System design interview question
Design a Distributed Feature Flag and A/B Testing Platform like LaunchDarkly at Scale
Design a platform that allows engineers to manage feature flags, conduct A/B tests, and perform gradual rollouts across millions of users and thousands of services globally. The system must handle 50M daily active users, support 100K+ feature flags across 500+ client applications, and serve flag evaluation requests at sub-millisecond latency (p99 < 5ms) with 99.99% availability. Flags must be evaluated on the client-side with minimal network overhead, support complex targeting rules (user segments, geographic regions, custom attributes), and propagate updates globally within 2-3 seconds. The system must audit all flag changes, support real-time analytics on flag impact, and handle both synchronous (REST/gRPC) and streaming flag update mechanisms.
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