← Openai Interview Insights

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

SeniorPrefer not to say
May 2026

Summary

System design round at OpenAI for a software engineer role, focused entirely on designing an online chess platform from scratch. The question had a lot of moving parts and the depth they expected was pretty serious.

Questions Asked (1)

Q1

Design an online chess service that supports matchmaking and real-time gameplay. Cover queue design, Elo-based skill matching, game state management over WebSocket, server-side move validation, clocks, disconnects, and how you'd scale and shard game servers.

System DesignTechnical Trade-offsData Modeling
Author's notes

This one sprawled fast.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying requirements (e.g., expected concurrent users, latency targets, game variants) and then outline the high-level architecture: matchmaking service, game server fleet, and persistence layer. Dive into each component, explaining trade-offs and how you'd handle edge cases like disconnects and clock synchronization.

Pro tip: Emphasize idempotency and reconciliation: moves should be idempotent, and game state should be periodically snapshotted to recover from failures. Also, consider using a monotonic clock for game timers to avoid issues with system clock adjustments.

1. Clarify Requirements and Scope

Ask about expected scale (concurrent players, games per second), latency requirements, game rules (e.g., standard chess, variants), and client platforms. This shapes design decisions.

2. High-Level Architecture

Sketch the main components: matchmaking service, game servers, WebSocket gateway, database, and cache. Explain how they interact and the data flow.

3. Matchmaking and Elo

Describe the queue design (e.g., Redis sorted sets by rating), matchmaking algorithm (expanding rating range over time), and how Elo updates are computed and persisted.

4. Real-Time Gameplay

Detail WebSocket handling: connection management, move validation on server, clock synchronization, and state updates. Discuss disconnect handling and reconnection.

5. Scaling and Sharding

Explain how to scale game servers horizontally, shard by game ID or player ID, and ensure consistency. Cover load balancing, service discovery, and failure recovery.

Key Points to Mention

  • Use Redis sorted sets for matchmaking queue with rating-based bucketing and dynamic range expansion.
  • Server-side move validation using a chess library (e.g., chess.js) and authoritative game state to prevent cheating.
  • WebSocket protocol design: message types (move, resign, draw offer), sequence numbers, and acknowledgments.
  • Clock management: server-side timers with periodic sync to clients, handling latency and disconnects (e.g., auto-resign after timeout).
  • Disconnect handling: grace period, reconnection with game state recovery, and opponent notification.
  • Sharding strategy: consistent hashing by game ID to distribute load, with game servers stateless except for active games, and state persisted to a database (e.g., Redis with persistence).

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