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 third-party clusterware, then the css process interfaces with your clusterware to manage node membership information.
Cluster Ready Services (CRS)—The primary program for managing high availability operations within a cluster. Anything that the crs process manages is known as a cluster resource which could be a database, an instance, a service, a Listener, a virtual IP (VIP) address, an application process, and so on. The crs process manages cluster resources based on the resource's configuration information that is stored in the OCR. This includes start, stop, monitor and failover operations. The crs process 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. By default, the crs process makes five attempts to restart a resource and then does not make further restart attempts if the resource does not restart.
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.
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