A runtime failover for high-availability environments, such as Oracle9i Real Application Clusters and Oracle Fail Safe, that refers to the failover and re-establishment of application-to-service connections. It enables client applications to automatically reconnect to the database if the connection fails, and, optionally, resume a SELECT statement that was in progress. This reconnect happens automatically from within the Oracle Call Interface (OCI) library.

Transparent Application Failover (TAF) is a client-side feature that allows for clients to reconnect to surviving databases in the event of a failure of a database instance. Notifications are used by the server to trigger TAF callbacks on the client-side.

TAF is configured using either client-side specified TNS connect string or using server-side service attributes. However, if both methods are used to configure TAF, the server-side service attributes will supersede the client-side settings. The server-side service attributes are the preferred way to set up TAF.
TAF can operate in one of two modes, Session Failover and Select Failover. Session Failover will recreate lost connections and sessions. Select Failover will replay queries that were in progress.

TAF automatically reestablishes the connection using the same connect string or an alternate connect string that you specify when configuring failover. TAF automatically logs a user in with the same user ID as was used prior to 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, however, be restored by invoking a callback function. If a command was completely executed upon connection failure, and it changed the state of the database, TAF does not resend the command. If TAF reconnects in response to a command that may have changed the database, TAF issues an error message to the application. 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 is submitted.

A sample configuration of a failover would look like

sales.us.acme.com= (DESCRIPTION= (ADDRESS= (PROTOCOL=tcp) (HOST=sales1-server) (PORT=1521)) (CONNECT_DATA= (SERVICE_NAME=sales.us.acme.com) (FAILOVER_MODE= (TYPE=select) (METHOD=basic) (RETRIES=20) (DELAY=15))))

It is important to note that you can pre-establish a connection to reduce the failover time. This is done with the failover time. You can do this with the METHOD=preconnect option instead of the METHOD=basic definition.