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This document proposes the introduction of APIs that allow source operators (e.g., HybridSource, MySQL CDC Source) to send watermarks to downstream operators, indicating that the watermark should start increasing according to the system time. In addition to supporting processing-time temporal joins, this FLIP provides the fundation to simplifies DataStream APIs such as KeyedStream#window(...), such that users would no longer need to explicitly differentiate between TumblingEventTimeWindows and TumblingProcessingTimeWindows, leading to a more intuitive experience.

This document proposes the introduction of APIs that enable source operators (e.g., HybridSource, MySQL CDC Source) to send watermarks to downstream operators, signaling the start of increasing watermarks based on the system time. Apart from supporting processing-time temporal joins, this FLIP lays the groundwork for simplifying DataStream APIs, including KeyedStream#window(...), so that users would no longer be required to explicitly distinguish between TumblingEventTimeWindows and TumblingProcessingTimeWindows, resulting in a more intuitive user experience.

Terminology and Background

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And we can remove TemporalProcessTimeJoinOperator because its functionality is covered by  TemporalRowTimeJoinOperator.

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5) Update WatermarkToDataOutput to handle Watermark#useProcessingTime.

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