Byzantine fault
Fault in a computer system that presents different symptoms to different observers
A Byzantine fault is a condition of a system, particularly a distributed computing system, where a fault occurs such that different symptoms are presented to different observers, including imperfect information on whether a system component has failed. The term takes its name from an allegory, the "Byzantine generals problem", developed to describe a situation in which, to avoid catastrophic failure of a system, the system's actors must agree on a strategy, but some of these actors are unreliable in a way which causes other (good) actors to disagree on the strategy and they may be unaware of the disagreement.
A Byzantine fault is also known as a Byzantine generals problem, a Byzantine agreement problem, or a Byzantine failure.
Byzantine fault tolerance (BFT) is the resilience of a fault-tolerant computer system or similar system to such conditions.
Definition
A Byzantine fault is any fault presenting different symptoms to different observers. A Byzantine failure is the loss of a system service due to a Byzantine fault in systems that require consensus among multiple components.
The Byzantine allegory considers a number of generals who are attacking a fortress. The generals must decide as a group whether to attack or retreat; some may prefer to attack, while others prefer to retreat. The important thing is that all generals agree on a common decision, for a half-hearted attack by a few generals would become a rout, and would be worse than either a coordinated attack or a coordinated retreat.
The problem is complicated by the presence of treacherous generals who may not only cast a vote for a suboptimal strategy; they may do so selectively.
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