May 2020
Beginner to intermediate
319 pages
7h 3m
Chinese
Content preview from 数据库系统内幕
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194
｜
第
11
章
性和可用性，还要从同步代价的方面考虑一致性 [ATTIYA94]。同步代价可能包括时延、
执行额外操作所花费的 CPU 周期、用于持久化恢复信息的磁盘 I/O、等待时间、网络 I/
O，以及一切可以通过避免同步而节省的代价。
首先，我们将重点关注操作结果的可见性和传播。回到并发读写的例子，通过将写入按
照依赖关系一个接一个地放置，或定义一个新值被传播出去的时间点，我们就能限制可
能的历史数量。
我们从执行数据读写操作的
进程
（客户端）的角度来讨论一致性模型。由于我们在数据
复制的上下文中讨论一致性，因此我们假设数据库可以有多个副本。
11.5.1
严格一致性
严格一致性
（strict consistency）相当于完全透明的复制：任何进程的任何写入都可
以立即被任何进程的后续的读操作读取。它涉及全局时钟的概念，如果在时刻 t
1
有
write(x, 1)，则任何 read(x) 的操作将在时刻 t
2
>t
1
时返回新写入的值 1。
不幸的是，这只是一个理论模型，且不可能实现，因为物理定律和分布式系统的工作方
式限制了事情发生的速度 [SINHA97]。
11.5.2
可线性化
可线性化
（linearizability）是最强的单对象、单操作一致性模型。该模型下，在写操作
开始和结束之间的某个时间点，写操作的效果严格一次性地对所有读取者可见，没有客
户端会观察到状态转移、部分（即未完成的、仍在进行中的）或不完全（即在完成之前
中断的）的写入操作 [LEE15]。
并发操作表现为可见性属性所支持的可能的顺序历史记录之一。可线性化有一定的不确 ...
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