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.