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• The users need not provide other information except for KeySelector. It is keeping clean for users to use this API. 
• The wrapping StreamRecord could help to reduce duplicated extraction and computation of key and keyGroupKeyGroup. Since the load of CPU is usually the bottleneck of a job comparing with the network consumption.

The con is that the implementation could be complicated and sensitive since wrapping the StreamRecord with a new field. But I believe it is acceptable since the modification is limited in the SourceTask. 

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