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

SeniorPrefer not to say

Jun 2026

Summary

Meta system design round for a software engineer role. The question was a full chess platform design with some tricky constraints around undo mechanics and a live leaderboard. Lot of ground to cover in one session.

Questions Asked (1)

Design an online chess platform where users can challenge friends, undo their last move before the opponent responds, and view a live global leaderboard with ELO rankings at any time.

System DesignTechnical Trade-offsData Modeling

Author's notes

This question has a lot of moving parts and I don't think I scoped it well at the start.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and scale, then design the core components: real-time game service, undo mechanism, and leaderboard. Focus on trade-offs between consistency, latency, and complexity, and explain how you would handle the undo feature without breaking game integrity.

Pro tip: Emphasize that the undo feature must be implemented as a server-side state rollback with proper validation, not just a client-side UI trick, to prevent cheating and ensure consistency.

1. Clarify Requirements and Scale

Ask about expected user base, concurrent games, latency requirements, and whether undo is allowed only before opponent's move. Establish functional and non-functional requirements.

2. High-Level Architecture

Outline main components: API gateway, game service (WebSocket for real-time), matchmaking, leaderboard service, and data stores. Sketch data flow for a game move and leaderboard update.

3. Design Undo Mechanism

Explain how to implement undo: store move history per game, allow undo only if opponent hasn't moved, validate on server, and rollback state. Discuss trade-offs (e.g., using event sourcing vs. snapshot).

4. Design Leaderboard and ELO

Choose a data store for leaderboard (e.g., Redis sorted sets for real-time, or a relational DB with caching). Describe ELO calculation and update frequency, and how to handle high read volume.

5. Address Trade-offs and Scalability

Discuss consistency vs. availability, latency, and cost. Mention sharding, caching, and how to handle failures (e.g., game state recovery).

Key Points to Mention

Real-time communication using WebSockets for game moves and undo requests.
Server-side validation of undo to prevent cheating and ensure game integrity.
Data modeling for game state: move history, board state, and undo stack.
ELO rating calculation and update strategy (e.g., after game completion).
Leaderboard implementation using Redis sorted sets for O(log N) updates and fast reads.
Scalability considerations: sharding by game ID, caching leaderboard, and handling concurrent games.

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