8.11 Design a URL Shortener at Scale

A URL shortener stores one mapping, from a short code such as aZ3kP9 to a long URL, and turns every visit to sho.rt/aZ3kP9 into a redirect. Creating a link is rare and simple. Redirects arrive constantly, must be fast, and are very unevenly spread: a handful of viral links receive most of the traffic, while scanners and bots request codes that do not exist.

The design is therefore built around the read path. Cache-aside serves the popular links, a Bloom filter rejects nonexistent codes cheaply, and click counters stay off the path that the redirect waits for.