Major Oracle Clusterware components

The Oracle Clusterware comprises several background processes that facilitate cluster operations. These processes or components are the main communication links between the Oracle Clusterware high availability components and the Oracle Database as well as they monitor and manage database operations.

Here is the list of major oracle clusterware components or processes.

1)Cluster Synchronization Services (CSS): It manages and controls which nodes are members of the cluster and notify members when a node joins or leaves the cluster.

2)Cluster Ready Services (CRS): It manages high availability operations within a cluster. The CRS process start, stop, monitor and failover operations. It generates events when a resource status changes. When you have installed Oracle RAC, crs monitors the Oracle instance, Listener, and so on, and automatically restarts these components when a failure occurs.

3)Event Management (EVM): It is a background process that publishes events that crs creates.

4)Oracle Notification Service (ONS): It publishes and subscribes service for communicating Fast Application Notification (FAN) events.

5)RACG: It runs server callout scripts when FAN events occur.

6)Process Monitor Daemon (OPROCD): This process is locked in memory to monitor the cluster and provide I/O fencing. OPROCD performs its check, stops running, and if the wake up is beyond the expected time, then OPROCD resets the processor and reboots the node. An OPROCD failure results in Oracle Clusterware restarting the node. OPROCD uses the hangcheck timer on Linux platforms.