Interlude. The Machine That Agrees

Dusk: a walled city on a hill surrounded by several army camps with fires, a lone messenger riding between them. — The Byzantine generals problem: agreement is possible if fewer than a third are traitors. Satoshi solved it for computers, not for people.

For many years I built systems where several servers have to store the same data and must never disagree. If one server thinks there are a hundred rubles in an account and another is sure there are fifty, the system is broken, even if both servers are working fine. Engineers have a boring name for this, state consistency, and a whole library of scientific papers on how to achieve it.

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