Transaction Processing

A transaction delimits the beginning and end of a series of data access operations executed across a connection. Subject to the transactional capabilities of your data source, the Connection object also allows you to create and manage transactions. For example, using the Microsoft OLE DB Provider for SQL Server to access a database on Microsoft SQL Server, you can create multiple nested transactions for the commands you execute.

ADO ensures that changes to a data source resulting from operations in a transaction occur successfully together or not at all.

If you cancel the transaction, or if one of its operations fails, the result will be as if none of the operations in the transaction occurred. The data source will remain as it was before the transaction began.

ADO provides the following methods for controlling transactions: BeginTrans, CommitTrans, and RollbackTrans. Use these methods with a Connection object when you want to save or cancel a series of changes made to the source data as a single unit. For example, to transfer money between accounts, you subtract an amount from one and add the same amount to the other. If either update fails, the accounts no longer balance. Making these changes within an open transaction ensures that either all or none of the changes go through.

After you call the BeginTrans method, the provider will no longer instantaneously commit changes you make until you call CommitTrans or RollbackTrans to end the transaction.

Calling the CommitTrans method saves changes made within an open transaction on the connection and ends the transaction. Calling the RollbackTrans method reverses any changes made within an open transaction and ends the transaction. Calling either method when there is no open transaction generates an error.

Depending on the Connection object's Attributes property, calling either the CommitTrans or RollbackTrans method may automatically start a new transaction. If the Attributes property is set to adXactCommitRetaining, the provider automatically starts a new transaction after a CommitTrans call. If the Attributes property is set to adXactAbortRetaining, the provider automatically starts a new transaction after a RollbackTrans call.

Transaction Isolation Level

Use the IsolationLevel property to set the isolation level of a transaction on a Connection object. The setting does not take effect until the next time you call the BeginTrans method. If the level of isolation you request is unavailable, the provider may return the next greater level of isolation. Refer to the IsolationLevel property in the ADO Programmer's Reference for more details on valid values.

Nested Transactions

For providers that support nested transactions, calling the BeginTrans method within an open transaction starts a new, nested transaction. The return value indicates the level of nesting: a return value of "1" indicates you have opened a top-level transaction (that is, the transaction is not nested within another transaction), "2" indicates that you have opened a second-level transaction (a transaction nested within a top-level transaction), and so forth. Calling CommitTrans or RollbackTrans affects only the most recently opened transaction; you must close or roll back the current transaction before you can resolve any higher-level transactions.