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Different Notions of Time

Some operations, such a as time-windows and ordering by time, require a notion of time. In these operations the user has the choice between two approaches:

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When using element timestamps to order elements or perform windowing, it becomes necessary to keep track of the progress of time. For example, in a windowing operator the window from t to t + 5 can only be processed once the operator is certain that no more elements with a timestamp lower than t + 5 will arrive from any of its input streams. Flink uses so called watermarks to keep track of the timestamp of tuples passing through the system: when a source knows that no elements with a timestamp lower than t1 will be emitted in the future it will emit a watermark with timestamp t1. Watermarks are broadcast to downstream operators. The watermark at once one specific operator at any point in time is the minimum watermark time over all its inputs. Once that watermark increases the operator can react to that and further broadcast the watermark to downstream operators. In our windowing example from earlier, the operator would wait for the watermark for time t + 5, then process the window and then broadcast watermark t + 5 itself.

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