Design a Notification System

You're designing a notification system for an e-commerce platform. Users receive notifications for order updates (shipped, delivered), promotional emails, SMS reminders, and push notifications on mobile. The system must be reliable (at-least-once delivery), support multiple channels (email, SMS, push), and throttle notifications to avoid overwhelming users.

Scale assumptions:

• 100 million daily active users (DAU)
• 20% of DAU receive at least one notification per day (20M notifications/day)
• Notification types: email (60%), SMS (20%), push (20%)
• Average latency: <5 seconds for push, <10 seconds for SMS, <1 hour for email
• At-least-once delivery guarantee
• Deduplication window: 1 hour (same event, same user, same channel = 1 delivery)
• Retention: notification delivery logs for 90 days

Derived numbers:

• Total notifications/day: 20M
• Average throughput: 20M ÷ 10^5 sec ≈ 230 writes/sec, ~750 peak
• Storage for logs (90 days): 20M × 365 × 90 × 500 bytes ≈ 328.5 TB (with replication)
• Channels: ~12M emails/day, ~4M SMS/day, ~4M push/day