I Had No Idea What an Order Book Was. Now I've Built One That Handles 18k RPS.

18,210 requests per second. Zero failures. 1.46 million orders processed in 80 seconds on a single laptop that also has Chrome open. This post is for engineers who want to understand how a simple take-home assignment quietly became a study in distributed systems correctness, and how 2,000 req/s turned into 18,000 by fixing things that probably should have been right the first time.