Design Collaborative Document Editing

Design a web-based collaborative document editing system similar to Google Docs. The system should allow multiple users to create, edit, and share rich-text documents in real time. Users should be able to see other collaborators' edits, cursor positions, and presence indicators with low latency. The design should support document persistence, access control, comments, version history, and recovery from client or network failures. Clarify the product scope, propose APIs and data models, describe the high-level architecture, explain the real-time collaboration algorithm, and discuss scalability, reliability, and security trade-offs.