Think about a situation where you have an Oracle database with a physical standby configuration and you want you applications still be able to connect to the database after you switch-over the roles. Besides, you want this transition transparent to the application without any change in the configuration what so ever…
The below document is a sample configuration of that… The application layer will be unaware of the switch-over operation because it will still be able to connect to the service which also switched-over with the primary database. This is possible with virtual-service concept and some database triggers that will wake-up this virtual service or disable it according to the current role of the database.
So, here is the documentation: