What Is a Transaction?

A transaction is a set of related actions. A global transaction is a set of related actions that span multiple programs and resource managers. In this topic, whenever we use the term transaction, we are referring to a global transaction.

A simple example of a transaction is a withdrawal from a bank account, which can be described as a set of actions that changes the state of an account balance (by reducing it). For this transaction, the system must execute a procedure that consists of three operations.

Procedure for Any Transaction	Procedure for Bank Withdrawal Example
1. Verify the activity to be performed	1. Verify that a withdrawal will be made
2. Perform the work of the transaction	2. Withdraw a specified amount from the account
3. Create a permanent record of the completed work	3. Update the record of the balance of the account

These steps are performed by a discrete software module created expressly for the purpose of executing this transaction. The module must also include or use code that launches and ends the transaction. If the code sections that launch and end the transaction are not part of the main transaction software module, then they are usually packaged together in a separate module.

A transaction coordinator is a software module that executes the logic to manage a transaction among all participating resources.

What Are the ACID Properties?

When a transaction such as a bank withdrawal is performed, it is imperative that all its constituent operations either succeed or fail together. Consider the problems that can occur if one operation in a transaction succeeds while another operation in the same transaction fails: a bank that allows a customer to withdraw money without recording the reduced balance in an updated account record will not stay in business for long!

A transaction that adheres to the rule that all constituent operations either succeed or fail is characterized by atomicity. The BEA Tuxedo system requires all transactions to be characterized by atomicity and three related attributes: consistency, isolation, and durability. These four attributes are known collectively as the ACID properties of transactions performed within the BEA Tuxedo system.

ACID Properties of BEA Tuxedo Transactions

This Property . . .	Means That . . .
Atomicity	A transaction is a discrete unit of work: all constituent operations must either succeed or fail. These operations may include queuing messages, updating databases, and displaying the results of a transaction on a screen.
Consistency	A transaction must either (a) leave the system in a correct state or (b) abort. If a transaction cannot achieve a stable state, it must return to its initial state.
Isolation	The behavior of a transaction is not affected by other transactions being executed simultaneously. A transaction must serialize all access to shared resources and guarantee that concurrent programs do not corrupt each other's operations.
Durability	The effects of a committed transaction are permanent. Even if the system fails, the changes resulting from a transaction are permanent and durable.

If . . .	Then . . .
Any operation within the transaction fails for any reason	The transaction aborts, that is, it terminates abruptly. The transaction rolls back, that is, it undoes its own work and restores the state of the enterprise to its pre-transaction state. For example, after an attempt to withdraw money from a bank account fails and is rolled back, the bank account contains the same amount of money it contained before the transaction, and the record of the account balance shows the same amount that it showed before the transaction.
All operations within the transaction succeed	The client commits the transaction. In other words, it formally signals that it is ready to terminate and the effects of the transaction should be preserved: the order database is updated permanently and the order sent to the shipping department is kept as a permanent record in that department's queue.

Whether a transaction succeeds or fails depends on the requirements of atomicity.