Lock contention on hot row
A real-time ad-bidding service runs fine at 8k req/s but at a marketing event traffic ramps to 11k req/s and latency doesn't degrade gracefully — it falls off a cliff: median goes from 12ms to 900ms over about 90 seconds, then stays there even as you add pods. The DB shows enormous `lock_wait` time concentrated on UPDATEs to a single `campaign_budgets` row for the headline campaign that's getting 70% of the event traffic. CPU per pod actually drops as you scale out. Adding capacity made it worse. Triage and explain the dynamics.
What a strong answer looks like
Stop the bleeding first (mitigate), then form hypotheses from real signals. Separate root cause from symptom, communicate status as you go, and close with what prevents a repeat.
0:00 of about 40 min
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.