【71-75】
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 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 successfully completed（committed） transaction are permanently recorded in the database and must not be lost because of 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.
