A new controller class is instantiated for each request. But class variables are global for the whole inheritance chain of classes that contain them. And rails is not thread safe so there is always a mutex that ensures that only one request is ever running through the rails dispatcher at once. So as long as you make absolutely certain that the @@classvariable gets set on each and every request, you will not run into any problems.
-- Ezra Zygmuntowicz-- Lead Rails Evangelist
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