This is one of those questions where the scope is so wide you can easily spend 20 minutes on matchmaking and then realize you haven't touched clocks at all.
Start by clarifying requirements and scale, then design the high-level architecture with separate services for matchmaking, game state, and persistence. Focus on the real-time gameplay loop, detailing move validation, timer management, and consistency mechanisms. Discuss trade-offs between consistency, latency, and complexity, and how to handle failures and reconnections.
Pro tip: Emphasize idempotency and server-authoritative design: moves should be validated and applied exactly once, and timers should be managed server-side to prevent cheating. Also, discuss how to handle reconnections gracefully to maintain user experience.
Ask about expected user base, concurrent games, latency requirements, and features like spectating or chat. Define functional and non-functional requirements to scope the design.
Outline core services: user service (accounts, ratings), matchmaking service, game service (real-time gameplay), and storage (game history, user data). Choose communication patterns (WebSockets for real-time, REST for other APIs).
Detail the game service: how moves are validated (using a chess engine library), how timers are managed (server-side with increments), and how state is synchronized between players. Discuss consistency models and conflict resolution.
Explain skill-based matchmaking using Elo/Glicko ratings, with a queue system that pairs players within a rating range. Discuss how to handle rating updates after games and prevent abuse.
Describe storage for game history (e.g., append-only log or database), user profiles, and ratings. Discuss sharding, replication, and caching to handle scale. Address failure recovery and reconnection.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.