Further Reading
See RAC home page on Oracle's website.
Oracle Real Application Clusters (RAC) is a database clustering technology offering increased availability and scalability. RAC is included in the price of the Standard Edition of the Oracle Database, making it affordable for even quite modest production systems.
History
Oracle has offered a clustered database solution since the introduction of Oracle Parallel Server (OPS) in the late eighties. This technology forms the basis of RAC which was introduced in 2001 with Oracle 9i Release 1. The product has continued to evolve at a rapid rate to the current version which is Oracle 10g Release 2.
A RAC cluster consists of a database which is located on shared storage and two or more servers (nodes) each of which runs an instance. An instance is an area of shared memory together with a set of processes which interact to manipulate the data in the database. The instances communicate with each other via a dedicated private network known as the “interconnect” to ensure the integrity of the shared data.
Why use RAC?
What RAC allows you to do is loosely couple a set of servers either to improve scalability, availability or both. In our experience, most RAC customers are using it to improve reliability. If one of the servers fails for some reason (e.g. hardware, network etc) the database continues to be accessible to sessions running on the remaining nodes.
In a similar way that you would buy a redundant power supply for your server, you can now have a redundant server for your database.