Design a Global Notification Service

"Design a distributed notification system capable of sending 100 million notifications daily across iOS/Android push, SMS, and email, ensuring guaranteed delivery for critical transactional alerts without overloading downstream vendors."

Notifications span critical two-factor authentication codes (which must arrive within 5 seconds) to bulk promotional marketing campaigns (which can take hours).

A production design must address disparate latency SLAs, third-party vendor rate limits, message deduplication, and user opt-out preferences.

Step 1 of 5

Requirements gathering

Which architectural and business requirements must the notification system satisfy?

This scenario leans on Message Queues, Idempotency and Retries and Scaling Basics. Reading one first is a recommendation, never a requirement — and doing the lab cold then reading is a perfectly good order.