Design collaborative document editing backend

You are asked to design the backend for a real-time collaborative document editor similar to Google Docs. The interviewer is specifically interested in how you **handle parallel (concurrent) editing** of the same document by multiple users. Design and explain a system that supports the following: - Many users can open the same document and edit it at the same time. - Each user should see other users' changes in near real-time (e.g., within a few hundred milliseconds). - The document should remain logically consistent: no characters are randomly lost or duplicated, and all users eventually converge to the same content. - The system should be resilient to network latency and temporary disconnections. In your answer, cover: 1. **High-level architecture** - How clients connect to the service and receive real-time updates. - The main backend services involved and storage choices for documents and edit history. 2. **Data model for edits** - How you represent document content (e.g., as a string, sequence of operations, or a more complex data structure). - How individual editing operations (insert, delete, formatting changes) are modeled. 3. **Handling concurrent edits** (core part) - How you serialize or merge edits coming from multiple users. - Discuss at least one of the following approaches in some depth: - Centralized global sequence of operations assigned by the server. - Operational Transformation (OT). - Conflict-Free Replicated Data Types (CRDTs). - How your chosen approach ensures that all users eventually see the same document, even when operations arrive out of order due to network delays. 4. **Consistency and performance trade-offs** - What kind of consistency guarantees you provide (e.g., eventual vs. strong consistency at the document level). - How you keep latency low while avoiding conflicts. 5. **Scalability and reliability** (briefly) - How you scale to millions of documents and thousands of concurrent users per document. - How you handle server failures and recovery without losing edits. Assume a typical web/mobile client using WebSockets or similar technology is acceptable. Focus emphasis on the **design of the collaboration/concurrency mechanism** rather than on UI.