Github user eliaslevy commented on the issue:
https://github.com/apache/flink/pull/6076
Thanks, that helps.
Regarding debugging watermarks, at least in 1.4.2 sources did not display
their emitted watermarks, although this may have been fixed in 1.5.0. In turn,
that makes jobs with task chaining turned on more difficult to debug. It may
be useful to suggest turning off task chaining for debugging.
Also, the Kafka connector page discusses the fact that the Kafka source
only emits watermarks as the minimum of the per partition watermark, which
means an idle partition may delay watermark emission. It may be good to
reiterate that behavior here, as Kafka is the most commonly used connector.
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