Step 1: System Definition

At exactly noon, a sneaker release opens with 5,000 pairs in stock, and several hundred thousand people already have the product page open. In the first second, far more purchase requests arrive than there are pairs. A minute later, the stock is gone. In that minute, the system has to decide, fairly and without a single mistake, which 5,000 requests win.

A flash sale system is the part of an e-commerce site that runs sales like this one: a small, fixed stock sold to a very large crowd in a short window