Kazuho at Work

Normally, a cache entry has a single lifetime. The problem is that if a cache entry is read frequently and if it takes time to update the entry, a situation known as thundering herd would occur on expiration; i.e. many cache consumers will detect expiration and send same update requests to the backend, causing a performance decline. There are two solutions to the problem:

• use a proxy that combines identical update requests as a single request
• initiate an update request prior to expiration, when lifetime being left gets below a certain threshold

In version 0.05, as I did in Cache::Adaptive, I implemented the latter approach^*1 to let applications start updating cache entries prior to their expiration. When the refresh_before property is set to nonzero, swifty would set the do_refresh flag when a cache entry is read for the first time after its left lifetime becomes below refresh_before.

The application logic can check if the flag is being set, and start updating the cache entry. Here's the example code.

my $swifty = Cache::Swifty->new({
    dir            => ...,
    lifetime       => 300, # cache lifetime is 5 minutes
    refresh_before => 10,  # one of the consumer will be notified when left lifetime<10
});

(snip)

$entry = $swifty->get(...);
if ($swifty->do_refresh) {
    # start updating the cache entry
}