::Serializer defaults) hex-encoded
               my $cache = CHI->new(serializer => { serializer => 'Data::Dumper', compress => 1 });

               # Serialize using custom object
               my $cache = CHI->new(serializer => My::Custom::Serializer->new())

           The default is to use raw Storable.

       traits [LISTREF]
           List of one or more roles to apply to the "CHI::Driver" class that is constructed. The
           roles will automatically be prefixed with "CHI::Driver::Role::" unless preceded with a
           '+'. e.g.

               traits => ['StoresAccessedAt', '+My::CHI::Driver::Role']

INSTANCE METHODS

       The following methods can be called on any cache handle returned from CHI->new(). They are
       implemented in the CHI::Driver package.

   Getting and setting
       get( $key, [option => value, ...] )
           Returns the data associated with $key. If $key does not exist or has expired, returns
           undef. Expired items are not automatically removed and may be examined with
           "get_object" or "get_expires_at".

           $key may be followed by one or more name/value parameters:

           expire_if [CODEREF]
               If $key exists and has not expired, call code reference with the CHI::CacheObject
               and CHI::Driver as the parameters. If code returns a true value, "get" returns
               undef as if the item were expired. For example, to treat the cache as expired if
               $file has changed since the value was computed:

                   $cache->get('foo', expire_if => sub { $_[0]->created_at < (stat($file))[9] });

           busy_lock [DURATION]
               If the value has expired, the get will still return undef, but the expiration time
               of the cache entry will be set to the current time plus the specified duration.
               This is used to prevent multiple processes from recomputing the same expensive
               value simultaneously. The problem with this technique is that it doubles the
               number of writes performed - see "expires_variance" for another technique.

           obj_ref [SCALARREF]
               If the item exists in cache (even if expired), place the CHI::CacheObject object
               in the provided SCALARREF.

       set( $key, $data, [$expires_in | "now" | "never" | options] )
           Associates $data with $key in the cache, overwriting any existing entry.  Returns
           $data.

           The third argument to "set" is optional, and may be either a scalar or a hash
           reference. If it is a scalar, it may be the string "now", the string "never", or else
           a duration treated as an expires_in value described below. If it is a hash reference,
           it may contain one or more of the following options. Most of these options can be
           provided with defaults in the cache constructor.

           expires_in [DURATION]
               Amount of time from now until this data expires. DURATION may be an integer number
               of seconds or a duration expression.

           expires_at [INT]
               The epoch time at which the data expires.

           expires_variance [FLOAT]
               Controls the variable expiration feature, which allows items to expire a little
               earlier than the stated expiration time to help prevent cache miss stampedes.

               Value is between 0.0 and 1.0, with 0.0 meaning that items expire exactly when
               specified (feature is disabled), and 1.0 meaning that items might expire anytime
               from now until the stated expiration time. The default is 0.0. A setting of 0.10
               to 0.25 would introduce a small amount of variation without interfering too much
               with intended expiration times.

               The probability of expiration increases as a function of how far along we are in
               the potential expiration window, with the probability being near 0 at the
               beginning of the window and approaching 1 at the end.

               For example, in all of the following cases, an item might be considered expired
               any time between 15 and 20 minutes, with about a 20% chance at 16 minutes, a 40%
               chance at 17 minutes, and a 100% chance at 20 minutes.

                   my $cache = CHI->new ( ..., expires_variance => 0.25, ... );
                   $cache->set($key, $value, '20 min');
                   $cache->set($key, $value, { expires_at => time() + 20*60 });

                   my $cache = CHI->new ( ... );
                   $cache->set($key, $value, { expires_in => '20 min', expires_variance => 0.25 });

               CHI will make a new probabilistic choice every time it needs to know whether an
               item has expired (i.e. it does not save the results of its determination), so you
               can get situations like this:

                   my $value = $cache->get($key);     # returns undef (indicating expired)
                   my $value = $cache->get($key);     # returns valid value this time!

                   if ($cache->is_valid($key))        # returns undef (indicating expired)
                   if ($cache->is_valid($key))        # returns true this time!

