Requirements

Functional Requirements

• Send push notifications (iOS/Android), SMS, and email
• Support different notification types (transactional, marketing, alerts)
• Users can opt-in/opt-out of notification channels
• Rate limiting per user to prevent notification fatigue
• Template-based notifications with personalization
• Support scheduled notifications

Non-Functional Requirements

• Latency: Push notifications delivered within 1 second
• Throughput: 10M notifications per minute peak
• Delivery: At-least-once delivery with deduplication
• Availability: 99.9% uptime
• Scalability: Support 500M registered devices

Back-of-the-Envelope Estimation

API Design

POST /api/v1/notifications/send
Request: {
  "user_id": "user_123",
  "type": "push",
  "template": "new_follower",
  "data": { "follower_name": "Alice" },
  "channels": ["push", "email"]
}
Response: { "notification_id": "n_456", "status": "queued" }

POST /api/v1/notifications/bulk
Request: {
  "user_ids": ["u1", "u2", ...],
  "type": "marketing",
  "template": "weekly_digest",
  "scheduled_at": "2026-06-21T09:00:00Z"
}
Response: { "batch_id": "b_789", "total": 100000 }

High-Level Architecture

Detailed Design

Notification Processing Pipeline

Rate Limiting

Prevent notification fatigue with per-user rate limiting:

Deduplication

Prevent duplicate notifications using idempotency keys:

Multi-Channel Delivery

Route notifications to the appropriate channel based on user preferences:

User Preferences:
{
  user_id: "u_123",
  channels: {
    push: { enabled: true, device_tokens: ["tok1", "tok2"] },
    sms: { enabled: true, phone: "+1234567890" },
    email: { enabled: true, address: "user@example.com" }
  },
  quiet_hours: { start: "22:00", end: "08:00", timezone: "America/New_York" }
}

Retry and Dead Letter Queue

For failed notifications, implement retry logic:

1. Immediate retry: Retry up to 3 times with exponential backoff
2. Dead Letter Queue (DLQ): Notifications that fail after all retries
3. DLQ processing: Manual review, alerting, or bulk retry
4. Fallback channels: If push fails, try email as fallback

Practice Exercises

1. Design: How would you implement a notification preference center where users can configure channel preferences, quiet hours, and notification types?

2. Scale: If the system needs to send 1M push notifications in 1 minute, estimate the number of worker instances needed assuming each worker can process 500 notifications/second.

3. Reliability: Design a system to guarantee exactly-once notification delivery. What challenges arise from network partitions and provider retries?

4. Analytics: How would you track notification delivery, open rates, and click-through rates? Design the analytics pipeline.

What to Learn Next