Failover
Oracle Net Services supports failover, which occurs when an instance fails, both at connection time and when an application is running. By allowing failover, you let a user connected to the failed instance fail over to an instance that is still up and running. You enable failover by simply listing multiple listeners in a DESCRIPTION entry in the TNSNAMES.ORA file. Oracle Net Services supports failover both at connection time and when an application is running.
Connect-time failover allows you to direct connection requests to a backup instance in the event that the primary instance is down and is perhaps most useful when used in connection with Real Application Clusters (where you have multiple instances running against a single database). You enable connect-time failover by simply listing multiple listeners in a DESCRIPTION entry in the TNSNAMES.ORA file. (Failover might also be used in standby database environments to redirect a connect request to a standby database in case the primary database is down.)
Transparent Application Failover (TAF) provides failover when an application is running. If a connection fails, and TAF is enabled, the user automatically fails over to another listener. You can even specify that the connection to the other listener be established when the user makes the initial connection to the first listener, to reduce the failover time. In addition, most queries currently running with TAF enabled will maintain their location, so that the next row retrieved ...