Concurrency is a technique that is used in an operating system in which the central processing unit--the CPU--is shared between several jobs that are independent of one another. This is also known as multitasking.

Concurrency is especially important in database management systems where there can be any number of database transactions taking place at the same time. The potential for error can be greater in a distributed database management system, where data may be replicated and partitioned in various sites throughout the database file. If a user wants unique access to the data, to read or update the data for example, then unique access must be guaranteed, even with copies of the data throughout the database. A concurrency control mechanism helps ensure that the simultaneous, or concurrent, transactions do not interfere with each other.

Although a transaction is conducted properly, it may produce the wrong answer because of...