Byzantine fault tolerance

The property of a distributed system that keeps working correctly even when some components fail arbitrarily or act maliciously.

Byzantine Generals Problem: The 1982 problem of loyal generals agreeing on a plan when some commanders may be traitors. The formal statement of the problem every Byzantine fault tolerance protocol is built to solve.
Leslie Lamport: Computer scientist who formalized distributed agreement and co-invented the Byzantine Generals framing. He and Shostak and Pease posed the traitor problem that named the entire field.
Boeing 777 fly-by-wire: A flight control system computing on redundant channels that tolerate faulty processors. An early high-stakes deployment where arbitrary hardware faults had to be outvoted in real time.
Crash fault tolerance: Tolerance of processes that simply stop, the weaker failure model of Paxos-style systems. Crash tolerance needs 2f+1 replicas; the arbitrary-fault model is what makes BFT costly and distinct.
3f+1 replica bound: The minimum replica count needed to tolerate f Byzantine faults with unforgeable messages. This arithmetic result dictates the cost of every Byzantine system built since 1980.
Practical Byzantine Fault Tolerance: A 1999 state machine replication protocol that reaches Byzantine agreement with low overhead. The first protocol efficient enough to make Byzantine fault tolerance practical, not just theoretically possible.
Marshall Pease: SRI researcher who proved early results on achieving reliable consensus with faulty processors. His 1980 work with Lamport and Shostak established the 3m+1 processor bound.
Bitcoin: A peer-to-peer payment system reaching agreement without identities via proof of work. Its Nakamoto consensus solved open-membership Byzantine agreement, a problem classical protocols never addressed.
Paxos: Leslie Lamport's consensus algorithm for asynchronous systems tolerating crash failures. The dominant crash-tolerant alternative; comparing it clarifies exactly what BFT pays extra for.
Blockchain consensus: The family of protocols by which decentralized ledgers agree on transaction order. Modern cryptocurrencies are Byzantine fault tolerance applied to money, its largest commercial audience.

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