System design interview question
Design a Distributed Real-Time Notification Delivery System like Firebase Cloud Messaging at Scale
Design a system that delivers billions of push notifications daily to 500M mobile devices across 150+ countries with <5 second latency guarantees and 99.95% delivery reliability. The system must handle variable traffic patterns (peak 2M notifications/second), support rich notification types (text, images, deep links), enable user targeting based on complex criteria (device type, location, user segments), and maintain delivery guarantees even during partial infrastructure failures. You must architect for multi-tenant isolation, compliance with regional data residency requirements, and cost-efficient operation across heterogeneous device types (iOS, Android, Web).
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