Quiz Platform (Kahoot / Duolingo) — Simple

Problem statement (interviewer prompt)

Design a live quiz platform (Kahoot / Duolingo classrooms): a host runs a multi-question quiz, hundreds of participants answer in real time on their phones, scores update live, and an end-of-game leaderboard is shown. Latency < 500ms on the answer-reveal cycle.

flowchart LR
  HOST[Host]
  PLR[Players]
  WS[Realtime gateway]
  GS[Quiz session]
  SC[(Score store)]
  HOST --> WS --> GS
  PLR --> WS --> GS
  GS --> SC

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class WS edge;
    class HOST,PLR,GS service;
    class SC datastore;