O que é System-design interview, capacity estimate?

A system-design interview asks the candidate to design a system from a one-line description within a fixed time, and assesses how requirements are established, how the design is sized, how its components and interfaces are chosen, and how it fails. A capacity estimate is the back-of-envelope calculation of the load a design must carry (messages a second at peak, bytes a day, memory, connections) from stated assumptions, used to choose between designs.