May 2020
Beginner to intermediate
319 pages
7h 3m
Chinese
Content preview from 数据库系统内幕
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166
｜
第
8
章
定要怀疑最后一个确认消息是否已送达目的地。
注意我们没有做任何时序上的假设：将军间的通信是完全异步的。并没有一个上限约束
将军必须在多长时间内做出回应。
8.5 FLP
不可能定理
Fisher、Lynch 和 Paterson 在论文中描述了一个著名的问题：
FLP 不可能问题
[FISCHER85]
（FLP 是作者姓氏的首字母），论文讨论了一种共识形式：各进程启动时有一个初始值，
并尝试就新值达成共识。算法完成后，所有正常进程上的新值必须相同。
如果网络完全可靠，很容易对特定值达成共识。但实际上，系统容易出现各式各样的故
障，例如消息丢失、重复、网络分区，以及进程缓慢或崩溃。
共识协议描述了这样一个系统：给定
初始状态
的多个进程，它将所有进程带入
决定状态
。
一个正确的共识协议必须具备以下三个属性：
一致性
协议达成的决定必须是一致的：每个进程都做出了决定且所有进程决定的值是相同
的。否则我们就尚未达成共识。
有效性
达成共识的值必须由某一个参与者
提出
，这意味着系统本身不能“提出”值。这也
意味着这个值不是无关紧要（trivial）的：进程不能总是决定某个预定义的默认值。
终止性
只有当所有进程都达到决定状态时，协议才算完成。
文献 [FISCHER85] 假定处理过程是完全异步的，进程之间没有共享的时间概念。这样的
系统中的算法不能基于超时，并且一个进程无法确定另一个进程是崩溃了还是仅仅运行
太慢。论文表明，在这些假设下，不存在任何协议能保证在有限时间内达成共识。完全
异步的共识算法甚至无法容忍一个远程进程无通知地突然崩溃。 ...
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.