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Design Google Docs

A collaborative document editor where many people edit the same document at once and see changes live.

Requirements

  • Real-time collaborative editing with live updates.
  • Converge to the same document for all editors.
  • Persist documents and support offline edits that sync later.
  • Presence and cursors.

Key ideas

  • The hard problem is concurrent editing: two people typing at once must merge without losing or corrupting text.
  • Two well-known approaches: operational transformation (transform each edit against concurrent ones) and CRDTs (data structures that merge automatically). Both give eventual convergence.
  • Architecture: clients send small operations to a document server that orders them, applies the merge strategy, and broadcasts the result to all editors.
  • Persistence: store the document plus an operation log so edits can replay and offline changes can sync.

High-level design

flowchart LR
    U1[User] ---|edits| DS[Document Server]
    U2[User] ---|edits| DS
    DS -->|OT / CRDT merge| Store[(Document + Op Log)]
    DS -->|broadcast| U1
    DS -->|broadcast| U2
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