Oracle Clusterware

Oracle Clusterware is the cross platform cluster software required to run the Real Application Clusters (RAC) option for Oracle Database. It provides the basic clustering services at the operating system level that enable Oracle software to run in clustering mode. In earlier versions of Oracle (version 9i and earlier), RAC required a vendor supplied clusterware like Sun Cluster or Veritas Cluster Server with the exception of Linux and Windows.

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 a 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.

The following functions are provided by the Oracle Cluster Ready Services daemon (CRSd):

Oracle Cluster Synchronization Services daemon (OCSSd) provides basic ‘group services’ support. Group Services is a distributed group membership system that allows the applications to coordinate activities to achieve a common result. As such, it provides synchronization services between nodes, access to the node membership information, as well as enabling basic cluster services, including cluster group services and cluster locking. It can also run without integration with vendor clusterware. Failure of OCSSd causes the machine to reboot to avoid a split-brain situation. This is also required in a single instance configuration if Automatic Storage Management (ASM) is used. ASM was a new feature in Oracle 10g. OCSSd runs as the "oracle" user.

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