Windowing and Watermark in Apache beam : Google dataflow

Question

I have a fixed window of 1 minute. I am considering event time.

beam.WindowInto(window.FixedWindows(300))

When I deploy this code ,is the window created instantly even if I have not published any message .suppose I deployed at 6:30 , is it like the windows are automatically created as 6:30 to 6:35, 6:35 to 6:40 and so on ?

If I publish a message to topic having event timestamp = 6:31 (unix seconds i.e 10,176589653) when system time = 6:36 ..does it mean the watermark for that specific message is at 6:31 and it will miss the window as system time is at 6:36 and allowed lateness=0 and will be rejected.

Answer 1

Windows are always created using UNIX time 0 as a base, meaning, no matter if you start the pipeline at 6:31, 6:32 or 6:35, the windows would always be [6:30, 6:35), [6:35, 6:40).... Note that this also applies for days, the windows would start at 00:00 UTC.

If you want to change this, there's an offset parameter.