Memoize::Expire(3Perl Programmers Reference GuMemoize::Expire(3p)NAMEMemoize::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 ];
DESCRIPTIONMemoize::Expire is a plug-in module for Memoize. It allows
the cached values for memoized functions to expire automati-
cally. 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 for-
getful 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 ordi-
nary 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:
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# 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 imple-
ments, 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
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(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. Oth-
erwise, 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 => $package;
}
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sub EXISTS {
my ($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".
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.
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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 ALSOperl(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@Plover.com.
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