Design an online chess game with undo and leaderboard

## Scenario Design a real-time online chess game service. ## Core requirements - Users can **challenge friends** and start a game session. - Two players take turns making moves. - A player may **undo (take back) moves only until the opponent makes their next move**. - Interpreting this strictly: after you make a move, you can retract it as long as the opponent has not responded with their move yet. - Provide a **leaderboard** that can show **all users’ rankings by score at any time**. ## Non-functional requirements (clarify assumptions) - Support many concurrent games. - Low-latency move delivery (near real time). - Correctness: legal move validation, turn ordering, no double moves. - Basic reliability and auditability (game history, dispute resolution). ## Out of scope (unless you choose to include) - Anti-cheat engine detection, spectating, chat, tournaments, puzzles.