On-callMediumoc-g017

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

Question

At 15:00 your MongoDB primary's CPU hits 95% and stays there; query latency across the whole app degrades, not just one feature. mongostat shows a surge in scanned documents far exceeding documents returned, the page-fault and disk-read rate is up, and the working set no longer fits in RAM. A product change last week added a new filter on an existing collection (filtering by a 'region' field). No index was added for it. Traffic is normal. How do you triage and mitigate?

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

Loading whiteboard…

Run or narrate your approach, then ask the coach.