               Typical applications won't be affected by this, since the object is recomputed as
               soon as it is determined to be expired. But it's something to be aware of.

       compute( $key, $options, $code )
           Combines the "get" and "set" operations in a single call. Attempts to get $key; if
           successful, returns the value. Otherwise, calls $code and uses the return value as the
           new value for $key, which is then returned. Caller context (scalar or list) is
           respected.

           $options can be undef, a scalar, or a hash reference. If it is undef, it has no
           effect. If it is a scalar, it is treated as the "expires_in" duration and passed as
           the third argument to "set". If it is a hash reference, it may contain name/value
           pairs for both "get" and "set".  e.g.

               # No expiration
               my $value = $cache->compute($key, undef, sub {
                   # compute and return value for $key here
               });

               # Expire in 5 minutes
               my $value = $cache->compute($key, '5min', sub {
                   # compute and return value for $key here
               });

               # Expire in 5 minutes or when a particular condition occurs
               my $value = $cache->compute($key,
                   { expires_in => '5min', expire_if => sub { ... } },
                   sub {
                      # compute and return value for $key here
               });

               # List context
               my @value = $cache->compute($key, '5min', sub {
                   ...
                   return @some_list;
               });

           This method will eventually support the ability to recompute a value in the background
           just before it actually expires, so that users are not impacted by recompute time.

           Note: Prior to version 0.40, the last two arguments were in reverse order; both will
           be accepted for backward compatibility. We think the coderef looks better at the end.

   Removing and expiring
       remove( $key )
           Remove the data associated with the $key from the cache.

       expire( $key )
           If $key exists, expire it by setting its expiration time into the past. Does not
           necessarily remove the data. Since this involves essentially setting the value again,
           "remove" may be more efficient for some drivers.

   Inspecting keys
       is_valid( $key )
           Returns a boolean indicating whether $key exists in the cache and has not expired.
           Note: Expiration may be determined probabilistically if "expires_variance" was used.

       exists_and_is_expired( $key )
           Returns a boolean indicating whether $key exists in the cache and has expired.  Note:
           Expiration may be determined probabilistically if "expires_variance" was used.

       get_expires_at( $key )
           Returns the epoch time at which $key definitively expires. Returns undef if the key
           does not exist or it has no expiration time.

       get_object( $key )
           Returns a CHI::CacheObject object containing data about the entry associated with
           $key, or undef if no such key exists. The object will be returned even if the entry
           has expired, as long as it has not been removed.

   Atomic operations (ALPHA)
       These methods combine both reading and writing of a cache entry in a single operation. The
       names and behaviors were adapted from memcached <http://memcached.org/>.

       Some drivers (e.g.  CHI::Driver::Memcached::libmemcached, CHI::Driver::DBI) may implement
       these as truly atomic operations, and will be documented thusly.  The default
       implementations are not atomic: the get and set occur discretely and another process could
       potentially modify the cache in between them.

       These operations are labeled ALPHA because we haven't yet figured out how they integrate
       with other CHI features, in particular "SUBCACHES". APIs and behavior may change.

       add( $key, $data, [$expires_in | "now" | "never" | options] )
           Do a set, but only if $key is not valid in the cache.

       replace( $key, $data, [$expires_in | "now" | "never" | options] )
           Do a set, but only if $key is valid in the cache.

       append( $key, $new_data)
           Append $new_data to whatever value is currently associated with $key. Has no effect if
           $key does not exist in the cache.

           Returns true if $key was in the cache, false otherwise.

           This is intended for simple string values only. For efficiency's sake, CHI won't
           attempt to check for, or handle, the case where data is serialized or compressed; the
           new data will simply be appended, and an error will most probably occur when you try
           to retrieve the value.

           Does not modify expiration or other metadata. If $key exists but is expired, it will
           remain expired.

           If you use a driver with the non-atomic (default) implementation, some appends may be
           lost due to race conditions.

   Namespace operations
       clear( )
           Remove all entries from the namespace.

       get_keys( )
           Returns a list of keys in the namespace. This may or may not include expired keys,
           depending on the driver.

