8 units · 31 bite-size lessons · a boss round after every unit. Built for full-stack interviews, where the question is rarely “how do you write it?” and almost always “why this and not that — what does it cost, and where does it break?” Every lesson teaches one decision, names its limits (latency, throughput, throttles, scale ceilings), applies it to a fictional case study, and ends with what a strong spoken answer sounds like.