Facebook News Feed — Simple

Problem statement (interviewer prompt)

Design the Facebook News Feed: aggregate posts from friends, pages, groups, and recommendations into a personalised feed. Optimise for p99 feed-open <300ms across 3B MAU with extreme variance in friend counts (a few to 5000+).

flowchart LR
  U([User])
  W[Write: post]
  POST[(Posts)]
  AGG([Aggregator])
  RANK([Ranker])
  TAO[(TAO graph cache)]
  FEED[Feed API]
  U -->|post| W --> POST
  W --> TAO
  U -->|read| FEED --> AGG --> TAO
  AGG --> RANK --> 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 U client;
    class W,FEED service;
    class POST,TAO datastore;
    class AGG,RANK compute;