Oracle RAC database instance is a set of processes and memory that operates on the database files. Oracle RAC database instance communicates through the database processes and Oracle Clusterware with other Oracle RAC database instances in the same cluster. Oracle RAC database instance has the same memory and background processes as a single instance database, with the addition of Cache Fusion protocol implemented by database processes. Cache Fusion is a disk-less cache coherency mechanism in Oracle RAC that provides copies of blocks directly from a holding instance's memory cache to a requesting instance's memory cache. This provides a logical merging of the System Global Area (SGA) across the database instances in the cluster. Global processes (not all are shown) provide the necessary services to maintain the cache coherency, and recovery in the event of a node failure.
Cache Fusion use the private network (interconnect) for communication between cluster nodes. The private network facilitates communication between Oracle RAC instances. This includes the exchange of information related to data consistency, cache management, and lock management.