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Design a notification system for a large-scale application (100M users).
Notification storage: Cassandra partition by (userId, type); most recent first Unread count: Redis HINCRBY notifications:{userId} total 1 Push notifications: SNS/FCM for mobile push; separate queues per channel type Real-time delivery: Redis Pub/Sub or WebSocket for in-app notifications Preference management: MongoDB for user notification preferences (complex nested) Rate limiting: Redis per-user
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