May 2020
Beginner to intermediate
319 pages
7h 3m
Chinese
Content preview from 数据库系统内幕
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86
｜
第
5
章
5.3
并发控制
当在 1.1 节中讨论数据库的架构时，我们提到事务管理器和锁管理器协同工作以处理并
发控制。并发控制是一组用于处理并发事务间交互的技术。这些技术可以大致分为以下
几类：
乐观并发控制
乐观并发控制（OCC）允许多个事务执行并发的读取和写入操作，最后确定其执行
结果能否被串行化（serializable
译注 1
）。换句话说，事务不会彼此阻塞，而是保留其
操作历史，并在提交前检查这些历史操作是否存在冲突的可能。如果会产生冲突，
则中止其中某一个冲突的事务。
多版本并发控制
多版本并发控制（MVCC）允许一条记录同时存在多个时间戳的版本，通过这种方
式保证事务读到的是数据库过去某个时刻的一致视图。MVCC 可以使用验证技术来
实现，即只允许多个更新或事务提交中的某一个获胜，也可以使用无锁技术（例如
时间戳排序）或基于锁的技术（例如两阶段锁）。
悲观并发控制
悲观（也称为保守）并发控制（PCC）既有基于锁的实现，也有不加锁的实现，它们
的主要区别在于如何管理和授权对共享资源的访问。基于锁的实现要求事务维护数
据库记录上的锁，以防止其他事务修改被加锁的记录或访问当前事务正在修改的记
录，直到锁被释放为止。不加锁的实现根据未完成事务的调度，维护读取与写入的
操作列表以限制事务的执行。悲观的调度可能导致死锁：多个事务需要互相等待对
方释放锁才能继续执行。
在本章中，我们重点介绍节点本地的并发控制技术。第 13 章将会介绍分布式事务以及
其他方法，例如确定性并发控制（参见 13.4 节 ）。
在进一步讨论并发控制之前，我们需要明确要解决的问题集，并讨论事务操作会以怎样 ...
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