Design LeetCode with concurrent contests
PremiumProduct architectureHard
- Online judge
- Code execution sandbox
- Judging queue
Quick answer
This product architecture interview question asks you to design LeetCode with concurrent contests. 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
Design LeetCode with concurrent contests: an online judge where many timed coding contests run at once. Users read problems, submit code that runs in a sandbox against hidden tests, and watch a live leaderboard. The hard parts are the spike at contest start and judging fairly under heavy load.
A weekly contest has 60,000 participants and 4 problems over 90 minutes, while company and college contests run alongside it. At the start second everyone opens the problems at once, and in the last 15 minutes submissions triple. A slow queue must not cost anyone rank, and problems must stay hidden until the start.
What must the system do?
- Contest lifecycle: Users register before the start; problems unlock for them at the exact start time and submissions close at the end.
- Submit and judge: Code runs in a sandbox against hidden tests and returns a verdict: accepted, wrong answer, time limit, memory limit, runtime error or compile error.
How well must it do it?
- Fairness: Penalty time uses the moment the server received a submission, never the moment judging finished.
- Verdict latency: p95 verdict within 10 seconds during contests, including the final rush.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Online judge
- Code execution sandbox
- Judging queue
- Live leaderboard
- Redis sorted sets
- Traffic spikes
How you’ll practise
- Requirements
- Core entities
- API design
- Client and server flow
- Scale and edge cases
- 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 product architecture 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 product architecture question. You design LeetCode with concurrent contests and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Hard. It suits engineers preparing for senior interviews, where you are expected to drive the discussion and go deep on trade-offs.
- 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