System Design: Chess Game

Question

# Design Chess.com (Online Chess Game)

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## Problem Statement

Design the backend system for an online chess platform supporting real-time play similar to Chess.com. The system must handle matchmaking, real-time move processing, and accurate clock management.

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## Requirements

### Functional Requirements
- Players can create accounts and have Elo ratings
- Players can join a matchmaking queue and be paired with an opponent
- Matched players play a game in real time (both players connected via WebSocket)
- Each move is validated server-side before being applied
- Each player has a clock (e.g., 10 min blitz); the server is the authoritative clock
- Games end on checkmate, draw, resignation, or timeout
- Game history is persisted and viewable later

### Non-Functional Requirements
- **Latency:** Move processing < 100ms round-trip
- **Scale:** 1M DAU, 100K concurrent games
- **Correctness:** Authoritative server-side clock; no client clock manipulation
- **Durability:** Complete game history stored permanently

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## Design Areas to Cover

1. **Matchmaking** — How to pair players by Elo rating quickly
2. **Game Session** — How a game state is stored and updated
3. **Move Pro

Accepted Answer

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