在并发控制中丢失更新？ [英] Lost update in Concurrency control?

问题描述

我有两个事务T和U在DB中同时执行。如何提供丢失更新问题的示例？

I have two transactions T and U which are executed simultaneously in a DB. How does one provide an example of the lost update problem?

我们可以假设我们有三个帐户A，B，C，他们每个有100英镑，200英镑£300。

We can assume that we have three accounts A,B,C and they each have £100,£200 and £300 respectively.

推荐答案

丢失更新问题涉及并发读取和更新数据，在读者不阻止写入者的系统中。

The "lost update" problem relates to concurrent reads and updates to data, in a system where readers do not block writers. It is not necessary for the transactions to be exactly simultaneous.

1. 会话＃1读取帐户A，获取100。


2. 会话＃2读取帐户A，获取100。


3. 会话＃2将帐户A更新为150（+50）并提交。


4. 会话＃1将帐户A更新为120（+20）并提交。

1. Session #1 reads Account A, gets 100.
2. Session #2 reads Account A, gets 100.
3. Session #2 updates Account A to 150 (+50) and commits.
4. Session #1 updates Account A to 120 (+20) and commits.

知道另一个会话已经修改了帐户，会话＃2的更新被覆盖（丢失）。

In this scenario, because Session #1 does not know that another session has already modified the account, the update by Session #2 is overwritten ("lost").

有几种方法可以解决这个问题，例如版本号或前后比较。

There are several ways to solve this, e.g. version numbers or before-and-after compares.

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