AI & Technical question
Design a URL shortener like Bit.ly
- AI & Technical
- Medium
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What this question tests
Tests systems design fluency for a well-known, simple product: identifying the core technical components and the product decisions layered on top of them.
How to approach it
- Clarify the core function: take a long URL and generate a short, unique alias that redirects to it when visited.
- Cover the technical core: a hashing or counter-based method to generate a short, unique key, and a key-value database mapping the short key to the original URL.
- Cover redirection: when a user visits the short URL, the service looks up the original URL and issues an HTTP redirect, which needs to be extremely fast and highly available.
- Cover product-layer decisions: custom aliases, link expiration, click analytics (a major value driver for a product like Bit.ly, not just a technical utility), and QR code generation.
- Cover abuse prevention: detecting and blocking malicious or spam links, since a URL shortener can be exploited to mask harmful destinations.
- Define success: redirect latency (should be near-instant), uptime, and for the product side, link creation and click-through engagement from analytics users.
What a strong answer includes
- Separates the technical core (hashing, key-value lookup, redirect) from the product layer (custom aliases, analytics, expiration), showing awareness this is a product, not just infrastructure.
- Names click analytics as a major value driver, since that is actually what differentiates a product like Bit.ly from a purely technical URL-shortening utility.
- Flags abuse prevention as a real requirement, since URL shorteners are a known vector for phishing and malicious link masking.
- Prioritizes redirect latency and uptime as the core guardrail metrics, since the entire product fails if redirects are slow or unreliable.
Common mistakes
- Describing only the technical hashing mechanism without addressing the product value layer like analytics and custom links.
- Ignoring abuse and spam prevention, a real and well-known risk for URL shortening services.
Likely follow-up questions
- How would you generate short keys to avoid collisions at scale?
- How would you prioritize analytics features against expanding into a mobile app for the product?
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Learn the skill behind it
Chapters of the AI PM course that teach what this question tests.
- Chapter 1: Foundations: the model and the decisions it forces on you
- Chapter 8: Evals: define good and make the number defensible
- Chapter 6: Agents and agentic architecture