May 2020
Beginner to intermediate
319 pages
7h 3m
Chinese
Content preview from 数据库系统内幕
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152
｜
第
8
章
加法线程
加法线程
结果a
）
c
）
b
）
d
）
结果
结果
结果
乘法线程
乘法线程
加法线程
加法线程
乘法线程
乘法线程
图
8-1
：并发执行中可能的交错情形
为了精确定义执行历史并减少可能的结果数量，我们需要
一致性模型
。一致性模型描述
并发执行的过程，并且确定了运算执行以及对其他参与者可见的顺序。使用不同的一致
性模型，我们可以约束或放松系统可能的状态数量。
分布式系统和并发计算在术语和学术研究上有许多重叠之处，但也存在一些差异。并发
系统中存在
共享内存
，进程可以用它来交换信息。在分布式系统中，各个进程拥有自己
的本地状态，参与者之间通过传递消息进行通信。
并发和并行
我们经常互换使用
并发
和
并行
计算这两个术语，但是这两个概念在语义上有细微的
差异。当两个步骤序列并发执行时，二者都在进行中，但任意时刻都只有其中一个
在执行。当两个步骤序列并行执行时，它们的步骤可以（在某一时刻）同时执行。并
发的操作时间上存在重叠，而并行的操作由多个处理器执行 [WEIKUM01]。
Erlang 编程语言的创建者 Joe Armstrong 举过一个例子：并发执行就像一台咖啡机前
排了两队，而并行执行就像两台咖啡机前排了两队。即便如此，绝大部分资料都用
术语“并发”来描述拥有多个并行执行线程的系统，而“并行”这个词则很少见。
分布式系统中的共享状态
我们可以尝试在分布式系统中引入共享内存的概念，例如，单一信息源（比如数据库）。
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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.