memoize::expire(3)

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 famil
iar 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 Memo ize::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 automati cally. 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_CRE; AT|O_RDWR, 0666];; # Set up expiration policy, supplying persistent hash as; a target
tie my %cache => 'Memoize::Expire',
LIFETIME => $lifetime, # In sec

onds
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 pol
icy 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 func
tion 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 func
tion 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)". MyEx pirePolicy::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)". MyEx pirePolicy::FETCH should return the cached value. Other wise, 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 spec ify that the cache is also persistent or that it has some other interesting semantics. The example above demon strates how to do this, as does "Memoize::Expire".

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 leastrecently used data is expired. See
<http://search.cpan.org/search?mode=mod
ule&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. Memo
ize 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 con
clude 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.

docs.sk

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