13.3.2 What Transparent Application Failover Restores

TAF automatically restores some or all of these elements associated with active database connections. Other elements may need to be embedded in the application code to enable TAF and recover a connection.

• Client-server database connections: TAF automatically reestablishes the connection using the same connect string or an alternate connect string that you specify when configuring failover.

• Users' database sessions: TAF automatically logs a user in with the same user ID as was used before the failure. If multiple users were using the connection, then TAF automatically logs them in as they attempt to process database commands. Unfortunately, TAF cannot automatically restore other session properties. These properties can be restored by invoking a callback function.

• Completed commands: If a command was completed at the time of connection failure, and it changed the state of the database, then TAF does not resend the command. If TAF reconnects in response to a command that may have changed the database, then TAF issues an error message to the application.

• Open cursors used for fetching: TAF allows applications that began fetching rows from a cursor before failover to continue fetching rows after failover. This is called select failover. It is accomplished by re-running a SELECT statement using the same snapshot, discarding those rows already fetched and retrieving those rows that were not fetched initially. TAF verifies that the discarded rows are those that were returned initially, or it returns an error message.

• Active transactions: Any active transactions are rolled back at the time of failure because TAF cannot preserve active transactions after failover. The application instead receives an error message until a ROLLBACK command is submitted.

• Server-side program variables: Server-side program variables, such as PL/SQL package states, are lost during failures, and TAF cannot recover them. They can be initialized by making a call from the failover callback.