A URL shortener — grow it only as far as its requirements justify.
Answer it out loud before you open anything. The value of the flags below is in comparing them to what you actually said — including whether you asked who uses it before naming what you would build it with.
The situation behind the question
The interviewer gives you the classic: "Design a URL shortener." Then adds: "Most candidates draw a distributed system in five minutes. I want you to start from one sentence and add each component only when a requirement forces it. Tell me when you stop."
What it is really testing
Whether the candidate can run the whole loop on a small problem: frame, requirements, invariants, data, first slice, and then grow the architecture only against stated requirements — and stop.