RetellAI·Software Engineer·Onsite - System Design / Architecture·Senior

SeniorPrefer not to say

Jul 2026Remote

Summary

System design round at Retellai for a software engineer role, pretty much one big question about building a collaborative doc editor. The whole session was focused on concurrency, which I was not fully prepared for at that depth.

Questions Asked (1)

Design the backend for a real-time collaborative document editor. Focus on how multiple users can edit the same document simultaneously without losing or duplicating content, and how all clients eventually converge to the same state.

System DesignTechnical Trade-offsData Modeling

Author's notes

This was the whole interview basically.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements (real-time collaboration, conflict resolution, scalability) and then propose a conflict-free replicated data type (CRDT) or operational transformation (OT) based architecture. Walk through the system components: clients, WebSocket servers, document state store, and persistence, explaining how they interact to ensure eventual consistency. Finally, discuss trade-offs between CRDT and OT, and how to handle offline editing and scaling.

Pro tip: Mention that CRDTs are often preferred for peer-to-peer and offline-first scenarios due to their simplicity in ensuring eventual consistency, but OT can be more efficient for centralized systems with frequent small edits. Show awareness of real-world implementations like Yjs or ShareDB.

1. Clarify Requirements and Constraints

Ask about expected number of concurrent users, document size, latency requirements, and whether offline editing is needed. This shapes the choice of conflict resolution algorithm and system architecture.

2. Choose Conflict Resolution Strategy

Decide between Operational Transformation (OT) and Conflict-free Replicated Data Types (CRDTs). Explain the trade-offs: OT requires a central server for transformation, while CRDTs allow decentralized updates but may have higher metadata overhead.

3. Design System Architecture

Outline components: clients, WebSocket servers for real-time communication, a document state store (e.g., Redis or in-memory), and persistent storage (e.g., database). Describe how updates flow from clients to server and are broadcast to other clients.

4. Ensure Eventual Consistency and Fault Tolerance

Explain how the chosen algorithm guarantees convergence (e.g., CRDTs merge automatically, OT transforms operations). Discuss handling network partitions, offline edits, and server failures with replication and persistence.

5. Address Scalability and Performance

Discuss scaling WebSocket servers horizontally, sharding documents, and optimizing state synchronization (e.g., using snapshots and delta updates). Mention monitoring and load balancing.

Key Points to Mention

Operational Transformation (OT) vs. Conflict-free Replicated Data Types (CRDTs): trade-offs in complexity, latency, and consistency guarantees.
WebSocket for real-time bidirectional communication, with fallback to long-polling if needed.
Document state management: using a central server for OT or decentralized for CRDTs, and how to persist and retrieve document history.
Handling offline editing and reconnection: queuing operations locally and merging upon reconnection.
Scalability considerations: sharding by document ID, using pub/sub (e.g., Redis) for broadcasting updates across servers.
Eventual consistency: ensuring all clients converge to the same state despite concurrent edits, and techniques like vector clocks or Lamport timestamps for ordering.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.