About Oracle Real Application Clusters

Oracle RAC extends Oracle Database so that you can store, update, and efficiently retrieve data using multiple database instances on different servers at the same time. Oracle RAC provides the software that manages multiple servers as and instances as a single group. The data files that make up the database must reside on shared storage that is accessible from all servers that are part of the cluster. Each server in the cluster runs the Oracle RAC software.

An Oracle Database database has a one-to-one relationship between data files and the instance. An Oracle RAC database, however, has a one-to-many relationship between data files and instances. In an Oracle RAC database, multiple instances access a single set of database files.

Each database instance in an Oracle RAC database uses its own memory structures and background processes. Oracle RAC uses Cache Fusion to synchronize the data stored in the buffer cache of each database instance. Cache Fusion moves current data blocks (which reside in memory) between database instances, rather than having one database instance write the data blocks to disk and requiring another database instance to reread the data blocks from disk. When a data block located in the buffer cache of one instance is required by another instance, Cache Fusion transfers the data block directly between the instances using the interconnect, enabling the Oracle RAC database to access and modify data as if the data resided in a single buffer cache.

Oracle RAC is also a key component for implementing the Oracle enterprise grid computing architecture. Having multiple database instances accessing a single set of data files prevents the server from being a single point of failure. Any packaged or custom application that scales on an Oracle Database will scale well on Oracle RAC without requiring changes to the application code.

You will learn more about the operation of the Oracle RAC database in a cluster, how to build the cluster, and the structure of an Oracle RAC database in other sections of this guide.