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Memoize::Expire(3pm)   Perl Programmers Reference Guide   Memoize::Expire(3pm)




NAME

       Memoize::Expire - Plug-in module for automatic expiration of memoized
       values


SYNOPSIS

         use Memoize;
         use Memoize::Expire;
         tie my %cache => 'Memoize::Expire',
                            LIFETIME => $lifetime,    # In seconds
                            NUM_USES => $n_uses;

         memoize 'function', SCALAR_CACHE => [HASH => \%cache ];


DESCRIPTION

       Memoize::Expire is a plug-in module for Memoize.  It allows the cached
       values for memoized functions to expire automatically.  This manual
       assumes you are already familiar with the Memoize module.  If not, you
       should study that manual carefully first, paying particular attention
       to the HASH feature.

       Memoize::Expire is a layer of software that you can insert in between
       Memoize itself and whatever underlying package implements the cache.
       The layer presents a hash variable whose values expire whenever they
       get too old, have been used too often, or both. You tell "Memoize" to
       use this forgetful hash as its cache instead of the default, which is
       an ordinary hash.

       To specify a real-time timeout, supply the "LIFETIME" option with a
       numeric value.  Cached data will expire after this many seconds, and
       will be looked up afresh when it expires.  When a data item is looked
       up afresh, its lifetime is reset.

       If you specify "NUM_USES" with an argument of n, then each cached data
       item will be discarded and looked up afresh after the nth time you
       access it.  When a data item is looked up afresh, its number of uses is
       reset.

       If you specify both arguments, data will be discarded from the cache
       when either expiration condition holds.

       Memoize::Expire uses a real hash internally to store the cached data.
       You can use the "HASH" option to Memoize::Expire to supply a tied hash
       in place of the ordinary hash that Memoize::Expire will normally use.
       You can use this feature to add Memoize::Expire as a layer in between a
       persistent disk hash and Memoize.  If you do this, you get a persistent
       disk cache whose entries expire automatically.  For example:

         #   Memoize
         #      |
         #   Memoize::Expire  enforces data expiration policy
         #      |
         #   DB_File  implements persistence of data in a disk file
         #      |
         #   Disk file

         use Memoize;
         use Memoize::Expire;
         use DB_File;

         # Set up persistence
         tie my %disk_cache => 'DB_File', $filename, O_CREAT|O_RDWR, 0666];

         # Set up expiration policy, supplying persistent hash as a target
         tie my %cache => 'Memoize::Expire',
                            LIFETIME => $lifetime,    # In seconds
                            NUM_USES => $n_uses,
                            HASH => \%disk_cache;

         # Set up memoization, supplying expiring persistent hash for cache
         memoize 'function', SCALAR_CACHE => [ HASH => \%cache ];


INTERFACE

       There is nothing special about Memoize::Expire.  It is just an example.
       If you don't like the policy that it implements, you are free to write
       your own expiration policy module that implements whatever policy you
       desire.  Here is how to do that.  Let us suppose that your module will
       be named MyExpirePolicy.

       Short summary: You need to create a package that defines four methods:

        TIEHASH
           Construct and return cache object.

        EXISTS
           Given a function argument, is the corresponding function value in
           the cache, and if so, is it fresh enough to use?

        FETCH
           Given a function argument, look up the corresponding function value
           in the cache and return it.

        STORE
           Given a function argument and the corresponding function value,
           store them into the cache.

        CLEAR
           (Optional.)  Flush the cache completely.

       The user who wants the memoization cache to be expired according to
       your policy will say so by writing

         tie my %cache => 'MyExpirePolicy', args...;
         memoize 'function', SCALAR_CACHE => [HASH => \%cache];

       This will invoke "MyExpirePolicy->TIEHASH(args)".
       MyExpirePolicy::TIEHASH should do whatever is appropriate to set up the
       cache, and it should return the cache object to the caller.

       For example, MyExpirePolicy::TIEHASH might create an object that
       contains a regular Perl hash (which it will to store the cached values)
       and some extra information about the arguments and how old the data is
       and things like that.  Let us call this object `C'.

