Github user zentol commented on a diff in the pull request:
    --- Diff: docs/dev/ ---
    @@ -213,10 +213,36 @@ arrive after the system's event time clock (as 
signaled by the watermarks) has a
     timestamp. See [Allowed Lateness]({{ site.baseurl 
}}/dev/stream/operators/windows.html#allowed-lateness) for more information on 
how to work
     with late elements in event time windows.
    +## Idling sources
    +Currently with pure event time watermarks generators, watermarks can not 
progress if there are no elements
    +to be processed. That means in case of gap in the incoming data, even time 
will not progress and for
    +example window operator will not be triggered and thus existing windows 
will not be able produce any
    +output data.
    +To circumvent this one can use periodic watermark assigners that don't 
only assign based on
    +element timestamps. Example solution could be an assigner that switches to 
using current processing time
    +as the time basis after not observing new events for a while.
    +Sources can be marked as idle using 
`SourceFunction.SourceContext#markAsTemporarilyIdle`. For details please refer 
to Javadoc of
    --- End diff --
    to `the` Javadoc


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