THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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As a result, the mapping of a key to a KeyGroup could be established inside the Source Operator, but the problem is that we have to pass down the key group of a record to the downstream operators with the record itself together. One solution is that we can create a new class called KeyedStreamRecord, which wrap wraps the StreamRecord with a new field, KeyGroup.
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