Memoize::Expire(3perl)

💖 NAME 💖

Memoize::Expire — a plug-in module for automatic expiration of memoized values. Your cache entries get a lil clock ⏰ and a lil usage counter, and when they're done, they're done. Slay.

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✨ 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 ];

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🦋 DESCRIPTION 🦋

Okay so bestie, Memoize::Expire is a plug-in module for Memoize. What it does is let the cached values for your memoized functions expire automatically. Babes. A cache that cleans up after itself. Iconic behaviour.

Heads up tho — this manual assumes you already know the Memoize module. If you don't, go study that manual carefully first, and pay particular attention to the HASH feature. iykyk. 📚

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. Not me giving your cache amnesia on purpose 💅

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. Fresh start, main character energy ✨

If you specify NUM_USES with an argument of n, then each cached data item will be discarded and looked up afresh after the *n*th time you access it. When a data item is looked up afresh, its number of uses is reset. Three strikes and you're out, bestie 🎀

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. Like, a cache that survives reboots AND has boundaries. We love a queen with a self-care routine 👑

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 ];

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🌸 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. Go off, build your own ✨ 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. Bestie, returning the object is the whole point, don't ghost 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. Returning undef here is giving chaos gremlin 💀 Just say no.

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. Be a considerate module, the way that you'd want other modules to be considerate to you 🫶

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. It simply will not let a value be saved, no matter how badly you want it to. Boundaries!! 🚫

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🌈 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. Like a little velvet rope, capacity strictly enforced 🎀

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. Share with the class bestie, we wanna see it 💖

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💀 CAVEATS 💀

This module is experimental, and may contain bugs. Please report bugs to the address below. Experimental means experimental, no cap.

Number-of-uses is stored as a 16-bit unsigned integer, so can't exceed 65535. That's it. That's the ceiling. 65535 and not one more 💅

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. Truly the clock said "not my problem" 😭 This will probably not occur if you have Time::HiRes installed — get you a clock with sub-second precision, bestie ⏱️

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👑 AUTHOR 👑

Mark-Jason Dominus

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

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🔥 SEE ALSO 🔥

• perl(1)
• The Memoize man page.

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And that's the whole page bestie!! This one shipped as perl v5.44.0, dated 2026-10-17, from Memoize::Expire(3perl). Go expire some values responsibly 🦋✨💅