One shard CPU at 100%
Your ledger DB is sharded by account_id into 64 shards. At 22:15 about 1.5% of users get 'service unavailable' on every action while everyone else is fine. Dashboards: one shard (shard-41) has CPU at 100% and query latency at 20s; its connection pool is exhausted; a single very-high-volume account (a large marketplace merchant) lives on shard-41 and a payout batch at 22:12 sent it a flood of writes; the other 63 shards are at baseline. No deploy. How do you triage and mitigate?
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.