CHAPTER 3
Oracle RAC Architecture

Oracle Real Application Clusters allows multiple instances to access a single database. Instances running on multiple server nodes access the common set of datafiles comprising a single database. In a single-instance environment, one Oracle database is used by only one instance running on a server machine; thus, users accessing the database can connect to the database via that single server only. The processing resources (CPU memory and so on) available for the database work are limited to the single server’s processing resources. In an Oracle RAC environment, more than one instance can use the same database. This scenario presents multiple processing resources for database users.

NOTE

An instance is a set of memory structures in a machine associated with the database. A database is a collection of physical files. Database and instance share one-to-many relationships. A database can be concurrently mounted by more than one instance in Oracle RAC, and at any point, one instance will be part of only one database.