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Byzantine fault tolerance - Wikipedia
Lamport devised a colorful allegory in which a group of army generals formulate a plan for attacking a city. [...] This formulation of the problem, together with some additional results, were presented by the same authors in their 1982 paper, "The Byzantine Generals Problem".[9]
In its simplest form, the generals must decide only 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 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. 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 having to 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 majority agreement on their strategy. There can be a default vote value given to missing messages. For example, missing messages can be given the value <Null>. Further, if the agreement is that the <Null> votes are in the majority, a pre-assigned default strategy can be used (e.g., retreat).
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A similar problem faces honeybee swarms. They have to find a new home, and the many scouts and wider participants have to reach consensus about which of perhaps several candidate homes to fly to. And then they all have to fly there, with their queen.[12]
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Bitcoin, Boeing 777
Some spacecraft flight systems such as that of the SpaceX Dragon[30] consider Byzantine fault tolerance in their design.
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Byzantine fault tolerance (BFT) is the dependability of a fault-tolerant computer system, particularly distributed computing systems, where components may fail and there is imperfect information on whether a component has failed.
In a "Byzantine failure", a component such as a server can inconsistently appear both failed and functioning to failure-detection systems, presenting different symptoms to different observers.
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. The term is derived from the Byzantine Generals' Problem,[1] where actors must agree on a concerted strategy to avoid catastrophic system failure, but some of the actors are unreliable.
Byzantine fault tolerance has been also referred to with the phrases
interactive consistency or
source congruency,
error avalanche,
Byzantine agreement problem,
Byzantine generals problem, and
Byzantine failure.
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january 2019 by andreasbuzzing
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This section of the website is aimed at introducing pupils to mechanisms and how they work. CAM profiles are discussed in detail as well as the design of a CAM toy. Linkages are also considered. Click on the titles below to view the section of your choice.
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february 2017 by andreasbuzzing