Github user StephanEwen commented on the pull request:
https://github.com/apache/flink/pull/1518#issuecomment-172863106
I think this is expected and similar to the behavior of the batch API.
Two operators are connected with "forward" if they have the same
parallelism, and with "rebalance" if not. Since the source is forced to
parallelism 1 (non parallel collection), a parallelism 2 program needs to
rebalance.
I would leave that as it is.
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