Oracle RAC is composed of two or more DATABASE instances. They are composed of Memory structures and background processes same as the single instance database.Oracle RAC instances use two processes GES(Global Enqueue Service), GCS(Global CACHE Service) that enable cache fusion.Oracle RAC instances are composed of following background processes:

• ACMS—Atomic Controlfile to Memory Service (ACMS).
• GTX0-j—Global Transaction Process.
• LMON—Global Enqueue Service Monitor.
• LMD—Global Enqueue Service Daemon.
• LMS—Global Cache Service Process.
• LCK0—Instance Enqueue Process.
• RMSn—Oracle RAC MANAGEMENT Processes (RMSn).
• RSMN—Remote Slave Monitor.

Oracle RAC is composed of two or more database instances. They are composed of Memory structures and background processes same as the single instance database.Oracle RAC instances use two processes GES(Global Enqueue Service), GCS(Global Cache Service) that enable cache fusion.Oracle RAC instances are composed of following background processes: