On-callHardoc-g100

Subject ThrottlingLevel Senior–Staff~40 minCommon in Reliability & on-call interviewsIndustries Technology

Question

Your API gateway fronts a fleet of services during an unexpected 3x traffic surge. Everything degrades uniformly: health checks, login, paid-customer API calls, and a cheap free-tier polling endpoint all slow to multi-second responses and timeouts, so even health checks fail and instances get killed and restarted, worsening the surge. There is no per-tier throttling — every request competes equally. CPU is pinned fleet-wide. You can't add capacity fast enough. How do you triage and stabilize service for what matters most?

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.