May 2020
Beginner to intermediate
319 pages
7h 3m
Chinese
Content preview from 数据库系统内幕
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简介与概述
｜
167
8.6
系统同步性
从 FLP 不可能定理中可以看出时序假设是分布式系统的关键特征之一。在
异步系统
中，
我们不知道进程运行的相对速度，也不能保证在有限时间内或以特定顺序传递消息。进
程可能要花无限长的时间来响应，而且无法总是可靠地检测到进程故障。
对异步系统的主要批评在于上述假设不切实际：进程不可能具有
任意
不同的处理速度，
链路传递消息的时间也不会
无限
长。依赖时间能够简化推理，并提供时间上限的保证。
在异步模型中不一定能解决共识问题 [FISCHER85]。而且，不一定能设计出高效的异步
算法。对于某些任务，切实可行的解决方案很可能需要依赖时间 [ARJOMANDI83]。
我们可以放宽一些假设，认为系统是
同步
的。为此我们引入了时间的概念。在同步模型
下对系统进行推理要容易得多。它假定各进程的处理速度相近、传输延迟是有限的，并
且消息传递不会花任意长的时间。
同步系统也可以表示为同步的进程本地时钟：两个进程本地时间源之间的时间差存在上
限 [CACHIN11]。
在同步模型中设计系统可以使用超时机制。我们可以构建更复杂的抽象，例如领导者选
举、共识、故障检测以及基于它们的其他抽象。这使得最佳情况的场景更加健壮，但是
如果时序假设不成立则可能导致故障。例如：Raft 共识算法（参见 14.4 节）中，可能最
终有多个进程认为它们是领导者，为了解决该问题，我们强制滞后的进程接受其他进程
成为领导者；故障检测算法（参见第 9 章）可能会错误地将活动进程标记为故障，反之
亦然。设计系统时，我们必须考虑这些可能性。 ...
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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.