Design a URL shortener
Understanding the problem
In two or three sentences: what does a URL shortener do, and who uses it? Give one real example.
Requirements
Functional (in scope)
What are the 2 or 3 things a user must be able to do? (Hint: create, and…?)
Out of scope
Which features will you deliberately skip and say so? (custom aliases, analytics, expiry, accounts…)
Non-functional
Put numbers on these: how many new links per day? How many redirects per day? What redirect latency is acceptable? Is availability or consistency more important here, and why?
Entities and API
What are the core entities? Sketch the fields of each.
Write the API for each functional requirement: method, path, request, response.
POST /urls
GET /{code}High-level design
Start with the simplest design that meets the functional requirements. Draw it, then walk through one create request and one redirect request.
Question to answer: should the redirect be a 301 or a 302? What changes for you?
Deep dives
1. Generating short codes
How do you produce a unique short code? Fill in the options you know and what breaks at scale.
| Option | Pros | Cons | Pick when |
|---|---|---|---|
| Hash the URL and truncate | |||
| Random string + uniqueness check | |||
| Counter + base62 |
How long must the code be for your scale? Show the maths.
2. Making redirects fast
Reads far outnumber writes. What do you add to keep redirects quick, and what do you have to think about when data changes?
3. Scaling storage and traffic
How much data is this after a few years? When does one database stop being enough, and how would you split it? Where do the load balancer and consistent hashing fit in?
What’s expected at each level
- Mid-level: what must they cover?
- Senior: what extra depth do you expect?
- Staff+: what would set them apart?
What I got wrong
Fill this in after you compare your design with other write-ups. Mistakes here are the most useful part of the page.
Changelog
- 2026-09-21: Skeleton created.