About Transactions

A transaction is a series of discreet actions in an application that must all complete successfully. By enclosing one or more actions in an indivisible unit of work, a transaction ensures data integrity and consistency. If all actions do not complete, the changes are rolled back.

For example, to transfer funds from a checking account to a savings account, the following steps typically occur:

1. Check to see if the checking account has enough money to cover the transfer.

2. Debit the amount from the checking account.

3. Credit the amount to the savings account.

4. Record the transfer to the checking account log.

5. Record the transfer to the savings account log.

These steps together are considered a single transaction.

If all the steps complete successfully, the transaction is committed. If any step fails, all changes from the preceding steps are rolled back, and the checking account and savings account are returned to the states they were in before the transaction started. This type of event is called a rollback. A normal transaction ends in either a committed state or a rolled back state.

The following elements contribute to reliable transaction processing by implementing various APIs and functionalities:

• Transaction Manager. Provides the services and management functions required to support transaction demarcation, transactional resource management, synchronization, and transaction context propagation.

• Enterprise Server. Provides the infrastructure required to support the application runtime environment that includes transaction state management.

• Resource Manager. Through a resource adapter, the resource manager provides the application access to resources. The resource manager participates in distributed transactions by implementing a transaction resource interface used by the transaction manager to communicate transaction association, transaction completion, and recovery work. An example of such a resource manager is a relational database server.

• Resource Adapter. A system-level software library is used by Enterprise Server or a client to connect to a resource manager. A resource adapter is typically specific to a resource manager. The resource adapter is available as a library and is used within the address space of the client using it. An example of such a resource adapter is a Java Database Connectivity (JDBC) driver. For information on supported JDBC drivers, see Configuration Specifics for JDBC Drivers.

• Transactional User Application. In the Enterprise Server environment, the transactional user application uses Java Naming and Directory Interface (JNDI) to look up transactional data sources and, optionally, the user transaction). The application might use declarative transaction attribute settings for enterprise beans, or explicit programmatic transaction demarcation.