System design interview question
Design a Distributed Feature Flag and A/B Testing Platform (Like LaunchDarkly at Scale)
You are designing a feature flag and experimentation platform that serves 50,000 production clients across the globe, managing flags for 200M end-users with sub-100ms evaluation latency requirements. The system must support real-time flag updates with eventual consistency across all edge locations, handle 500K flag evaluations per second at peak, and provide detailed analytics on experiment results with strong consistency guarantees for billing and reporting. Clients evaluate flags locally with cached rules, but flag mutations must propagate globally and consistently. Design the complete system including flag evaluation SDKs, real-time sync mechanisms, distributed state management, analytics ingestion and querying, and failover strategies for mission-critical flag decisions.
Practice this question step by step with an AI interviewer that reviews your requirements, your architecture and your trade-offs, and scores the design.