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    <p>Welcome to one of the two hard problems in computer science, cache invalidation :)</p> <p>You would have to handle that manually since the logic for when a cached object, unlike a cached view which can be simply derived from the objects it displays, should be invalidated is application and situation dependent.</p> <p>You goto method for this is the <code>Rails.cache.fetch</code> method. <code>Rails.cache.fetch</code> takes 3 arguments; the cache key, an options hash, and a block. It first tries to read a valid cache record based on the key; if that key exists and hasn’t expired it will return the value from the cache. If it can’t find a valid record it instead takes the return value from the block and stores it in the cache with your specified key.</p> <p>For example:</p> <pre><code>@models = Rails.cache.fetch my_cache_key do Model.where(condition: true).all end </code></pre> <p>This will cache the block and reuse the result until something (tm) invalidates the key, forcing the block to be reevaluated. Also note the <code>.all</code> at the end of the method chain. Normally Rails would return an ActiveRecord relation object that would be cached and this would then be evaluated when you tried to use <code>@models</code> for the first time, neatly sidestepping the cache. The <code>.all</code> call forces Rails to eager load the records and ensure that it's the result that we cache, not the question.</p> <p>So now that you get all your cache on and never talk to the database again we have to make sure we cover the other end, invalidating the cache. This is done with the <code>Rails.cache.delete</code> method that simply takes a cache key and removes it, causing a miss the next time you try to fetch it. You can also use the <code>force: true</code>option with fetch to force a re-evaluation of the block. Whichever suits you.</p> <p>The science of it all is where to call <code>Rails.cache.delete</code>, in the naïve case this would be on update and delete for a single instance and update, delete, create on any member for a collection. There will always bee corner cases and they are always application specific, so I can't help you much there.</p> <p>I assume in this answer that you will set up some sane cache store, like memcached or Redis. </p> <p>Also remember to add this to config/environments/development.rb:</p> <pre><code>config.cache_store = :null_store </code></pre> <p>or you development environment will cache and you will end up hairless from frustration.</p> <p>For further reference read: <a href="http://robotmay.tumblr.com/post/23161612605/everyone-should-be-using-low-level-caching" rel="nofollow">Everyone should be using low level caching in Rails</a> and <a href="http://api.rubyonrails.org/classes/ActiveSupport/Cache/Store.html" rel="nofollow">The rails API docs</a></p> <p>It is also worth noting that functionality is not removed from Rails 4, merely extracted into a gem. If you need or would like the full features of the sweepers simply add it back to your app with a <code>gem 'rails-observers'</code> line in your Gemfile. That gem contains both the sweepers and observers that where removed from Rails 4 core.</p> <p>I hope that helpt you get started.</p>
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    1. POWhat is the right way to clear cache in Rails without sweepers
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    1. USJonas Schubert Erlandsson
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    1. COAbout the cache invalidation I guess it takes us back to the sweepers role. Instead of calling `Rails.cache.delete` in my controller I could use the sweepers from the externalized gem. I think in that case it makes sense, don't you ? Thank you for your answer :)
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    2. COYes, the role of the sweeper is hard to get out of since that is one of the prerequisites for a working cache strategy ;) I usually have a life cycle class for things like cache invalidation, messaging and interactions with external models. That way you have a clean controller, small core class and stick your extraneous behavior in another class that listens to the base class. In essence the sweeper is just a specialized case of one of these life cycle classes and there is nothing inherently wrong with them as a strategy.
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      1. USJonas Schubert Erlandsson
    3. COThat's why I feel strange about sweepers removal from Rails 4. I understand page/action caching is bad but since low level caching still needs sweepers or sweepers-like behavior, I don't see the point of removing them :(
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