| watermark too fast | lost/side-output late events |
| watermark too slow | delayed output and state growth |
| hot key | key splitting or two-stage aggregation |
| unbounded session | timeout and state TTL |
| non-idempotent sink | transactional or deduplicated commit |
| Track watermark lag, late-event rate, state bytes/keys, checkpoint duration, restore time, and backpressure. Test idle partitions because one silent source can hold back a global watermark. | |
| ## Test yourself | |
| 1. How can an idle partition stop progress? | |
| 2. What bounds state growth? | |
| 3. How do corrected late results reach a sink safely? | |
Continue to professional.md. | |