System design interview question
Design a Real-time Notification Delivery System with Guaranteed Delivery and User Preferences
Design a distributed notification delivery system that handles 500M active users across 3 global regions, processing 2M notification events per second with <500ms P99 latency from trigger to delivery. The system must support multiple delivery channels (push, email, SMS, in-app), respect user notification preferences and quiet hours, handle device roster changes in real-time, provide at-least-once delivery guarantees, and survive regional failures without data loss. Key constraints include managing 8B push subscriptions (15 devices per user on average), variable payload sizes (1KB500KB), burst loads during viral events (10x baseline), and the requirement to suppress duplicate notifications within a 5-minute window across all channels. You must design for extensibility to support new channels and preference types without re-architecting, and ensure that user preference changes take effect within 30 seconds globally.
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