Oracle Database 11g Real Application Clusters (Oracle RAC) enables the clustering of the Oracle Database on the Database Domain. Oracle RAC uses Oracle Clusterware for the infrastructure to cluster the Database Domain on the compute nodes.
Oracle Clusterware is a portable cluster management solution that is integrated with the Oracle database. Oracle Clusterware is also a required component for using Oracle RAC. Oracle Clusterware enables you to create a clustered pool of storage to be used by any combination of single-instance and Oracle RAC databases.
Single-instance Oracle databases have a one-to-one relationship between the Oracle database and the instance. Oracle RAC environments, however, have a one-to-many relationship between the database and instances. In Oracle RAC environments, the cluster database instances access one database. The combined processing power of the multiple servers can provide greater throughput and scalability than is available from a single server. Oracle RAC is the Oracle Database option that provides a single system image for multiple servers to access one Oracle database.
Oracle RAC technology provides high availability and scalability for all application types. The Oracle RAC infrastructure is also a key component for implementing the Oracle enterprise grid computing architecture. Having multiple instances access a single database prevents the server from being a single point of failure. Applications that you deploy on Oracle RAC databases can operate without code changes.