System design interview question
Design a Global Real-time Notification System with Delivery Guarantees (like Firebase Cloud Messaging at scale)
Design a distributed notification delivery system that serves 500M users across 6 continents, handling 2M notifications per second with strict latency requirements (<100ms for critical notifications) and at-least-once delivery semantics. The system must support multiple channels (push, email, SMS, in-app), handle device state changes dynamically, implement intelligent retry logic with exponential backoff, and provide real-time delivery status tracking. Consider a heterogeneous device ecosystem (iOS, Android, Web), network unreliability, device connectivity variations across regions, and the need to prevent notification storms while maintaining user engagement metrics.
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