Capstone: a global flash sale for MegaMart, in 45 minutes

A full mock interview: 5 million users chasing 10,000 units in 10 minutes. Clarify, estimate 150,000 requests a second and 80,000 notifications a second, sketch the architecture, then deep-dive into atomic claims in Redis, a checkout that never double-charges, 2.4 million notifications in seconds, and a sale scheduler that starts each sale once. Every Week 1 and Week 2 idea in one answer.