Design LaunchDarkly / Unleash (Real-Time Feature Flag Service at Scale)
Fifty thousand backend microservices evaluate flags on every HTTP request with zero network latency, while millions of mobile apps receive real-time toggles within 500 milliseconds. How a feature flag system splits server-side from client-side evaluation, prevents thundering herds with Server-Sent Events (SSE), and guarantees sub-microsecond local evaluations.