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POCaching & avoiding Cache Stampedes - multiple simultaneous calculations
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copied!<p>We have a very expensive calculation that we'd like to cache. So we do something similar to:</p> <pre><code>my $result = $cache->get( $key ); unless ($result) { $result = calculate( $key ); $cache->set( $key, $result, '10 minutes' ); } return $result; </code></pre> <p>Now, during <code>calculate($key)</code>, before we store the result in the cache, several other requests come in, that also start running <code>calculate($key)</code>, and system performance suffers because many processes are all calculating the same thing.</p> <p>Idea: Lets put a flag in the cache that a value is being calculated, so the other requests just wait for that one calculation to finish, so they all use it. Something like:</p> <pre><code>my $result = $cache->get( $key ); if ($result) { while ($result =~ /Wait, \d+ is running calculate../) { sleep 0.5; $result = $cache->get( $key ); } } else { $cache->set( $key, "Wait, $$ is running calculate()", '10 minutes' ); $result = calculate( $key ); $cache->set( $key, $result, '10 minutes' ); } return $result; </code></pre> <p>Now that opens up a whole new can of worms. What if $$ dies before it sets the cache. What if, what if... All of them solvable, but since there is nothing in <a href="http://search.cpan.org/search?m=all&q=cache" rel="nofollow noreferrer">CPAN</a> that does this (there is something in CPAN for <em>everything</em>), I start wondering:</p> <p>Is there a better approach? Is there a particular reason e.g. Perl's <code>Cache</code> and <code>Cache::Cache</code> classes don't provide some mechanism like this? Is there a tried and true pattern I could use instead?</p> <p>Ideal would be a CPAN module with a debian package already in squeeze or a eureka moment, where I see the error of my ways... :-)</p> <p>EDIT: I have since learned that this is called a <a href="https://en.wikipedia.org/wiki/Cache_stampede" rel="nofollow noreferrer">Cache stampede</a> and have updated the question's title.</p>

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