51.2 What Is a Transaction?

To emulate a business transaction, a program may need to perform several steps. A financial program, for example, might transfer funds from a checking account to a savings account by using the steps listed in the following pseudocode:

begin transaction
    debit checking account
    credit savings account
    update history log
commit transaction

Either all or none of the three steps must complete. Otherwise, data integrity is lost. Because the steps within a transaction are a unified whole, a transaction is often defined as an indivisible unit of work.

A transaction can end in two ways: with a commit or with a rollback. When a transaction commits, the data modifications made by its statements are saved. If a statement within a transaction fails, the transaction rolls back, undoing the effects of all statements in the transaction. In the pseudocode, for example, if a disk drive were to crash during the credit step, the transaction would roll back and undo the data modifications made by the debit statement. Although the transaction fails, data integrity would be intact because the accounts still balance.

In the preceding pseudocode, the begin and commit statements mark the boundaries of the transaction. When designing an enterprise bean, you determine how the boundaries are set by specifying either container-managed or bean-managed transactions.