Design an HTTP router
PremiumMachine codingMedium
- Radix tree
- Path parameters
- Route priority
Quick answer
This machine coding interview question asks you to design an HTTP router. Aim for a design that meets its requirements at the stated scale, then explain its main components, data flow and trade-offs. Interviewers want to see how you scope the problem, size it and defend each choice.
The problem
Build an HTTP router, the part of web frameworks that maps a request like GET /users/42/orders to the handler for /users/:id/orders. It matches static paths, parameters and wildcards, picks the most specific route and runs middleware. The hard parts are priority rules, 404 versus 405 and fast matching across thousands of routes.
No server is started. Tests build a request object (method, path, headers) and call router.handle(req), which returns a response object.
What must an HTTP router do?
- Register: add(method, pattern, handler) with static segments (/users), parameters (/:id) and one trailing wildcard (/*path).
- Match: handle(req) finds the route and passes params, such as {id: '42'} for /users/42. The query string is not part of the path, and a trailing slash is ignored.
How well must it do it?
- Fast: Matching cost depends on path length, not route count; 10,000 routes still match in microseconds.
- Order-independent: The same routes give the same matches whatever order they were added in.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Radix tree
- Path parameters
- Route priority
- Middleware chain
- HTTP methods
- Trie
How you’ll practise
- Requirements
- Entities and interfaces
- Core implementation
- Edge cases and tests
- Extensions
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any machine coding design round.
- Clarifies the requirements before drawing anything
- Uses a few numbers that actually change the design
- Draws a clear path from the client to storage
- Picks storage and caching with reasons, not by habit
- Explains what breaks first under load and how to fix it
- States trade-offs openly instead of claiming a perfect design
Questions people ask about this interview question
- Which design type is this question?
- It is a machine coding question. You design an HTTP router and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Medium. Entry-level engineers can practise it, but it expects you to reason about requirements and trade-offs, not only list components.
- What should I cover first?
- Start with the requirements: what the system must do, how well it must do it and which numbers change the design. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
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Updated · Written by the SevyDevy team