System Design FAQ: Top Questions

A Document Collaboration Platform enables multiple users to edit documents simultaneously in real time, with features like autosave, change tracking, access control, and conflict resolution.

📋 Functional Requirements

• Create/edit/share documents with others
• Multi-user real-time editing
• Access control (view/edit/comment)
• Autosave and version history

📦 Non-Functional Requirements

• Low-latency updates
• Conflict-free concurrent editing
• Scalable storage and collaboration

🏗️ Key Components

• Document Store: Stores content snapshots and metadata (e.g., MongoDB, PostgreSQL)
• Real-time Sync Service: WebSocket-based diff engine
• CRDT/OT Engine: Operational Transformation (OT) or Conflict-Free Replicated Data Type (CRDT)
• Versioning Service: Diffs stored as deltas with timestamps

📄 Document Schema (Simplified)

{
  "doc_id": "abc123",
  "title": "Team Strategy",
  "content": "This is the initial draft...",
  "owners": ["user123"],
  "editors": ["user234", "user345"],
  "history": [ { "ts": 1718131912, "diff": "+Hello" }, ... ]
}
        

📶 Real-time Collaboration (WebSocket)

// WebSocket connection
ws.onmessage = (event) => {
  const op = JSON.parse(event.data);
  applyOperationToDocument(op);
  broadcastToOtherUsers(op);
};
        

🔁 Operational Transformation (OT)

function transformInsertOps(opA, opB) {
  if (opA.pos <= opB.pos) opB.pos += opA.length;
  else opA.pos += opB.length;
}
        

🧪 Autosave and Versioning

• Snapshot every 60s or major change
• Diff-based versioning for rollback

🔐 Access Control

• JWT-based user auth
• Per-document ACL (read/edit/comment)

📈 Metrics and Monitoring

• Active collaborators per document
• Latency of updates (ms)
• Conflict resolution stats

🧰 Tools/Infra Used

• Sync: WebSocket, Redis Pub/Sub
• Storage: PostgreSQL, S3, MongoDB
• CRDT: Automerge, Yjs

📌 Final Insight

Designing collaborative editing involves solving distributed consistency and latency challenges. Use a CRDT/OT engine, real-time transport, and delta-based versioning to provide a Google Docs-like experience.