The Oracle Real Application Clusters (RAC) option was first released in a blaze of publicity as part of Oracle 9.0.1 in the summer of 2001. It is considered by many within the Oracle community to be the most significant feature introduced in Oracle 9i.
Although RAC has been marketed as an entirely new feature, much of the technology was
inherited from Oracle Parallel Server (OPS), which was introduced in Oracle 6.0 and developed
steadily over the next decade.
A cluster consists of multiple interconnected servers that appear to end users and applications as if they are one single server. A RAC database allows multiple instances residing on different servers in the cluster to access a common database residing on shared storage. The combined processing power of the multiple servers in the cluster can provide greater throughput and scalability than is available from a single server. A basic RAC setup is depicted in Figure 1-1.
Oracle defines the database as being the structures that reside in files permanently on disk, including the datafiles, the control files, and the online redo logs. In the case of a RAC cluster, the database must be located on shared physical disk storage that all servers in the cluster must be able to access equally. The servers communicate with the shared storage via a storage network. The instance is defined as the structures that are created in memory within the operating system running on a server, including the Shared Global Area (SGA) and the associated background processes. An instance must be started before a database can be created. All persistent data is stored in the database files and will remain on disk when the instance or the operating system is shut down.
An instance can only mount and open a single database. In a single instance environment, as the name suggests, there is a one-to-one mapping between an instance and a database. A database, however, may be mounted by one or more instances on separate servers. The database may be accessed as long as at least one instance is started and has mounted it.
The servers in the cluster are bound together using cluster management software called Oracle
Clusterware, which enables the servers to appear as though they are a single server. Servers in the cluster communicate with each other using a dedicated private network known as the cluster
interconnect. In a RAC cluster, each instance can execute transactions concurrently because database access is coordinated to ensure data consistency and integrity. Instances exchange messages with each other across the interconnect to manage the state of individual Oracle data blocks and locks. Each server is also connected to a public network that allows the cluster to communicate with the outside world, including clients and application servers.
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