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3) StateRequestHandler will be created in the Java operator to process the state access requests from the Python side. It’s a callback which will be called by Beam’s portability framework upon receiving a state request. It’s called in a separate callback thread instead of the main thread in the Java operator. 

4) The state data will be serialized into byte array and then sent to the Java operator. It will be stored as primitive byte array in the underlying state backend in the Java operator no matter what's the actual Python type it represents.

Current key in state backend

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