TikTok — Simple

Problem statement (interviewer prompt)

Design TikTok: 60–90s vertical video upload + a recommendation-first For-You feed with sub-100ms next-video latency. Cover ranking signals (watch-time, completion, replays), cold-start, audio licensing, and 1B+ MAU.

flowchart LR
  U([Creator])
  UP([Upload + Transcode])
  S3[(Object store)]
  CDN
  CG([Candidate Gen<br/>recommendation-first])
  RANK([Ranker])
  F[For-You Feed]
  V([Viewer])
  U --> UP --> S3 --> CDN
  V --> F --> CG --> RANK --> V
  V -.signals.-> CG

    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 U,V client;
    class F service;
    class UP,CG,RANK compute;
    class S3 storage;