Flash sale. Concert tickets

Concert tickets sell out in minutes: a waiting room admits as many fans as the hot row can take, a conditional UPDATE and holds with a TTL prevent overselling, and a second date gets its own row and its own queue.

Diagram of a flash sale: a waiting room holds back the crowd and admits a steady stream to one hot row, where holds with a TTL stop overselling.

System Design — Load balancers, caches, queues and regions: build a system that holds the load