Design a multi-channel notification system
Company: DoorDash
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Take-home Project
Design a notification system that receives alert events from other internal systems and notifies all subscribed users through multiple channels (e.g., in-app/push, email, SMS).
Your design should address:
- Core requirements and APIs (publish alert, manage subscriptions/preferences, send notifications)
- Fanout to many subscribers
- Multiple delivery channels with different providers
- Reliability (retries, dead-letter queues), ordering/deduplication, and idempotency
- Rate limiting and user preferences (opt-out, quiet hours)
- Observability and operational concerns
Assume high volume (millions of alerts/day) and that different alert types may have different urgency and SLAs.
Quick Answer: This System Design question evaluates distributed-systems and API-design skills—scalability, reliable fanout, multi-channel integration, delivery guarantees (retries, idempotency, deduplication), rate-limiting, subscription/preferences modeling, and observability.