           The keys may not look the same as they did when passed into "set"; they may have been
           serialized, utf8 encoded, and/or digested (see "KEY AND VALUE TRANSFORMATIONS").
           However, they may still be passed back into "get", "set", etc. to access the same
           underlying objects. i.e. the following code is guaranteed to produce all key/value
           pairs from the cache:

             map { ($_, $c->get($_)) } $c->get_keys()

       purge( )
           Remove all entries that have expired from the namespace associated with this cache
           instance. Warning: May be very inefficient, depending on the number of keys and the
           driver.

       get_namespaces( )
           Returns a list of namespaces associated with the cache. This may or may not include
           empty namespaces, depending on the driver.

   Multiple key/value operations
       The methods in this section process multiple keys and/or values at once. By default these
       are implemented with the obvious map operations, but some cache drivers (e.g.
       Cache::Memcached) can override them with more efficient implementations.

       get_multi_arrayref( $keys )
           Get the keys in list reference $keys, and return a list reference of the same length
           with corresponding values or undefs.

       get_multi_hashref( $keys )
           Like "get_multi_arrayref", but returns a hash reference with each key in $keys mapping
           to its corresponding value or undef. Will only work with scalar keys.

       set_multi( $key_values, $set_options )
           Set the multiple keys and values provided in hash reference $key_values.  $set_options
           is a scalar or hash reference, used as the third argument to set. Will only work with
           scalar keys.

       remove_multi( $keys )
           Removes the keys in list reference $keys.

       dump_as_hash( )
           Returns a hash reference containing all the non-expired keys and values in the cache.

   Property accessors
       chi_root_class( )
           Returns the name of the root class under which this object was created, e.g.  "CHI" or
           "My::CHI". See "SUBCLASSING AND CONFIGURING CHI".

       driver_class( )
           Returns the full name of the driver class. e.g.

               CHI->new(driver=>'File')->driver_class
                  => CHI::Driver::File
               CHI->new(driver=>'+CHI::Driver::File')->driver_class
                  => CHI::Driver::File
               CHI->new(driver=>'+My::Driver::File')->driver_class
                  => My::Driver::File

           You should use this rather than "ref()". Due to some subclassing tricks CHI employs,
           the actual class of the object is neither guaranteed nor likely to be the driver
           class.

       short_driver_name( )
           Returns the name of the driver class, minus the CHI::Driver:: prefix, if any.  e.g.

               CHI->new(driver=>'File')->short_driver_name
                  => File
               CHI->new(driver_class=>'CHI::Driver::File')->short_driver_name
                  => File
               CHI->new(driver_class=>'My::Driver::File')->short_driver_name
                  => My::Driver::File

       Standard read-write accessors
               expires_in
               expires_at
               expires_variance
               label
               on_get_error
               on_set_error

       Standard read-only accessors
               namespace
               serializer

   Deprecated methods
       The following methods are deprecated and will be removed in a later version:

           is_empty

DURATION EXPRESSIONS

       Duration expressions, which appear in the "set" command and various other parts of the
       API, are parsed by Time::Duration::Parse.  A duration is either a plain number, which is
       treated like a number of seconds, or a number and a string representing time units where
       the string is one of:

           s second seconds sec secs
           m minute minutes min mins
           h hr hour hours
           d day days
           w week weeks
           M month months
           y year years

       e.g. the following are all valid duration expressions:

           25
           3s
           5 seconds
           1 minute and ten seconds
           1 hour

KEY AND VALUE TRANSFORMATIONS

       CHI strives to accept arbitrary keys and values for caching regardless of the limitations
       of the underlying driver.

   Key transformations
       ·   Keys that are references are serialized - see "key_serializer".

       ·   Keys with wide (>255) characters are utf8 encoded.

       ·   Keys exceeding the maximum length for the underlying driver are digested - see
           "max_key_length" and "key_digester".

       ·   For some drivers (e.g. CHI::Driver::File), keys containing special characters or
           whitespace are escaped with URL-like escaping.

