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What Is Oracle Real Application Clusters?
Real Application Clusters is a software that enables you to use clustered hardware by running multiple instances against the same database. The database files are stored on disks that are either physically or logically connected to each node, so that every active instance can read from or write to them.
The Real Application Clusters software manages data access, so that changes are coordinated between the instances and each instance sees a consistent image of the database. The cluster interconnect enables instances to pass coordination information and data images between each other.
This architecture enables users and applications to benefit from the processing power of multiple machines. RAC architecture also achieves redundancy in the case of, for example, a system crashing or becoming unavailable; the application can still access the database on any surviving instances.