Oracle Clusterware Components
Oracle Clusterware is the software which enables the nodes to communicate with each other, allowing them to form the cluster of nodes which behaves as single logical server. Oracle Clusterware is run by Cluster Ready Services (CRS) consisting of two key components: Oracle Cluster Registry (OCR), which records and maintains the cluster and node membership information; voting disk, which polls for consistent heartbeat information from all the nodes when the cluster is running, and acts as a tiebreaker during communication failures.
The CRS service has four components, each handling a variety of functions: Cluster Ready Services daemon (CRSd), Oracle Cluster Synchronization Service Daemon (OCSSd), Event Volume Manager Daemon (EVMd), and Oracle Process Clusterware Daemon (OPROCd). Failure or death of the CRS daemon can cause node failure, which triggers automatic reboots of the nodes to avoid the corruption of data (due to the possible failure of communication between the nodes), also known as fencing. The CRS daemon runs as "root" (super user) on UNIX platforms and runs as a service on Windows platforms.
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