Step 3: Capacity Estimation

A flash sale lasts minutes, so daily averages tell us nothing. Every estimate below describes the opening burst.

Concurrent users. Assume a peak of about 5 million users on the site at once. If half of them act in a given second, that is about 2.5 million operations a second. Browsers and the CDN absorb part of that, so plan for up to 1 million requests a second reaching our servers in the opening burst.

Reads and writes. Assume 90% reads and 10% writes. At 1 million requests a second, about 900,000 are reads: product pages, stock status, and queue position checks