Those who received a retreat vote from the ninth general will retreat, while the rest will attack (which may not go well for the attackers). For instance, if nine generals are voting, four of whom support attacking while four others are in favor of retreat, the ninth general may send a vote of retreat to those generals in favor of retreat, and a vote of attack to the rest. 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.
The important thing is that all generals agree on a common decision, for a halfhearted attack by a few generals would become a rout, and would be worse than either a coordinated attack or a coordinated retreat. The generals must decide as a group whether to attack or retreat some may prefer to attack, while others prefer to retreat. If two generals falsely declare that they intend to attack, but instead retreat, the battle is lost (right).Īs an analogy of the fault's simplest form, consider a number of generals who are attacking a fortress. If all generals attack in coordination, the battle is won (left). Byzantine fault tolerance ( BFT) is the resiliency of a fault-tolerant computer system to such conditions. It is difficult for the other components to declare it failed and shut it out of the network, because they need to first reach a consensus regarding which component has failed in the first place. In a Byzantine fault, a component such as a server can inconsistently appear both failed and functioning to failure-detection systems, presenting different symptoms to different observers. The term takes its name from an allegory, the "Byzantine generals problem", developed to describe a situation in which, in order to avoid catastrophic failure of the system, the system's actors must agree on a concerted strategy, but some of these actors are unreliable. For the political structure and complexity, see Byzantinism.Ī Byzantine fault (also Byzantine generals problem, interactive consistency, source congruency, error avalanche, Byzantine agreement problem, and Byzantine failure ) is a condition of a computer system, particularly distributed computing systems, where components may fail and there is imperfect information on whether a component has failed.
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