In a RAC setup, each node has its own memory; however, they share physical datafiles. As we know that data blocks are read into memory for updates or query in a single instance database. Similarly, in a RAC set up, a node reads a block from datafile when the node needs it. However, in RAC setting, the required block may be already available in the memory of another node and it is faster to read block from the memory of another block than from a datafile. Therefore, RAC provides a mechanism to read block from memory of one node to the memory of another node. This mechanism is called cache fusion.
Oracle uses high-speed interconnect to communicate between nodes. GES monitors and the Instance Enqueue process manages the cache fusion.