On-callMediumoc-g249

Subject Slow dependencyLevel Mid–Senior~30 minCommon in Networking & APIs · Code quality & review interviewsIndustries Technology, Software development

Question

A previously-fast Node.js payment-callback service starts showing intermittent +200ms to +5000ms latency added to a fraction of requests, clustered in waves every ~30-60s. The slow requests all spend the extra time before the first byte is sent to the downstream payment API — the downstream itself reports fast service times for those same requests. CPU and memory are fine. The only recent change was migrating from a hardcoded IP for the payment API to a hostname. The internal DNS resolver's query rate has jumped. How do you 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.