System Design FAQ: Top Questions

A Real-Time Collaborative Document Editor allows multiple users to edit a document concurrently with low latency, high availability, and strong consistency guarantees.

📋 Functional Requirements

• Real-time updates with minimal latency
• Concurrent editing with conflict resolution
• Offline editing and synchronization
• Document versioning and access control

📦 Non-Functional Requirements

• Low latency synchronization (~100ms)
• Conflict-free state with consistency
• Multi-region availability

🏗️ Core Components

• Editor Client: Rich text interface with WebSocket connection
• Sync Service: Broadcasts updates to all participants
• Conflict Resolver: CRDT or OT (Operational Transform) engine
• Persistence Layer: Stores document versions and edit history
• Auth Layer: Role-based permissions and login management

🔄 WebSocket Collaboration Example

// Simple WebSocket event broadcast
socket.on("edit", (update) => {
  doc.applyUpdate(update);
  socket.broadcast.emit("edit", update);
});
        

🧠 Conflict Resolution: CRDT Sample

// Using Yjs (a CRDT-based library)
import * as Y from 'yjs';

const doc = new Y.Doc();
const yText = doc.getText("editor");
yText.insert(0, "Hello world");

// Sync between peers using WebRTC, WebSocket, or BroadcastChannel
        

🗄️ Storage Layer (MongoDB)

{
  "_id": "doc123",
  "title": "Project Plan",
  "content": "
Hello team
",
  "updates": [ /* CRDT or OT history */ ],
  "collaborators": ["alice", "bob"],
  "lastModified": "2025-06-11T10:15:00Z"
}
        

🔐 Access Control Schema

{
  "documentId": "doc123",
  "user": "bob",
  "permission": "editor"  // or "viewer", "commenter"
}
        

📈 Observability & Metrics

• Edit latency p95/p99
• Active collaborators per document
• Sync conflict rate

📌 Final Insight

Real-time collaboration systems rely on efficient sync protocols and data structures like OT/CRDT. Maintaining a consistent state across distributed editors with support for offline mode, low latency, and strong conflict resolution is key to success.