Event Time Windows and Watermarks

A streaming job counts vending machine revenue in 10-minute windows, and its newest sale so far was made at 10:11. The job has decided to wait 5 minutes for sales that are delayed on their way, so it treats 10:11 - 5 = 10:06 as the point it has reached. A sale made at 10:08 that arrives now is still counted, because its window ends at 10:10, after 10:06. When the newest sale reaches 10:17, the point moves to 10:12, and the window from 10:00 to 10:10 can be written.

That moving point is the watermark, and the 5 minutes is its delay