On-callMediumoc-g536

Subject Db unreachableLevel Entry–Mid~20 minCommon in Databases & SQL · Code quality & review interviewsIndustries Technology, Software development

Question

A single backend service starts returning 500s on every request that touches data. The logs are full of 'connection refused' / 'could not connect to database' errors, and the service's own health check (which pings the DB) is failing. The database dashboard shows the DB instance itself is up and other services are still querying it fine. Your service didn't deploy. What's your hypothesis, and how do you triage?

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.