       Note: All transformations above with the exception of escaping are one-way, meaning that
       CHI does not attempt to undo them when returned from "get_keys"; and idempotent, meaning
       that applying them a second time has no effect. So when you call "get_keys", the key you
       get may not be exactly what you passed in, but you'll be able to pass that key in to get
       the corresponding object.

   Value transformations
       ·   Values which are references are automatically serialized before storing, and
           deserialized after retrieving - see "serializer".

       ·   Values with their utf8 flag on are utf8 encoded before storing, and utf8 decoded after
           retrieving.

SUBCACHES

       It is possible to a cache to have one or more subcaches. There are currently two types of
       subcaches: L1 and mirror.

   L1 cache
       An L1 (or "level one") cache sits in front of the primary cache, usually to provide faster
       access for commonly accessed cache entries. For example, this places an in-process Memory
       cache in front of a Memcached cache:

           my $cache = CHI->new(
               driver   => 'Memcached',
               servers  => [ "10.0.0.15:11211", "10.0.0.15:11212" ],
               l1_cache => { driver => 'Memory', global => 1, max_size => 1024*1024 }
           );

       On a "get", the L1 cache is checked first - if a valid value exists, it is returned.
       Otherwise, the primary cache is checked - if a valid value exists, it is returned, and the
       value is placed in the L1 cache with the same expiration time. In this way, items fetched
       most frequently from the primary cache will tend to be in the L1 cache.

       "set" operations are distributed to both the primary and L1 cache.

       You can access the L1 cache with the "l1_cache" method. For example, this clears the L1
       cache but leaves the primary cache intact:

           $cache->l1_cache->clear();

   Mirror cache
       A mirror cache is a write-only cache that, over time, mirrors the content of the primary
       cache. "set" operations are distributed to both the primary and mirror cache, but "get"
       operations go only to the primary cache.

       Mirror caches are useful when you want to migrate from one cache to another.  You can
       populate a mirror cache and switch over to it once it is sufficiently populated. For
       example, here we migrate from an old to a new cache directory:

           my $cache = CHI->new(
               driver          => 'File',
               root_dir        => '/old/cache/root',
               mirror_cache => { driver => 'File', root_dir => '/new/cache/root' },
           );

       We leave this running for a few hours (or as needed), then replace it with

           my $cache = CHI->new(
               driver   => 'File',
               root_dir => '/new/cache/root'
           );

       You can access the mirror cache with the "mirror_cache" method. For example, to see how
       many keys have made it over to the mirror cache:

           my @keys = $cache->mirror_cache->get_keys();

   Creating subcaches
       As illustrated above, you create subcaches by passing the "l1_cache" and/or "mirror_cache"
       option to the CHI constructor. These options, in turn, should contain a hash of options to
       create the subcache with.

       The cache containing the subcache is called the parent cache.

       The following options are automatically inherited by the subcache from the parent cache,
       and may not be overridden:

           expires_at
           expires_in
           expires_variance
           serializer

       (Reason: for efficiency, we want to create a single cache object and store it in both
       caches. The cache object contains expiration information and is dependent on the
       serializer.  At some point we could conceivably add code that will use a single object or
       separate objects as necessary, and thus allow the above to be overridden.)

       The following options are automatically inherited by the subcache from the parent cache,
       but may be overridden:

           namespace
           on_get_error
           on_set_error

       All other options are initialized in the subcache as normal, irrespective of their values
       in the parent.

       It is not currently possible to pass an existing cache in as a subcache.

   Common subcache behaviors
       These behaviors hold regardless of the type of subcache.

       The following methods are distributed to both the primary cache and subcache:

           clear
           expire
           purge
           remove

       The following methods return information solely from the primary cache.  However, you are
       free to call them explicitly on the subcache. (Trying to merge in subcache information
       automatically would require too much guessing about the caller's intent.)

           get_keys
           get_namespaces
           get_object
           get_expires_at
           exists_and_is_expired
           is_valid
           dump_as_hash

   Multiple subcaches
       It is valid for a cache to have one of each kind of subcache, e.g. an L1 cache and a
       mirror cache.

