问题 问答题 简答题

试述两段锁协议的概念。

答案

参考答案:

所谓两段锁协议是指所有事务必须分两个阶段对数据项加锁和解锁。

1)在对任何数据进行读、写操作之前,首先要申请并获得对该数据的封锁;

2)在释放一个封锁之后,事务不再申请和获得对该数据的封锁。

所谓“两段”锁的含义是,事务分为两个阶段,第一阶段是获得封锁,也称为扩展阶段。在这个阶段,事务可以申请获得任何数据项上的任何类型的锁,但是不释放任何锁。第二阶段是释放封锁,也称为收缩阶段。在这个阶段,事务可以释放任何数据项上的任何类型的锁,但是不能再申请任何锁。

单项选择题
单项选择题

The standardized educational or psychological tests, that are widely used to aid in selecting, assigning, or promoting students, employees, and military personnel have been the target of recent attacks in books, magazines, the daily press, and even in Congress. The target is wrong, for in attacking the tests, critics divert attention from the fault that lies with ill-informed or incompetent users. The tests themselves are merely tools, with characteristics that can be measured with reasonable precision under specified conditions. Whether the results will be valuable, meaningless, or even misleading depends partly upon the tool itself but largely upon the user.

All informed predictions of future performance are based upon some knowledge of relevant past performance. How well the predictions will be validated by later performance depends upon the amount, reliability, and appropriateness of the information used and on the skill and wisdom with which it is interpreted. Anyone who keeps careful score knows that the information available is always incomplete and that the predictions are always subject to error.

Standardized tests should be considered in this context. They provide a quick, objective method of getting some kinds of information about what a person has learned, the skills he has developed, or the kind of person he is. The information so obtained has, qualitatively, the same advantages and shortcomings as other kinds of information. Whether to use tests, other kinds of information, or both in a particular situation depends, therefore, upon the empirical evidence concerning comparative validity, and upon such factors as cost and availability.

In general, the tests work most effectively when the traits or qualities to be measured can be most precisely defined (for example, ability to do well in a particular course of training program) and least effectively when what is to be measured or predicted cannot be well defined (for example, personality or creativity). Properly used, they provide a rapid means of getting comparable information about many people. Sometimes they identify students whose high potential has not been previously recognized, but there are many things they do not do. For example, they don’t compensate for gross social inequality, and thus don’t tell how able an underprivileged younger might have been had he grown up under more favorable circumstances.

According to the text, the author’s attitude toward the value of standardized tests seems to be()

A. critical

B. dubious

C. objective

D. ambiguous