       When Memoize needs to check to see if an entry is in the cache already,
       it will invoke "C->EXISTS(key)".  "key" is the normalized function
       argument.  MyExpirePolicy::EXISTS should return 0 if the key is not in
       the cache, or if it has expired, and 1 if an unexpired value is in the
       cache.  It should not return "undef", because there is a bug in some
       versions of Perl that will cause a spurious FETCH if the EXISTS method
       returns "undef".

       If your EXISTS function returns true, Memoize will try to fetch the
       cached value by invoking "C->FETCH(key)".  MyExpirePolicy::FETCH should
       return the cached value.  Otherwise, Memoize will call the memoized
       function to compute the appropriate value, and will store it into the
       cache by calling "C->STORE(key, value)".

       Here is a very brief example of a policy module that expires each cache
       item after ten seconds.

               package Memoize::TenSecondExpire;

               sub TIEHASH {
                 my ($package, %args) = @_;
                 my $cache = $args{HASH} || {};
                 bless $cache => $package;
               }

               sub EXISTS {
                 my ($cache, $key) = @_;
                 if (exists $cache->{$key} &&
                     $cache->{$key}{EXPIRE_TIME} > time) {
                   return 1
                 } else {
                   return 0;  # Do NOT return `undef' here.
                 }
               }

               sub FETCH {
                 my ($cache, $key) = @_;
                 return $cache->{$key}{VALUE};
               }

               sub STORE {
                 my ($cache, $key, $newvalue) = @_;
                 $cache->{$key}{VALUE} = $newvalue;
                 $cache->{$key}{EXPIRE_TIME} = time + 10;
               }

       To use this expiration policy, the user would say

               use Memoize;
               tie my %cache10sec => 'Memoize::TenSecondExpire';
               memoize 'function', SCALAR_CACHE => [HASH => \%cache10sec];

       Memoize would then call "function" whenever a cached value was entirely
       absent or was older than ten seconds.

       You should always support a "HASH" argument to "TIEHASH" that ties the
       underlying cache so that the user can specify that the cache is also
       persistent or that it has some other interesting semantics.  The
       example above demonstrates how to do this, as does "Memoize::Expire".

       Another sample module, Memoize::Saves, is available in a separate
       distribution on CPAN.  It implements a policy that allows you to
       specify that certain function values would always be looked up afresh.
       See the documentation for details.


ALTERNATIVES

       Brent Powers has a "Memoize::ExpireLRU" module that was designed to
       work with Memoize and provides expiration of least-recently-used data.
       The cache is held at a fixed number of entries, and when new data comes
       in, the least-recently used data is expired.  See
       <http://search.cpan.org/search?mode=module&query=ExpireLRU>.

       Joshua Chamas's Tie::Cache module may be useful as an expiration
       manager.  (If you try this, let me know how it works out.)

       If you develop any useful expiration managers that you think should be
       distributed with Memoize, please let me know.


CAVEATS

       This module is experimental, and may contain bugs.  Please report bugs
       to the address below.

       Number-of-uses is stored as a 16-bit unsigned integer, so can't exceed
       65535.

       Because of clock granularity, expiration times may occur up to one
       second sooner than you expect.  For example, suppose you store a value
       with a lifetime of ten seconds, and you store it at 12:00:00.998 on a
       certain day.  Memoize will look at the clock and see 12:00:00.  Then
       9.01 seconds later, at 12:00:10.008 you try to read it back.  Memoize
       will look at the clock and see 12:00:10 and conclude that the value has
       expired.  This will probably not occur if you have "Time::HiRes"
       installed.


AUTHOR

       Mark-Jason Dominus (mjd-perl-memoize+@plover.com)

       Mike Cariaso provided valuable insight into the best way to solve this
       problem.


SEE ALSO

       perl(1)

       The Memoize(3) man page.

       http://www.plover.com/~mjd/perl/Memoize/  (for news and updates)

       I maintain a mailing list on which I occasionally announce new versions
       of Memoize.  The list is for announcements only, not discussion.  To
       join, send an empty message to mjd-perl-memoize-request@Plover.com.



perl v5.24.0                      2016-02-05              Memoize::Expire(3pm)

perl 5.24 - Generated Thu Nov 17 18:24:15 CST 2016
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