# A 1-minute tumbling window over event_time on a streaming aggregation needs to close Canonical URL: Domain: Pipeline Design ยท Difficulty: medium ## Problem A 1-minute tumbling window over event_time on a streaming aggregation needs to close. The wall clock has reached 09:01:25 and the window covers [09:00, 09:01). One late event with event_time 09:00:46 is still in flight in the broker. If the window closes on the wall clock the late event is dropped silently. If it never closes, no result emits. The section's compromise is the watermark: a timestamp the engine emits declaring no events with event_time earlier than the watermark will be processed against an open window. The strategy that fits this source is bounded out-of-orderness: watermark = max event_time seen minus a fixed lag. Place the watermark by adding a watermark generator node between the source and the aggregation whose name states the strategy and the lag value the engine uses. ## Related - [All practice problems](https://datadriven.io/problems) - [Mock interview mode](https://datadriven.io/interview/a-1-minute-tumbling-window-over-eventtime-on-a-streaming-ag-cb0b1f4a) - [System Design Interview Questions](https://datadriven.io/data-engineering-system-design) - [Data Engineering Interview Prep Guide](https://datadriven.io/data-engineer-interview-prep) - [Daily Challenge](https://datadriven.io/daily) --- Source: DataDriven (https://datadriven.io). 100% free data engineering interview prep. Live code execution against Postgres 16, Python 3.11, and Spark sandboxes. No paywall, no premium tier, no signup gate.