The following list describes the processes that comprise CRS:
Cluster Ready Services (CRS): The primary program for managing high availability operations in a cluster.
The CRSD manages cluster resources based on the configuration information that is stored in OCR for each resource. This includes start, stop, monitor, and failover operations. The CRSD process generates events when the status of a resource changes. When you have Oracle RAC installed, the CRSD process monitors the Oracle database instance, listener, and so on, and automatically restarts these components when a failure occurs.
Cluster Synchronization Services (CSS): Manages the cluster configuration by controlling which nodes are members of the cluster and by notifying members when a node joins or leaves the cluster. If you are using certified third-party clusterware, then CSS processes interface with your clusterware to manage node membership information.
The cssdagent process monitors the cluster and provides I/O fencing. This service formerly was provided by Oracle Process Monitor Daemon (oprocd), also known as OraFenceService on Windows. A cssdagent failure may result in Oracle Clusterware restarting the node.
Oracle ASM: Provides disk management for Oracle Clusterware and Oracle Database.
Cluster Time Synchronization Service (CTSS): Provides time management in a cluster for Oracle Clusterware.
Event Management (EVM): A background process that publishes events that Oracle Clusterware creates.
Grid Naming Service (GNS): Handles requests sent by external DNS servers, performing name resolution for names defined by the cluster.
Oracle Agent (oraagent): Extends clusterware to support Oracle-specific requirements and complex resources. This process runs server callout scripts when FAN events occur. This process was known as RACG in Oracle Clusterware 11g release 1 (11.1).
Oracle Notification Service (ONS): A publish and subscribe service for communicating Fast Application Notification (FAN) events.
Oracle Root Agent(orarootagent): A specialized oraagent process that helps the CRSD manage resources owned by root, such as the network, and the Grid virtual IP address.
The Cluster Synchronization Service (CSS), Event Management (EVM), and Oracle Notification Services (ONS) components communicate with other cluster component layers on other nodes in the same cluster database environment. These components are also the main communication links between Oracle Database, applications, and the Oracle Clusterware high availability components. In addition, these background processes monitor and manage database operations.