Flink TumblingEventTimeWindows how achievement without overlap?

Question

There is this text in Stream Processing with Apache Flink page 211

“The WindowAssigner determines for each arriving element to which windows it is assigned.”

then I study source code of TumblingEventTimeWindows

public class TumblingEventTimeWindows extends WindowAssigner<Object, TimeWindow> {
    private static final long serialVersionUID = 1L;

...............................

    @Override
    public Collection<TimeWindow> assignWindows(
            Object element, long timestamp, WindowAssignerContext context) {
        if (timestamp > Long.MIN_VALUE) {
            if (staggerOffset == null) {
                staggerOffset =
                        windowStagger.getStaggerOffset(context.getCurrentProcessingTime(), size);
            }
            // Long.MIN_VALUE is currently assigned when no timestamp is present
            long start =
                    TimeWindow.getWindowStartWithOffset(
                            timestamp, (globalOffset + staggerOffset) % size, size);
            return Collections.singletonList(new TimeWindow(start, start + size));
        } else {
            throw new RuntimeException(
                    "Record has Long.MIN_VALUE timestamp (= no timestamp marker). "
                            + "Is the time characteristic set to 'ProcessingTime', or did you forget to call "
                            + "'DataStream.assignTimestampsAndWatermarks(...)'?");
        }
    }

...............................

from the source code I can found , It is true that elements are assigned to the window ,new TimeWindow(start, start + size) meanning each element be assigned a new TimeWindow.

but I am confused, TumblingEventTimeWindows how achievement without overlap?

if every element be assigned a new TimeWindow, the results are as follows

There is no guarantee that each window will not overlap , Can someone point me in the direction of TumblingEventTimeWindows how achievement without overlap?

Answer 1

The TimeWindow object isn't very important. It is a simple structure that holds the start and end timestamps for the window, and nothing else. It's name makes it sound important, but it's just used to encode a copy of the information describing the time interval the incoming event is being assigned to.

It's actually the WindowOperator that has the important window data. Logically it's keeping something like a map, where the keys are the intervals described by the TimeWindow objects, and the values are the lists of events assigned to those intervals.