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Design a backend for an online checkers game

xAISoftware Engineer · Onsite
MediumSoftware EngineerOnsiteSystem Design

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This problem tests your ability to design a state-aware, real-time multiplayer system that ensures data consistency, enforces complex game rules, and manages turn-based concurrency.

Scenario

Design the backend infrastructure for a real-time, online Checkers platform. The system should support multiple concurrent games, track game state, enforce move validation, and handle asynchronous player actions.

Requirements

Functional

  • Matchmaking: Players should be able to join a lobby and be paired with an opponent.
  • Game State Management: The system must validate moves (e.g., checking turn order, piece movement, and forced captures).
  • Real-time updates: Opponents must receive move notifications immediately.
  • Game History: Players should be able to view a replay of their previous moves.

Non-Functional

  • Consistency: The system must prevent illegal moves and race conditions (e.g., two moves sent at the same time).
  • Latency: Move updates should feel instantaneous for users.
  • Scalability: The system should support thousands of concurrent games.

What to provide

  • API Design: Define the necessary endpoints or WebSocket events for moves and game states.
  • Data Model: Propose a database schema for players, active games, and move history.
  • Concurrency Control: Explain how you ensure that only one valid move can be processed per turn.
  • Architecture: Describe the high-level system components (e.g., Load Balancers, Game Servers, Pub/Sub, Cache).

Hints

Hint 1

Consider using an authoritative server model where the client only sends intent, and the server validates the board state before broadcasting.

Hint 2

Think about how WebSockets or long-polling can be used to push state changes to clients in real-time.

Hint 3

Use a distributed lock or an atomic operation (like Redis Lua scripts) to manage turn-based concurrency to prevent double-moves.

Follow-up questions

  • How would you handle a player disconnecting mid-game, and how would you implement a 'rejoin' feature?
  • If the game grew to millions of concurrent users, how would you partition or shard the game servers?
  • How would you implement an undo feature that correctly reverts the state of the game, including captured pieces?