News Feed System — Simple

Problem statement (interviewer prompt)

Design a Twitter/Facebook-style home feed for 1B users with avg 200 followees. Show the latest posts from people they follow, ranked by relevance. Handle the celebrity fan-out problem and keep feed-open p99 under 200ms.

flowchart LR
  POST([User posts])
  FAN[[Fan-out service]]
  TLC[(Per-user Timeline Cache<br/>Redis lists)]
  GET[Feed API]
  RANK([Ranker])
  U([Reader])
  POST --> FAN
  FAN -->|push to followers| TLC
  U --> GET --> TLC
  TLC --> RANK --> GET --> U

    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 POST,U client;
    class GET service;
    class TLC cache;
    class FAN queue;
    class RANK compute;