Watermarks: The Completeness Heuristic

A watermark is the system's assertion: 'I believe all events with event_time <= W have been received.' When the watermark advances past the end of a window, the window closes and results are emitted. Events arriving after the watermark with event_time inside a closed window are 'late.' How Watermarks Work The Latency-Correctness Tradeoff The strongest insight: 'A watermark is not a guarantee. It is a heuristic. Events can arrive after the watermark, and the system must have a plan for them. That plan is allowed lateness + side outputs.' This shows you understand that watermarks are probabilistic, not absolute.