2. 计算机
  3. A transaction is an action，or a series of actions，carried out by a single user or (71)________________，that accesses or changes

A transaction is an action，or a series of actions，carried out by a single user or (71)________________，that accesses or changes

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问题     A transaction is an action，or a series of actions，carried out by a single user or  (71)________________，that accesses or changes the contents of the database．
    A transaction should possess the four basic or so-called ACID properties：
    Atomicity．The“all or nothing”property．A transaction is an (72)________________ unit that is either  performed in its entirety or is not performed at all．
    Consistency．A transaction must transform the database from one consistent state to another consistent state．
    Isolation．Transactions execute independently of one another．In other words，the partial effects of  (73)________________ transactions should not be visible to other transactions．
    Durability．The effects of a successfully completed(committed)transaction are permanently recorded in the database and must not be lost because of a subsequent failure．
    The four properties above，only (74)________________ property is the responsibility of the concurrency  control subsystem to ensure．It is the responsibility of both the (75)________________ and the application  developers to ensure consistency．To guarantee the Atomicity and Durability properties after a  failure，a log file is needed．
选项 A、DBMS
B、DBA
C、end user
D、DBS
答案A
解析     事务是由单个用户或应用程序执行的访问或更改数据库内容的操作或一系列操作。
    交易应具有四种基本或所谓的酸性性质：
    原子性。“全部或没有”属性。事务是一个不可分割的单元，要么全部执行，要么根本不执行。
    一致性。事务必须将数据库从一个一致状态转换为另一个一致状态。
    隔离。事务彼此独立执行。换句话说，不完整交易的部分影响对其他交易不应是可见的。
    持久性。成功完成(提交)事务的影响将永久记录在数据库中，并且不能由于随后的失败而丢失。
    上面的四个属性中，只有隔离属性是并发控制子系统的责任。DBMS和应用程序开发人员都有责任确保一致性。为了保证故障后的原子性和持久性属性，需要一个日志文件。
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