Practice: Design WhatsApp

Mobile messaging platform inspired by WhatsApp: persistent connections, store-and-forward delivery (the server keeps messages only until they're delivered), push for offline phones, server-side group fan-out and Signal-protocol end-to-end encryption with encrypted media. A teaching design, not Meta's internal architecture.

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 send and receive text messages in 1-on-1 chats with sub-second delivery when both parties are online
  • Users send and receive images, video, and voice messages; media is compressed and delivered efficiently
  • Messages are end-to-end encrypted — the server must never have access to plaintext message content
  • Offline users receive all missed messages in order when they come back online
  • Delivery receipts: single tick (sent to server), double tick (delivered to device), blue tick (read by recipient)
  • Typing indicators and online/last-seen presence are shown to contacts in real time
  • Group chats support up to 1,024 members with the same delivery and receipt guarantees
  • Users can search their message history, contacts, and groups by keyword
  • The system captures usage analytics and exposes operational metrics/logs for monitoring, without impacting message delivery latency

Non-functional requirements

  • Message delivery latency < 200ms p99 end-to-end when both users are online
  • Support 2 billion registered users; 100 million concurrent online users
  • Messages must be delivered at-least-once; deduplication on the client side
  • End-to-end encryption: server stores only ciphertext; keys never leave the device
  • Message history stored for 90 days; media stored for 30 days with CDN caching
  • Search queries return results within 300ms p99 via a dedicated, asynchronously-updated search index
  • All services and workers emit metrics/logs to a centralized observability pipeline for production monitoring and alerting
  • 99.99% uptime — users must be able to send messages even during partial outages
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Round 1 of 4 · Architecture Design · 60:00 remaining
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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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    • 30 min
    • 45 min
    • 60 min
    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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    SYSTEM BRIEF

    System Constraints

    What the system must do — every item is a user-facing behaviour your architecture must support.

      ↑ Engineering Constraints

      These are the failure modes you must design against — latency SLAs, durability targets, traffic ceilings.

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