Design a notification system that delivers messages across multiple channels: push notifications (iOS/Android), SMS, email, and in-app notifications. The system serves 100 million users and sends 1 billion notifications per day. Different notification types have different priorities (e.g., security alerts are urgent, marketing emails are low priority). Users can configure per-channel preferences (e.g., "email me for weekly digests, push for direct messages, never SMS").
In your 90-second answer, cover: - High-level architecture and the notification delivery pipeline - How you handle multiple delivery channels with different latency characteristics - User preference management and opt-out compliance - Rate limiting and throttling to avoid spamming users - Reliability: ensuring critical notifications are not lost
Constraints: push notifications must arrive within 5 seconds for high-priority events. The system must support at-least-once delivery semantics.
How to approach it
- Hint 1
Decouple notification producers from delivery using a message queue. Producers should not need to know which channels to use -- a routing layer handles that based on user preferences.
- Hint 2
Each delivery channel (push, SMS, email) has vastly different throughput, cost, and reliability characteristics. Use separate worker pools per channel so a slow email provider does not block push delivery.
- Hint 3
For at-least-once delivery, persist notification state in a database before attempting delivery, and use retries with exponential backoff for transient failures.
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