Design an online chess game server like Lichess
PremiumReal-timeMedium
- Server authoritative clock
- Move validation
- Move sync
Quick answer
This real-time interview question asks you to design an online chess game server like Lichess. Aim for a design that delivers updates to many users with low delay, then explain connections, fan-out, ordering and trade-offs. Interviewers want to see how you keep latency low, handle reconnects and stay correct under load.
The problem
Design the game server for an online chess platform like Lichess or Chess.com. Two players play with a time control (for example 3 minutes each plus 2 seconds per move); each move must reach the opponent fast, clocks must stay fair even at 300 ms of latency, and a player who drops briefly must resume without losing the game.
Spectators can watch live games. Matchmaking, ratings, puzzles and anti-cheat engines are out of scope; assume a move-validation library for the game rules exists.
What must an online chess game server like Lichess do?
- Moves: A player sends a move; the server validates it against the rules and the turn, applies it, and sends it to the opponent and spectators.
- Clocks: Each player has a clock that runs only on their turn; the server shows both clocks and decides when time runs out.
How well must it do it?
- Latency: A move reaches the opponent within 150 ms of reaching the server.
- Fairness: Clock error attributable to the system is under 100 ms per move.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Server authoritative clock
- Move validation
- Move sync
- Reconnect resume
- WebSocket
- Game state machine
How you’ll practise
- Requirements
- Estimate
- Connections
- Message flow
- Delivery guarantees
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any real-time design round.
- Clarifies latency, ordering and delivery guarantees
- Chooses a connection model with reasons
- Explains fan-out to many receivers
- Handles reconnects, missed messages and duplicates
- Plans for hot spots and sudden traffic spikes
- States trade-offs between consistency and speed
Questions people ask about this interview question
- Which design type is this question?
- It is a real-time question. You design an online chess game server like Lichess and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Medium. Entry-level engineers can practise it, but it expects you to reason about requirements and trade-offs, not only list components.
- What should I cover first?
- Start with who sends and receives updates, how fast they must arrive and what order matters. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
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Updated · Written by the SevyDevy team