Design a URL Shortener

A Goldman panel that hands you a URL shortener wants one small service built cleanly, with every choice inside it defended. Nobody is asking you to scale a planet. The prompt is short. Take a long URL, hand back a short code, and when someone visits that code, send them to the original. Two operations, one mapping, and a pile of decisions underneath that separate an engineer who has shipped a service from one who has only sketched one.

The shortener earns its place in this round because it forces the low-level judgment Goldman grades without ever needing a firehose of traffic. There is a real key-generation decision: how do you turn a URL into a short, unique code, and what happens when two codes want to be the same. There's a real storage decision. The redirect is read almost all the time and written rarely, so the store you pick and the cache in front of it are graded choices. There is a real correctness boundary: a code must map to exactly one URL forever, because a link that silently changes where it points is worse than a broken one. Get those right and the design shows you've owned a service. Reach for a giant distributed diagram and you have answered a question nobody asked.

The Goldman Sachs angle

Goldman runs this as a low-level design round, so build the object model and defend every choice inside it. The panel is a lead who has written services like this, and the round is senior-gated: it appears from Associate up and carries real weight at VP level. That shapes what lands. Name the pieces, a code generator, a mapping store, a cache, a rate limiter, and say why each one earns its place. Pick a store and say what it costs you. Then hold the line when the interviewer narrows in, because this round bleeds into fundamentals fast. How does the counter behave under many threads? What does the database guarantee on a unique constraint? Why is the redirect a cache-first read?

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