Actually I think Fabian initial message was incorrect. As far as I can see in the code of WindowOperator (last lines of org.apache.flink.streaming.runtime.operators.windowing.WindowOperator#processElement ), the element is sent to late side output if it is late AND it wasn’t assigned to any of the existing windows (because they were late as well). In other words, it should work as you Juho are wishing: element should be marked as late once they are overdue/late for the window after one full day.
I have tested it and it works as expected. Following program:
Prints only ONE number to the standard err:
And there is nothing on the side output.