DATA INTEGRITY - Database Design and Development: An Essential Guide for IT Professionals - page 443

BEGIN END

PARTIALLY
COMMITTED

ACTIVE

READ

WRITE

FAILED COMMITTED

TERMINATED

Figure 15-3 Operations and states of a transaction.

END Indicates end of transaction read or write operations.

Transaction proceeds to partially committed state.

COMMIT Signifies successful end of transaction so that any database

updates can be safely confirmed or committed to the database.
Updates cannot be undone.
Transaction moves to committed state and finally reaches the
terminated state.

ABORT Indicates failure of transaction so that effects of any database

updates must be undone.
Transaction proceeds to failed state from either active state
or partially committed state depending on the nature of the
failure.

ROLLBACK Indicates that the recovery manager needs to roll back database

updates and restore database to prior consistent state.
After rollback, transaction reaches the terminated state.

Processing of Transactions

According to C. J. Date, an eminent database expert, “The fundamental purpose of
the database system is to carry out transactions .” As already mentioned, a database
system is a transaction-processing environment. The database is there to support
processing of transactions.
In the process of supporting transactions, what is the impact of transactions on
the database itself? How do the users consider transactions? What purpose do trans-
actions serve for reading and changing database content? Let us go over a few
important aspects of transactions.

Transaction Structure Typically, a transaction consists of three major phases:

Input phase Receive messages and parameters to carry out the

necessary interaction with the database.

Database interaction Perform database read or write.