On-callMediumoc-g501

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

Question

Your app uses a cloud vendor API that enforces a SEPARATE quota per region. At 14:00 a feature degrades for users in one region (eu-west) — calls there error at ~60% with 429 'quota exceeded' — while the same feature works perfectly for US and APAC users. Dashboards: your GLOBAL call volume to the vendor is well under your overall quota, but the eu-west regional endpoint is pinned at its per-region cap; eu-west traffic grew after a product launch there last week. No code change today. 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.