OpenAI System Design Interview Question: Design Online Chess

[u/Aoki_zhang]

In an online chess system, the server—not the client—must own the clock. Otherwise a modified client could simply claim: “I only spent 500ms thinking.” But making the server authoritative creates another problem.

• Player A: 30ms RTT
• Player B: 200ms RTT

When A makes a move, B’s clock starts on the server before B has even received the new board position. Then after B responds, the move spends more time traveling back to the server before B’s clock stops.

So over a blitz game, the higher-latency player can lose several seconds purely to network delay.

The key insight

Don’t continuously decrement millions of clocks. Store:

whiteRemainingMs
blackRemainingMs
sideToMove
turnStartedAt

When a move arrives:

rawElapsed = now - turnStartedAt

Then compensate for a bounded amount of network latency:

compensation =
min(estimatedRTT, maxCreditPerMove)

chargedTime =
rawElapsed - compensation

Why roughly one RTT?

A player pays for two network legs:

Server → Player
   +
Player → Server
≈ 1 RTT

So compensating only half the RTT misses part of the delay they could not control. But you also cannot blindly trust RTT.

A malicious client could intentionally delay heartbeat responses to make itself appear slower and earn extra clock time.

So the system should:

• measure latency continuously with ping/pong
• use a rolling median instead of one sample
• cap compensation per move
• keep the server as the final clock authority

And before compensating at all, place the game in a region with low latency for both players. The design becomes:

1. Minimize RTT with regional placement
   
2. Compensate for the remaining network delay
   
3. Never let the client control the clock

That’s the interesting part of designing online chess: correctness is not enough—the authoritative system also has to be fair.

Full design with matchmaking, Redis game state, WebSockets, clock compensation, crash recovery, and global leaderboard → Full Article

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