Practice: Design an AI Code Assistant (GitHub Copilot)

Inline AI code completion inspired by GitHub Copilot and Cursor: debounced requests, a gateway that cancels superseded requests, a small fast code model with continuous batching, repository context from an index of symbols and code snippets, a completion cache, routing between a fast and a larger model, and acceptance telemetry. 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

  • Developers get inline code suggestions as they type.
  • Suggestions use the repository's own functions, types and conventions.
  • Multi-line and whole-function suggestions are available.
  • Suggestions that are no longer relevant are cancelled.
  • Teams can see how many suggestions developers keep.

Non-functional requirements

  • Suggestion latency: p99 < 300ms, typically about 100ms.
  • Scale: 2M daily developers, 10K requests/s at peak.
  • Repository changes reach the code index within minutes.
  • Code never leaves the customer's organisation boundary.
  • Availability: 99.9%; when degraded, suggestions slow or simplify instead of failing.
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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.
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Round 2 of 4
MCQ Round
Answer multiple-choice questions based on your design.

Exit Interview?

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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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