On-callHardoc-g210

Subject Split brainLevel Senior–Staff~45 minCommon in Reliability & on-call interviewsIndustries Technology, Software development

Question

Your inventory service runs active-active across two regions with async bidirectional replication. After a 4-minute cross-region link partition at 03:00, you oversell: an item with 1 unit in stock got sold to two different customers. Dashboards: during the partition, region-A and region-B each independently decremented the same item's stock to 0; after the link healed, replication 'merged' and the count went negative; both regions accepted a checkout for the last unit. No deploy. 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

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Run or narrate your approach, then ask the coach.