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Friday, November 23, 2007

What is a Transaction

What is Transaction Processing?
In Chapter 1, you were introduced to the concepts behind transaction processing. But you may still be wondering just what this is. Transaction processing has been around since the mainframe days of computing. You may have heard of, or have even used, products such as CICS, Tuxedo, or TopEnd. These are all examples of transaction processing systems, which provide transaction services to applications that use them. There are a number of attributes that make up transaction processing.

In order to discuss transaction processing, we must first agree on a definition of what a transaction is. A transaction is an atomic unit of work that either fails or succeeds. There is no such thing as a partial completion of a transaction. Since a transaction can be made up of many steps, each step in the transaction must succeed for the transaction to be successful. If any one part of the transaction fails, then the entire transaction fails. When a transaction fails, the system needs to return to the state that it was in before the transaction was started. This is known as rollback. When a transaction fails, then the changes that had been made are said to be "rolled back." In effect, this is acting similar to the way the Undo command works in most word processors. When you select undo, the change that you just may have made is reversed. The transaction processing system is responsible for carrying out this undo.
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- In computer programming, a transaction usually means a sequence of information exchange and related work (such as database updating) that is treated as a unit for the purposes of satisfying a request and for ensuring database integrity. For a transaction to be completed and database changes to made permanent, a transaction has to be completed in its entirety. A typical transaction is a catalog merchandise order phoned in by a customer and entered into a computer by a customer representative. The order transaction involves checking an inventory database, confirming that the item is available, placing the order, and confirming that the order has been placed and the expected time of shipment. If we view this as a single transaction, then all of the steps must be completed before the transaction is successful and the database is actually changed to reflect the new order. If something happens before the transaction is successfully completed, any changes to the database must be kept track of so that they can be undone.
A program that manages or oversees the sequence of events that are part of a transaction is sometimes called a transaction monitor. Transactions are supported by Structured Query Language, the standard database user and programming interface. When a transaction completes successfully, database changes are said to be committed; when a transaction does not complete, changes are rolled back. In IBM's Customer Information Control System product, a transaction is a unit of application data processing that results from a particular type of transaction request. In CICS, an instance of a particular transaction request by a computer operator or user is called a task.

Less frequently and in other computer contexts, a transaction may have a different meaning. For example, in IBM mainframe operating system batch processing, a transaction is a job or a job step.

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