Practice: Design an Online Judge (LeetCode)

Online coding judge inspired by LeetCode and Codeforces: submissions saved and queued per language, untrusted code run in throwaway container sandboxes with no network and fixed limits, hidden test cases in object storage, verdicts polled from a cache, live contest rankings in a sorted set, and judges scaled ahead of contests. A teaching design.

Build this system yourself on the canvas below. Add components, connect them, and run traffic simulations to see where your design holds up and where it breaks.

Functional requirements

  • Users can browse and read problems.
  • Users can submit code in several languages and get a verdict (accepted, wrong answer, time limit, runtime error).
  • Submitted code runs in isolation and can never read hidden test cases or reach the network.
  • Users can see their past submissions.
  • Contests show a live ranking by problems solved and penalty time.

Non-functional requirements

  • Problem page latency: p99 < 300ms.
  • Verdict within 10 seconds for most submissions, even during contests.
  • Scale: 2M daily users; contests of 50K people submitting in the final minutes.
  • Availability: 99.9%; a submission once accepted is never lost.
  • Security: untrusted code is fully isolated from other submissions and the platform.
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System Design Interview
Round 1 of 4 · Architecture Design · 60:00 remaining
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Phase
Build & Stress Test
Phase 01: Thinking in Systems
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Components

User
CDN
Load Balancer
Server
Cache
Database
Blob Storage
Search Index
Queue
Worker
Rate Limiter
Service
API Gateway
Reverse Proxy
WebSocket Server
Third-party API

Inspector

Notes
Use clear names so your design intent is easy to understand.
Good
Name by business meaning
"Order API", "Restaurant Service"
Avoid
Generic names = zero signal
"Server 1", "API", "Queue"
A short description for each component makes feedback much better.
100%
Start by identifying:
  • Users & entry points
  • APIs & services
  • Databases & storage
  • Traffic flow & scale

Round 1 of 4 Architecture Design

Run a simulation to see results.

Time Remaining
60:00
System Design Interview

What are the core functional requirements?
What are the key non-functional constraints?

Questions

Start Evaluation to unlock questions.

EVALUATE MODE

Test yourself like it's the real thing.

A structured 4-module evaluation that mirrors how top companies assess system design candidates.

Architecture Design
Draw your system on the canvas. Define components, connections, and data flow.
FR & NFR Requirements
Answer functional and non-functional requirement questions about your design.
MCQ Round
Multiple choice questions testing your depth on the chosen system.
Tradeoff Analysis
Justify your design decisions and defend your architectural tradeoffs.
AI Report Generated
Choose a Problem
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Round 2 of 4
MCQ Round
Answer multiple-choice questions based on your design.

Exit Interview?

You're in the middle of an interview session. Leaving now will end your current attempt.

Your progress will be saved.

Open a saved design

Select a design to load into the canvas.

My Evaluations

Your past evaluation sessions

Here’s a simple request flow that follows the expected layer order.

External User
→
Edge CDN → API Gateway → Load Balancer
→
Compute App Servers / Services
→
DataAccess Cache
→
Storage Database / Search Index
→
Async Queue → Worker

Tip: keep arrows moving forward through layers (Edge → Compute → Storage). Avoid sending storage back to compute.

Evaluation Instructions

Read the rules carefully before starting. The test auto-submits on refresh.

Before you start

  • Build your architecture on the canvas. The timer starts when you click Start Evaluation.
  • Don't forget to answer Functional Requirement and Non Functional Requirements.
  • When satisfied with your design, click Next to lock it and view the questions.
  • Please answer final step questions to complete the evaluation.

Dos

  • Do read each question carefully before answering.
  • Do include required components to maximize component coverage.
  • Do save a copy of your design if you want to keep it before submission.

Don'ts

  • Don't refresh or close the tab during an active evaluation — this will auto-submit your answers.
  • Don't switch app modes or open another tab while the evaluation is running.
  • Don't attempt to edit the design after clicking Next; the workspace will be locked.

All the best!!

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