Contexts are a way to structure the state and Configuration of an Evaluator. A Context exists on one and only one individual Evaluator. Each Evaluator has at least one Context, which we refer to as the root Context. This root context is special, as it is required on all Evaluators and because closing it is synonymous to releasing the Evaluator. In many simple REEF programs, this is the only Context used and it is therefore convenient to think of it as synonymous to "the Evaluator": The Driver can submit Tasks to it, is notifified notified when they are done and can close the root context when it wants to dispose of the Evaluator. 

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