       A cache cannot have more than one of each kind of subcache, but a subcache can have its
       own subcaches, and so on. e.g.

           my $cache = CHI->new(
               driver   => 'Memcached',
               servers  => [ "10.0.0.15:11211", "10.0.0.15:11212" ],
               l1_cache => {
                   driver     => 'File',
                   root_dir   => '/path/to/root',
                   l1_cache   => { driver => 'RawMemory', global => 1 }
               }
           );

   Methods for parent caches
       has_subcaches( )
           Returns a boolean indicating whether this cache has subcaches.

       l1_cache( )
           Returns the L1 cache for this cache, if any. Can only be called if has_subcaches is
           true.

       mirror_cache( )
           Returns the mirror cache for this cache, if any. Can only be called if has_subcaches
           is true.

       subcaches( )
           Returns the subcaches for this cache, in arbitrary order. Can only be called if
           has_subcaches is true.

   Methods for subcaches
       is_subcache( )
           Returns a boolean indicating whether this is a subcache.

       subcache_type( )
           Returns the type of subcache as a string, e.g. 'l1_cache' or 'mirror_cache'.  Can only
           be called if is_subcache is true.

       parent_cache( )
           Returns the parent cache (weakened to prevent circular reference).  Can only be called
           if is_subcache is true.

   Developing new kinds of subcaches
       At this time, subcache behavior is hardcoded into CHI::Driver, so there is no easy way to
       modify the behavior of existing subcache types or create new ones.  We'd like to make this
       more flexible eventually.

SIZE AWARENESS

       If "is_size_aware" or "max_size" are passed to the constructor, the cache will be size
       aware - that is, it will keep track of its own size (in bytes) as items are added and
       removed. You can get a cache's size with "get_size".

       Size aware caches generally keep track of their size in a separate meta-key, and have to
       do an extra store whenever the size changes (e.g. on each set and remove).

   Maximum size and discard policies
       If a cache's size rises above its "max_size", items are discarded until the cache size is
       sufficiently below the max size. (See "max_size_reduction_factor" for how to fine-tune
       this.)

       The order in which items are discarded is controlled with "discard_policy".  The default
       discard policy is 'arbitrary', which discards items in an arbitrary order.  The available
       policies and default policy can differ with each driver, e.g. the CHI::Driver::Memory
       driver provides and defaults to an 'LRU' policy.

   Appropriate drivers
       Size awareness was chiefly designed for, and works well with, the CHI::Driver::Memory
       driver: one often needs to enforce a maximum size on a memory cache, and the overhead of
       tracking size in memory is negligible.  However, the capability may be useful with other
       drivers.

       Some drivers - for example, CHI::Driver::FastMmap and CHI::Driver::Memcached - inherently
       keep track of their size and enforce a maximum size, and it makes no sense to turn on
       CHI's size awareness for these.

       Also, for drivers that cannot atomically read and update a value - for example,
       CHI::Driver::File - there is a race condition in the updating of size that can cause the
       size to grow inaccurate over time.

SUBCLASSING AND CONFIGURING CHI

       You can subclass CHI for your own application and configure it in a variety of ways, e.g.
       predefining storage types and defaults for new cache objects. Your configuration will be
       independent of the main CHI class and other CHI subclasses.

       Start with a trivial subclass:

           package My::CHI;

           use base qw(CHI);
           1;

       Then, just use your subclass in place of CHI:

           my $cache = My::CHI->new( ... );

           print $cache->chi_root_class;
              ==> 'My::CHI'

       This obviously doesn't change any behavior by itself. Here's an example with actual
       config:

           package My::CHI;

           use base qw(CHI);

           __PACKAGE__->config({
               storage   => {
                   local_file => { driver => 'File', root_dir => '/my/root' },
                   memcached  => {
                       driver  => 'Memcached::libmemcached',
                       servers => [ '10.0.0.15:11211', '10.0.0.15:11212' ]
                   },
               },
               namespace => {
                   'Foo' => { storage => 'local_file' },
                   'Bar' => { storage => 'local_file', depth => 3 },
                   'Baz' => { storage => 'memcached' },
               }
               defaults  => { storage => 'local_file' },
               memoize_cache_objects => 1,
           });

           1;

       Each of these config keys is explained in the next section.

