On-callHardoc-g220

Subject Partial outageLevel Senior–Staff~40 minCommon in Reliability & on-call · Distributed systems interviewsIndustries Technology, Software development

Question

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.

Learn the concepts

Diagram & narrate the incident

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Run or narrate your approach, then ask the coach.