Unit 43 · Module 11 · Putting it together
A full system design, end to end
One question, answered the way an interviewer wants to hear it.
Unit 43 of the free 49-unit computer-science course, in putting it together. 1 topic to watch or read and a short self-check.
Watch or read
One topic makes up this unit. Take each one whichever way suits you, then answer the questions below.
Design a URL shortener, end to end
'Design a URL shortener' is the classic system-design warm-up, and most candidates answer the wrong question. The interviewer is not testing your hash function — they want a data-store choice, defended. Here is the whole answer in four moves: base62-encode a hash into a short code, store code → URL as one key and one value in a KV store, handle collisions with a retry or a monotonic counter, and serve the redirect. Then the follow-up that separates a rehearsed answer from a real one: a 301 is cached by the browser, so your click counter flatlines and you can never repoint the link. A 302 costs a request per click — and that request is the product.
Self-check — 3 questions
Answer alone, at 2am, with no interviewer in the room.