   Configuration keys
       storage
           A map of names to parameter hashrefs. This provides a way to encapsulate common sets
           of parameters that might be used in many caches. e.g. if you define

               storage => {
                   local_file => { driver => 'File', root_dir => '/my/root' },
                   ...
               }

           then

               my $cache = My::CHI->new
                  (namespace => 'Foo', storage => 'local_file');

           is equivalent to

               my $cache = My::CHI->new
                  (namespace => 'Foo', driver => 'File', root_dir => '/my/root');

       namespace
           A map of namespace names to parameter hashrefs. When you create a cache object with
           the specified namespace, the hashref of parameters will be applied as defaults. e.g.
           if you define

               namespace => {
                   'Foo' => { driver => 'File', root_dir => '/my/root' },
                   'Bar' => { storage => 'database' },
                   ...
               }

           then

               my $cache1 = My::CHI->new
                  (namespace => 'Foo');
               my $cache2 = My::CHI->new
                  (namespace => 'Bar');

           is equivalent to

               my $cache1 = My::CHI->new
                  (namespace => 'Foo', driver => 'File', root_dir => '/my/root');
               my $cache2 = My::CHI->new
                  (namespace => 'Bar', storage => 'database');

       defaults
           A hash of parameters that will be used as core defaults for all cache objects created
           under this root class. e.g.

               defaults => {
                   on_get_error => 'die',
                   expires_variance => 0.2,
               }

           These can be overridden by namespace defaults, storage settings, or "new" parameters.

       memoize_cache_objects
           True or false, indicates whether "My::CHI->new" should memoize and return the same
           cache object if given the same parameters. This can speed things up if you create
           cache objects frequently. Will currently only work for 0- or 1- key parameter hashes.
           e.g.

               My::CHI->config({
                   memoize_cache_objects => 1,
               });

           then

               # $cache1 and $cache2 will be the same object, regardless of what
               # namespace and storage defaults are associated with 'Foo'
               #
               my $cache1 = My::CHI->new(namespace => 'Foo');
               my $cache2 = My::CHI->new(namespace => 'Foo');

               # $cache3 and $cache4 will be different objects
               #
               my $cache3 = My::CHI->new
                  (namespace => 'Bar', driver => 'File', root_dir => '/my/root');
               my $cache4 = My::CHI->new
                  (namespace => 'Bar', driver => 'File', root_dir => '/my/root');

           To clear the memoized cache objects, call

               My::CHI->clear_memoized_cache_objects;

   How defaults are combined
       Defaults are applied in the following order, from highest to lowest precedence:

       ·   Parameters passed in "new"

       ·   Namespace defaults, if any

       ·   Storage settings, if any

       ·   Core defaults defined under 'defaults'

   Inheritance of config
       A subclass will automatically inherit the configuration of its parent if it does not call
       "config" itself (ala Class::Data::Inheritable).

   Reading config from a file
           use YAML::XS qw(LoadFile);

           __PACKAGE__->config(LoadFile("/path/to/cache.yml"));

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 serializing/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 <http://redis.io/>

       ·   CHI::Driver::SharedMem - Cache in shared memory

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

PERFORMANCE COMPARISON OF DRIVERS

       See CHI::Benchmarks for a comparison of read/write times of both CHI and non-CHI cache
       implementations.

       "etc/bench/bench.pl" in the "CHI" distribution contains a script to run these types of
       benchmarks on your own system.

DEVELOPING NEW DRIVERS

       See CHI::Driver::Development for information on developing new drivers.

LOGGING

       "CHI" uses Log::Any for logging events. For example, a debug log message is sent with
       category "CHI::Driver" for every cache get and set.

       See Log::Any documentation for how to control where logs get sent, if anywhere.

STATS

       CHI can record statistics, such as number of hits, misses and sets, on a per-namespace
       basis and log the results to your Log::Any logger.  You can then use utilities included
       with this distribution to read stats back from the logs and report a summary. See
       CHI::Stats for details.

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.

       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. The notable exception is CHI::Driver::RawMemory which does no
           serialization.

       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

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) 2012 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.