ACID properties of transactions ACID 数据库事务标准

In the context of transaction processing, the acronym ACID refers to the four key properties of a transaction: atomicity, consistency, isolation, and durability.

Atomicity
All changes to data are performed as if they are a single operation. That is, all the changes are performed, or none of them are.
For example, in an application that transfers funds from one account to another, the atomicity property ensures that, if a debit is made successfully from one account, the corresponding credit is made to the other account.
Consistency
Data is in a consistent state when a transaction starts and when it ends.
For example, in an application that transfers funds from one account to another, the consistency property ensures that the total value of funds in both the accounts is the same at the start and end of each transaction.
Isolation
The intermediate state of a transaction is invisible to other transactions. As a result, transactions that run concurrently appear to be serialized.
For example, in an application that transfers funds from one account to another, the isolation property ensures that another transaction sees the transferred funds in one account or the other, but not in both, nor in neither.
Durability
After a transaction successfully completes, changes to data persist and are not undone, even in the event of a system failure.
For example, in an application that transfers funds from one account to another, the durability property ensures that the changes made to each account will not be reversed.



ACID，指数据库事务正确执行的四个基本要素的缩写.包含:原子性(Atomicity）、一致性（Consistency）、隔离性（Isolation）、持久性（Durability）。一个支持事务（Transaction）的数据库系统，必需要具有这四种特性，否则在事务过程（Transaction processing）当中无法保证数据的正确性,交易过程极可能达不到交易方的要求.
　　原子性
　　整个事务中的所有操作，要么全部完成，要么全部不完成，不可能停滞在中间某个环节。事务在执行过程中发生错误，会被回滚（Rollback）到事务开始前的状态，就像这个事务从来没有执行过一样。
　　一致性
　　在事务开始之前和事务结束以后，数据库的完整性约束没有被破坏。
　　隔离性
　　两个事务的执行是互不干扰的，一个事务不可能看到其他事务运行时，中间某一时刻的数据。
　　持久性
　　在事务完成以后，该事务所对数据库所作的更改便持久的保存在数据库之中，并不会被回滚。