how flink handles early event? ignore or create separate window?

Question

The watermark and late event handling is easy to understand, but how about early event? For example, if the original stream contains events happened from 3:00 to 4:00, but if I insert some events which happened from 6:00 to 7:00 into the stream, then how flink handles them? It would create separate window(s) for them and when the window expires, they get handled too?

Answer 1

Depending on the watermarking strategy, early events can advance the watermark and then cause subsequent "on time" events to be considered late.

Answer 2

Early events are not dropped but put into the corresponding window. The window is processed when the watermark passes the end timestamp of the window. So, Flink is able to maintain several windows at the same time.