Design a URL Shortening Service: Code Generation, Redirect Path, and Scaling
xAISoftware Engineer · Onsite
Last updated
A system design question asked as a 30-minute screen with no coding: design a URL shortening service that turns long URLs into short links and redirects visitors. It tests requirement scoping, capacity estimates, collision-free code generation, a fast read path, storage, scaling and failure handling.
Design a URL shortening service. A user submits a long URL and gets back a short link; anyone who opens the short link is redirected to the original URL.
This is the whole interview: a 30-minute technical screen with no coding, so the design discussion itself is what is evaluated. Aim for a complete end-to-end design first, then go deep where it matters most.
Constraints and Clarifications
- No code is expected. Requirements, estimates, APIs, a data model and an architecture are.
- The session lasts about 30 minutes, including requirements and questions.
Clarifying Questions
- How many links are created per day, and how many redirects are served? What is the ratio of reads to writes?
- Can users choose custom aliases? Do links expire, and can they be deleted?
- Should the same long URL always map to the same short code, or get a new code each time it is submitted?
- Are click analytics required, and how fresh must they be?
- How short must the codes be, and which characters may they use?
- Must the service defend against abuse, such as spam or links to malware?
Hints
Hint 1
Start from the traffic shape: Estimate how often links are created versus opened, and let that ratio decide where you spend most of your design time.
Hint 2
Where codes come from: Compare at least two ways of producing short codes, and ask what each one must coordinate across servers so that two long URLs never receive the same code.
Follow-up questions
- One short link suddenly receives a flood of clicks. What happens in each layer, and what would you change?
- How would you add per-link click analytics without slowing down redirects?
- How would you support links that expire, and should expired codes ever be reused?
- How would you stop the service from being used to disguise malicious URLs?
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