Twitter / X — Simple

Problem statement (interviewer prompt)

Design Twitter/X: users post 280-char tweets, follow each other, and see a home feed of tweets from people they follow (chronological + For-You). Support 500M tweets/day, 5B reads/day, hashtag search, trends, likes, retweets, replies, and DMs.

flowchart LR
  U([User])
  TW[Tweet Service]
  FAN[[Fan-out]]
  TL[(Home Timelines<br/>Redis ZSET)]
  TWDB[(Tweet DB)]
  GRAPH[(Social Graph)]
  HOME[Home Timeline API]
  SEARCH[Search]
  U -->|post| TW --> TWDB
  TW --> FAN
  GRAPH --> FAN
  FAN --> TL
  U -->|read| HOME --> TL
  TWDB -. index .-> SEARCH

    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 TW,HOME,SEARCH service;
    class TWDB datastore;
    class TL cache;
    class FAN queue;