System design interview question
Design a Global Real-Time Notification Delivery System (like Firebase Cloud Messaging at scale)
Design a distributed notification delivery system that serves 500M users across 6 continents, handling 2M push notifications per second with strict latency requirements (P99 < 100ms for delivery initiation). The system must support multiple notification types (push, email, SMS, in-app), handle user opt-in/opt-out preferences, provide delivery guarantees (at-least-once semantics), and maintain a complete audit trail of all notifications sent. Users expect near-real-time delivery on mobile devices with varied network conditions, and the system must gracefully handle spiky traffic (10x baseline during major events) while maintaining cost efficiency and enabling sophisticated targeting (by geography, user cohorts, A/B tests).
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