Why Rails.cache is not thread safe?

I know Rails.cache is ActiveSupport::Cache::MemoryStore, and it is not thread safe.

I don't understand, why rails use a thread-unsafe cache as its default? W开发者_StackOverflowhy not use ActiveSupport::Cache::SynchronizedMemoryStore? In my opinion, in a web site, if a cache is not thread-safe, it almost useless, because the requests are not handled in ONE thread.

Do you use Rails.cache in you webapp? And how do you use it?

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The default cache store in Rails is ActiveSupport::Cache::FileStore, not MemoryStore.

The memory store is of limited use in practice, since it is restricted to a single process, which makes it useless for Rails apps that are deployed using Passenger or a Mongrel cluster where requests are handled in separate processes, not in separate threads.

For small to medium-sized applications you'll probably do fine with the default file store. If you need to scale beyond that, you should have a look at ActiveSupport::Cache::MemCacheStore.

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Most deployment scenario's for Rails are actually single-threaded. Concurrency is achieved by spawning multiple processes, either automatically or beforehand. For many people, thread-safety won't matter that much.

Multi-threaded options do exist (especially with JRuby), so your question is still valid. Which is why in Rails 3, the old ActiveSupport::Cache::MemoryStore has been removed and replaced with ActiveSupport::Cache::SynchronizedMemoryStore, making it thread-safe by default.

If you need the thread-safety in a Rails 2 app, put the following somewhere in your environment.

ActionController::Base.cache_store = :synchronized_memory_store

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The default Rails cache (ActiveSupport::Cache MemoryStore) is thread-safe as of Rails version 3.1: http://api.rubyonrails.org/v3.1.0/files/activesupport/CHANGELOG.html As the CHANGELOG notes: "Make thread safe so that the default cache implementation used by Rails is thread safe."