Reels / Shorts — Simple

Problem statement (interviewer prompt)

Design Instagram Reels / YouTube Shorts: short vertical videos (≤90s) in a prefetch-driven for-you feed. Optimise for sub-100ms next-video first-frame latency, audio re-use (sound library), and integration with a parent platform.

flowchart LR
  U([Creator])
  UP([Upload + Transcode])
  S3[(Object Store)]
  CDN
  CG([Candidate Gen])
  RANK([Ranker])
  FEED[Vertical Feed]
  V([Viewer])
  U --> UP --> S3 --> CDN
  V --> FEED --> CG --> RANK --> V

    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 FEED service;
    class UP,CG,RANK compute;
    class S3 storage;