Assuming that you have created an RAC database with a single instance named RACDB, you may want to add a new instance to this RAC database to avoid a single point of failure or to distribute your application workload over more instances. One of the primary advantages of an RAC database in contrast to a standalone database is that an RAC database can consist of two or more instances accessing the same database. Having two or more instances enables you to achieve scalability by balancing the application workload across available instances to achieve better performance. It also provides a high availability solution by reestablishing the lost user connections rapidly from a failed instance to the other surviving instance in the RAC database.
Adding more instances to an RAC database is a very simple task and Oracle provides the following ways to achieve this:
DBCA
Enterprise Manager (grid control)
Manual method