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Design Collaborative Document Editing

VercelSoftware Engineer · Onsite
MediumSoftware EngineerOnsiteSystem Design

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This question tests your ability to design a distributed system capable of handling concurrent, low-latency document edits using conflict resolution strategies.

Problem Statement

Design a collaborative text editing service (similar to Google Docs or Notion) that allows multiple users to edit the same document simultaneously. The system must remain performant under high concurrency and ensure all participants eventually see the same document state.

Functional Requirements

  1. Real-time character-level or block-level synchronization between multiple users.
  2. Document persistence to long-term storage.
  3. Basic conflict resolution to handle simultaneous edits.
  4. Support for viewing version history or reverting to previous states.

Non-Functional Requirements

  1. Low latency: Changes should propagate to other users in under 200ms.
  2. Availability: Users should be able to continue typing even if their connection flickers.
  3. Scalability: The system should support thousands of active documents with varying numbers of concurrent users.

Scope of Response

  • Describe the high-level architecture and the choice of communication protocol.
  • Define the data model for representing document states and operations.
  • Explain the chosen synchronization algorithm (e.g., Operational Transformation vs. CRDT).
  • Discuss how you handle persistence, conflict resolution, and versioning.
  • Address how to handle offline-to-online transitions.

Hints

Hint 1

Consider the trade-offs between a centralized server-authoritative model using Operational Transformation (OT) and a decentralized/peer-to-peer approach using Conflict-free Replicated Data Types (CRDTs).

Hint 2

Think about how to use WebSockets for real-time communication while ensuring that the backend can handle the load of managing thousands of open socket connections.

Hint 3

For document history, consider how snapshots combined with an append-only operation log can optimize both performance and storage.

Follow-up questions

  • How would you optimize the system if a document grows to be extremely large (e.g., 500+ pages)?
  • How would you implement access control, such as distinguishing between 'viewers', 'commenters', and 'editors' in your data model?
  • If you chose a CRDT approach, what are the implications for metadata and memory usage as the document lifecycle increases?