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 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 AT Plover.com.
perl v5.16.3 2013-03-04 Memoize::Expire(3pm)
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