Rollback and replays for online play

The work: 24 May 2026 · Written 1 November 2026 · Entry 7 of 17

On 24 May 2026 two big pieces of engineering arrived together: a client-side prediction and rollback engine for online games, and a deterministic replay engine. They sound technical, but they answer two simple wishes: make online play feel instant, and let a player watch or share a run exactly as it happened.

The problem: late keys

When you play with other people online, their key presses reach you a moment late. If the game waited for every message before showing anything, it would feel slow. So the game predicts: it shows your own key presses at once and assumes the others will do what they did last.

When a late message arrives and it shows the guess was wrong, the game rolls back: it goes back to the last moment that was right, applies the real message, and plays forward again to the present, so quickly that the player does not notice.

Why the game must be deterministic

Rollback works only if the game is deterministic: given the same starting state and the same key presses, it always produces exactly the same result, on every computer. That rule shaped the whole game engine. Random numbers come from a seeded generator, the game never reads the clock in its rules, and it does not depend on the frame rate of the screen.

Replays for free

The same rule gives replays almost for free. A replay is not a video; it is the starting seed plus the list of key presses. Playing them again through the same game rules reproduces the run exactly. That is why a replay link is small enough to share, and why tests can run a whole game quickly and check that it ends the same way every time.