Design a Notification Service

"Design a notification service" is one of the prompts Amazon reaches for most. It fits how Amazon interviews. The prompt is a real service some team at Amazon runs day to day, rather than a textbook "design Twitter." One feature ships several things from the same backend: a marketing email, an order-shipped push, a delivery SMS, and an in-app badge. The interviewer hands you the broad shape, then picks one piece and pushes.

Anchor on the numbers first. Say the service serves 100 million daily active users, and each gets on the order of five notifications a day across channels. That is about 500 million sends a day, which averages to roughly 6,000 sends a second. Averages lie here. A single campaign ("your order shipped" to everyone who bought during a sale) can dump tens of millions of notifications into the system in one burst. So design for the spike rather than the average. Lead with that.

The Amazon angle

This prompt lands differently at Amazon for two reasons. The round is scored on the six dimensions Amazon publishes, and reliability and resiliency outweigh raw scale. So an interviewer cares more about "does the shipped-confirmation really arrive" than about a clever sharding trick. And Leadership Principles are graded inside the design round itself, never siloed into a separate slot. Customer Obsession shows up the moment you ask whether a dropped notification annoys a customer or strands a payment. Dive Deep is the interviewer asking exactly how a retry avoids sending the same SMS twice. Treat "what happens when this fails" as the main event, because at Amazon it is.