On-callMediumoc-g686

Subject Networking cdnLevel Mid–Senior~35 minCommon in Storage & CDN · Networking & APIs interviewsIndustries Technology

Question

Your origin servers suddenly see a 5x jump in request rate and CPU saturation, with origin latency climbing and some 5xx errors, even though end-user traffic (requests hitting the CDN) is flat. CDN dashboards show the cache hit ratio dropped from ~96% to ~70% over 30 minutes, and origin-facing requests spiked. A deploy 40 minutes ago changed how the app sets cache headers and added a request-correlation query parameter to asset URLs for tracing. An 'origin shield' tier sits between edge POPs and origin. Triage and fix.

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.