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Overall, comparing with hashing the key of a record and getting its key group, the key group would now be allocated by SourceOperator if the input stream is "pre-KeyedStream". The assignment of the key group would need to be propagated among the operators in the SourceTask. Because in general, the partitioning between two tasks is All-to-All, which leads to data shuffle. Therefore, the downstream operator that still wants to take advantage of KeyedStream's features can only has to repartition the data stream through keyBy().

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