On-callHardoc-g058

Subject Version skewLevel Senior–Staff~40 minCommon in Networking & APIs interviewsIndustries Software development

Question

At 15:00 you deploy backend v77, which changes a JSON API response: the field 'is_premium' (boolean) is replaced with 'subscription_tier' (string). Web clients update instantly and are fine. But over the next 6 hours, crash reports from the iOS app climb steadily, all on a null-pointer reading 'is_premium', and they're disproportionately from older app versions. App Store review for the new client build is still pending. 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

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Run or narrate your approach, then ask the coach.