Design a Notification System

The Brief

Design a system that reliably delivers notifications to users across push (FCM/APNs), email, SMS, and in-app channels. Users have preferences. Notifications must not duplicate. The system must handle massive fan-out (a single event can trigger millions of notifications) and gracefully retry failures.

Scale

Requirements

• Send push (FCM/APNs), email, SMS, and in-app notifications
• Users can set channel preferences per notification type
• Deduplicate: same notification never delivered twice
• Retry failed deliveries with exponential backoff
• Rate-limit per user (max N notifications per channel per hour)
• Support scheduled and triggered notifications

Constraints

• Third-party rate limits: FCM, Twilio, SendGrid all have caps
• No message loss — durability is required
• System must survive a single region going down
• Preference lookups must not become a bottleneck at fan-out