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We will introduce a new implementation of a StateBackend which will create a KeyedStateBackend that works with a single key at a time. That KeyedStateBackend will create state objects (ValueState, ListState, MapState) that will only keep a single value in the object's field, i.e. ValueState is a very thin wrapper for a value, MapState for a HashMap,
ListState for a List etc.  That way we get rid of a lookup to the StateTable for the current value for the current key. When the StateBackend observes a new incoming key it will reset all acquired state objects so far, setting the wrapped values to null. We will know that the key changed/finished because we will keep the current key and in the setCurrentKey we check if the new key is different then the current one.

TimerService

We will replace the InternalTimeServiceManager with a customized one which will keep timers only for the current key, and it will fire timers only at the end of each key. The value of InternalTimeService#getCurrentWatermark will be Watermark.MIN_VALUE when processing a key. and will be set to Watermark.MAX_VALUE when firing triggers.

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