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    <p>Rails does not have a built-in mechanism to prevent cache stampedes.</p> <p>According to the README for <code>atomic_mem_cache_store</code> (a replacement for <code>ActiveSupport::Cache::MemCacheStore</code> that mitigates cache stampedes):</p> <blockquote> <p>Rails (and any framework relying on active support cache store) does not offer any built-in solution to this problem</p> </blockquote> <p>Unfortunately, I'm guessing that this gem won't solve your problem either. It supports fragment caching, but it only works with time-based expiration.</p> <p>Read more about it here: <a href="https://github.com/nel/atomic_mem_cache_store" rel="nofollow">https://github.com/nel/atomic_mem_cache_store</a></p> <h3>Update and possible solution:</h3> <p>I thought about this a bit more and came up with what seems to me to be a plausible solution. I haven't verified that this works, and there are probably better ways to do it, but I was trying to think of the smallest change that would mitigate the majority of the problem.</p> <p>I assume you're doing something like <code>cache model do</code> in your templates as described by DHH (<a href="http://37signals.com/svn/posts/3113-how-key-based-cache-expiration-works" rel="nofollow">http://37signals.com/svn/posts/3113-how-key-based-cache-expiration-works</a>). The problem is that when the model's <code>updated_at</code> column changes, the <code>cache_key</code> likewise changes, and all your servers try to re-create the template at the same time. In order to prevent the servers from stampeding, you would need to retain the old cache_key for a brief time. </p> <p>You might be able to do this by (dum da dum) caching the cache_key of the object with a short expiration (say, 1 second) and a <code>race_condition_ttl</code>.</p> <p>You could create a module like this and include it in your models:</p> <pre><code>module StampedeAvoider def cache_key orig_cache_key = super Rails.cache.fetch("/cache-keys/#{self.class.table_name}/#{self.id}", expires_in: 1, race_condition_ttl: 2) { orig_cache_key } end end </code></pre> <p>Let's review what would happen. There are a bunch of servers calling <code>cache model</code>. If your model includes <code>StampedeAvoider</code>, then its <code>cache_key</code> will now be fetching <code>/cache-keys/models/1</code>, and returning something like <code>/models/1-111</code> (where 111 is the timestamp), which <code>cache</code> will use to fetch the compiled template fragment.</p> <p>When you update the model, <code>model.cache_key</code> will begin returning <code>/models/1-222</code> (assuming 222 is the new timestamp), but for the first second after that, <code>cache</code> will keep seeing <code>/models/1-111</code>, since that is what is returned by <code>cache_key</code>. Once 1 second passes, all of the servers will get a cache-miss on <code>/cache-keys/models/1</code> and will try to regenerate it. If they all recreated it immediately, it would defeat the point of overriding <code>cache_key</code>. But because we set <code>race_condition_ttl</code> to 2, all of the servers except for the first will be delayed for 2 seconds, during which time they will continue to fetch the old cached template based on the old cache key. Once the 2 seconds have passed, <code>fetch</code> will begin returning the new cache key (which will have been updated by the first thread which tried to read/update <code>/cache-keys/models/1</code>) and they will get a cache hit, returning the template compiled by that first thread.</p> <p>Ta-da! Stampede averted.</p> <p>Note that if you did this, you would be doing twice as many cache reads, but depending on how common stampedes are, it could be worth it.</p> <p>I haven't tested this. If you try it, please let me know how it goes :)</p>
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