# The Quiet Partition > One went quiet. All of them stopped. Canonical URL: Domain: ai_coding · Difficulty: hard · Seniority: L6 ## Problem Our Flink job aggregates user sessions and writes hourly rollups. Each midnight one source partition (the EU one) falls quiet for ~2 hours during low-traffic. During that window the global watermark stops advancing, no session windows close, and state size grows unbounded until the JM kills the job. Run tests/test_quiet_partition.py to reproduce. Find the watermarking pattern in jobs/sessions.py and fix it. Defend the new behavior: what's the maximum end-to-end latency you've now committed to? ## Related - [All practice problems](https://datadriven.io/problems) - [Mock interview mode](https://datadriven.io/interview/flink-watermark-stalls-on-quiet-partition) - [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.