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In fault-tolerant computer systems, and in particular distributed computing systems, Byzantine fault tolerance is the characteristic of a system that tolerates the class of failures known as the Byzantine Generals' Problem, which is a generalized version of the Two Generals' Problem. The phrases ''interactive consistency'' or ''source congruency'' have been used to refer to Byzantine fault tolerance, particularly among the members of some early implementation teams. The objective of Byzantine fault tolerance is to be able to defend against ''Byzantine failures'', in which components of a system fail with symptoms that prevent some components of the system from reaching agreement among themselves, where such agreement is needed for the correct operation of the system. Correctly functioning components of a Byzantine fault tolerant system will be able to provide the system's service, assuming there are not too many faulty components. The following practical, concise definitions are helpful in understanding Byzantine fault tolerance: ; Byzantine fault : Any fault presenting different symptoms to different observers ; Byzantine failure : The loss of a system service due to a Byzantine fault in systems that require consensus The terms ''fault'' and ''failure'' are used here according to the standard definitions originally created by a joint committee on "Fundamental Concepts and Terminology" formed by the IEEE Computer Society's Technical Committee on Dependable Computing and Fault-Tolerance and IFIP Working Group 10.4 on Dependable Computing and Fault Tolerance.〔(【引用サイトリンク】 url = http://www.dependability.org )〕 A version of these definitions is also described in the Dependability Wikipedia page. The type of system services which Byzantine faults affect are agreement (a.k.a consensus) services. ==Origin== ''Byzantine'' refers to the Byzantine Generals' Problem, an agreement problem (described by Leslie Lamport, Robert Shostak and Marshall Pease in their 1982 paper, ("The Byzantine Generals Problem" )) in which a group of generals, each commanding a portion of the Byzantine army, encircle a city. These generals wish to formulate a plan for attacking the city. In its simplest form, the generals must only decide whether to attack or retreat. Some generals may prefer to attack, while others prefer to retreat. The important thing is that every general agrees on a common decision, for a halfhearted attack by a few generals would become a rout and be worse than a coordinated attack or a coordinated retreat. The problem is complicated by the presence of traitorous generals who may not only cast a vote for a suboptimal strategy, they may do so selectively. 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. 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). The problem is complicated further by the generals being physically separated and must send their votes via messengers who may fail to deliver votes or may forge false votes. Byzantine fault tolerance can be achieved if the loyal (non-faulty) generals have a unanimous agreement on their strategy. Note that there can be a default vote value given to missing messages. For example, missing messages can be given the value The typical mapping of this story on to computer systems is that the computers are the generals and their digital communication system links are the messengers. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Byzantine fault tolerance」の詳細全文を読む スポンサード リンク
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