Design a Notification System

Problem Design a notification system that delivers push, SMS and email notifications to users for events such as order updates, promotional offers and reminders.

Functional requirements

• Accept notification-worthy events from many upstream services.
• Render per-type, per-locale templates.
• Respect user channel preferences and unsubscribe/opt-out.
• Route to the right channel(s): push (APNs/FCM), SMS gateway, email provider.
• Rate-limit per user so a burst of events doesn't spam them.
• Retry failed deliveries and expose delivery status/history for an in-app notification center.

Non-functional requirements

• ~50M registered users; ~500M notifications/day → ~6k/sec average, with marketing blasts spiking to ~100k/sec for minutes at a time.
• Transactional notifications (OTP, order status) p99 < 5 s end-to-end; marketing sends may take minutes.
• 99.9% delivery success to the provider, excluding invalid tokens/numbers.
• Store 90 days of delivery history: ~45B rows, time-partitioned with TTL/archival.
• Per-provider throughput caps (e.g. an SMS gateway allowing 2k/sec) are a hard external constraint.

Areas to go deep

• Decoupling ingestion from delivery and isolating transactional from bulk traffic.
• Per-user ordering, rate-limiting, and idempotency under at-least-once delivery.
• Retry/backoff policy, invalid-token pruning, and delivery-status write amplification.