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Design Real-Time Collaborative Editing

OpenAIMachine Learning Engineer · Technical Screen
MediumMachine Learning EngineerTechnical ScreenSystem Design

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This question tests your ability to design a low-latency, distributed synchronization system that manages concurrent text edits from multiple users while maintaining eventual consistency.

Problem Statement

Design a back-end system for a real-time collaborative text editor similar to Google Docs or Notion. Multiple users should be able to edit the same document simultaneously, with changes appearing for others with minimal latency.

Functional Requirements

  1. Real-time updates: When user A makes a change, other active users must see that change shortly after.
  2. Conflict resolution: The system must handle concurrent edits to the same character positions without data loss or corruption.
  3. Persistence: The document state must be saved to a database, and users should be able to see the edit history.

Non-Functional Requirements

  1. Latency: Targeted sub-100ms synchronization latency.
  2. Scalability: Support for up to 100 concurrent editors on a single document.
  3. Reliability: The system must reach a consistent state across all clients even if the network is unstable.

Deliverables

  1. High-level architecture: Describe the interaction between clients, the server, and the database.
  2. Synchronization mechanism: Explain the strategy used to resolve concurrent edits (e.g., Operational Transformation (OT) or Conflict-free Replicated Data Types (CRDT)).
  3. Data model: Propose a schema for storing document states and edit operations.
  4. Trade-offs: Discuss the pros and cons of your chosen synchronization approach regarding server load versus client complexity.

Hints

Hint 1

Consider how you would uniquely identify the position of an insertion or deletion if other users are inserting text at the same time.

Hint 2

Think about the difference between a centralized server-authoritative model versus a decentralized peer-to-peer approach.

Hint 3

Evaluate whether your chosen synchronization logic handles offline scenarios, where a user reconnects after a long duration and needs to sync missed changes.

Follow-up questions

  • How would you implement 'presence' features, such as showing the cursor position and name of other active users?
  • If the document grows to be millions of characters long, what strategies would you use to optimize storage and transmission?
  • How does your system handle 'undo' functionality across a shared session?