Design a Notification System

Create a multi-channel notification system that can send messages via email, SMS, push notifications, and in-app alerts.

Functional Requirements

• Send notifications across multiple channels (email, SMS, push, in-app)
• Support templated messages with personalization
• Allow user preferences for notification channels and frequency
• Batch and digest notifications to prevent spam
• Track delivery status and engagement metrics

Non-Functional Requirements

• Handle 1M+ notifications per day
• Delivery within seconds for urgent notifications
• At-least-once delivery guarantee
• Support for scheduled and recurring notifications

Questions to Consider

• How will you handle channel-specific formatting and rate limits?
• What queue architecture will ensure reliable delivery?
• How will you deduplicate notifications?