System design interview question
Design a Real-time Notification Service like Firebase Cloud Messaging
Design a notification delivery system that sends push notifications to mobile devices at scale. Your system must handle 1 million active users, with 100,000 notifications sent per second during peak hours. Notifications must be delivered reliably within 5 seconds of being triggered, and the system must support multiple platforms (iOS, Android, Web). Users may be offline, so you need to queue notifications and retry delivery when they reconnect. The system should track delivery status (sent, delivered, failed) and allow clients to query their notification history. Consider how you'd handle failures, ensure no duplicate notifications are sent, and maintain low latency for the critical path.
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