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? Why 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?
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.