On-callMediumoc-g009

Subject Cache incidentsLevel Mid–Senior~30 minCommon in Reliability & on-call interviewsIndustries Technology, Software development

Question

At 18:00 exactly, every minute on the minute, your homepage feed service spikes: Redis cache-hit ratio briefly craters, the Postgres origin sees a burst of identical heavy 'trending posts' queries, p99 latency triples for ~10 seconds, then recovers. This started after yesterday's deploy that introduced a 'trending' module cached with a 60-second TTL and a fixed expiry aligned to the wall clock. How do you triage and fix this?

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.