Google Docs — Simple

Problem statement (interviewer prompt)

Design Google Docs: many users edit the same document simultaneously and see each others' edits + cursors in real time. Pick between Operational Transform (OT) and CRDTs, handle offline edits + sync, comments + suggestions, and revision history.

flowchart LR
  A([User A])
  B([User B])
  WS[WebSocket Gateway]
  COL[Collab Service<br/>OT / CRDT]
  DOC[(Doc state + history)]
  A --> WS --> COL --> DOC
  B --> WS
  COL --> WS --> B

    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 A,B client;
    class WS edge;
    class COL service;
    class DOC datastore;