什么是事务？

1.   什么是事务

2.1              事务定义

百度百科：

事务，一般是指要做的或所做的事情。在计算机术语中是指访问并可能更新数据库中各种数据项的一个程序执行单元(unit)。

数据库事务(Database Transaction) ，是指作为单个逻辑工作单元执行的一系列操作。 事务处理可以确保除非事务性单元内的所有操作都成功完成，否则不会永久更新面向数据的资源。通过将一组相关操作组合为一个要么全部成功要么全部失败的单元，可以简化错误恢复并使应用程序更加可靠。一个逻辑工作单元要成为事务，必须满足所谓的ACID(原子性、一致性、隔离性和持久性)属性。

维基百科：

A transaction is an agreement, communication, or movement carried out between separate entities or objects, often involving the exchange of items of value, such as information, goods, services, and money.

A database transaction comprises a unit of work performed within a database management system (or similar system) against a database, and treated in a coherent and reliable way independent of other transactions. Transactions in a database environment have two main purposes:

To provide reliable units of work that allow correct recovery from failures and keep a database consistent even in cases of system failure, when execution stops (completely or partially) and many operations upon a database remain uncompleted, with unclear status.

To provide isolation between programs accessing a database concurrently. Without isolation the programs' outcomes are possibly erroneous.

A database transaction, by definition, must be atomic, consistent, isolated and durable. Database practitioners often refer to these properties of database transactions using the acronym ACID.

Transactions provide an "all-or-nothing" proposition, stating that each work-unit performed in a database must either complete in its entirety or have no effect whatsoever. Further, the system must isolate each transaction from other transactions, results must conform to existing constraints in the database, and transactions that complete successfully must get written to durable storage.

2.2              事务场景

网上购物的一次交易中付款过程包括如下几步：

1．  创建支付订单

2．  更新客户的现金账户余额

3．  更新商户的现金账户余额

4．  更新支付订单状态

5．  更新交易订单状态

6．  发送消息，通知商户发货：用户支付成功

正常情况下，这些操作顺利完成，最终交易成功，与交易相关的数据库信息成功更新，并且发送消息成功。但是如果这个过程中任何一个环节出错（第6项可选），比如客户账户余额不足，商户账户异常，都将导致交易失败，一旦交易失败，数据库中数据要保持状态不变，否则会出现很多状态不一致的情况。事务正是为了保证这种情况下交易的平稳性和可预测性的技术。

2.3              事务的工作原理

事务确保数据的ACID，工作原理如下：