System design interview question
Design a Real-time Notification System like Firebase Cloud Messaging
Design a distributed notification system that delivers messages to mobile and web clients in real-time. The system must support 1 million active users across multiple geographic regions, handle 50,000 concurrent connections, and deliver notifications with <5 second latency 99% of the time. Clients connect via WebSocket or long-polling and can be offline; offline messages should be persisted and delivered when the client reconnects. The system must reliably deliver notifications even during server failures and support different notification priorities and delivery guarantees (at-least-once vs. at-most-once).
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