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Theoretically, we can just keep use a Map[SourceTopicPartition/TargetTopicPartition, AckedOffset], but in some scenarios, that might not work well. For example, if we have messages 1,2,3,4,5 from a source partition 1, message 1,2,4,5 are produced to destination partition 1, and message 3 is produced to destination partition 2. If we just use a Map[SourceTopicPartition/TargetTopicPartition, AckedOffset], it would hold { [(SourceTopicPartition=1, TargetTopicParitition=1),5], [(SourceTopicPartition=1, TargetTopicParitition=2),3] }. From the map itself, we cannot tell whether message 4 has been acked or not. And if there is no more message sent to target partition 2, the offset for source partition 1 will block on 3 forever.

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