Software developer interview: how would you design a URL shortener like bit.ly?

Model answer

Treat this as a structured system-design conversation, not a code dump. Start by clarifying requirements and scale: shorten a long URL to a short code, redirect quickly, and note it is read-heavy since reads far outnumber writes. Sketch the core flow — generate a unique short code, store the mapping from code to long URL, and on a request, look up the code and redirect. Discuss how to generate the code: a base-62 encoding of an auto-increment id, or a hash with collision handling; explain the trade-offs. Cover the data store — a key-value or relational store keyed by the short code — and why. Then add the parts that show maturity: caching hot URLs to speed up redirects, handling collisions and custom aliases, expiry, and basic analytics like click counts. Mention scaling levers such as read replicas and a CDN or cache layer. Talk about edge cases — invalid codes, very high traffic, and abuse. Throughout, state assumptions and reason aloud; interviewers grade how you structure the problem and handle trade-offs far more than getting one perfect answer.

This is a guide to structure and approach, not a script to memorise — say it in your own words.

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