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 and with a seperate software for Linux and Windows.
Oracle clusterware naming Evolution:
In 9i – Third Party softwares (Sun Cluster or Veritas Cluster Server)
In 10gR1-Cluster Ready services (Oracle corporation)
In 10gR2- Oracle Clusterware (Oracle corporation)
Oracle Clusterware stores configuration information in the Oracle Cluster Repository (OCR),
which must be located on shared storage available to all nodes. The Membership of the cluster is determined using a file known as the voting disk, which must also be available to all nodes. Voting Disk acts a tiebreaker during communication failures and Consistent heartbeat information from all the nodes is sent to voting disk when the cluster is running .
Oracle Clusterware comprises a set of daemons(Background process).
1.Cluster Ready Services Daemon(crsd)
This component manages high availability operations within the cluster. Objects managed by CRS are known as resources and can include databases, instances, services, listeners, virtual IP addresses, and application processes. By default, CRS manages four application process resources: Oracle Net listeners, virtual IP addresses, the Global Services Daemon (GSD), and the Oracle Notification Service (ONS).
Configuration information about each resource is stored in the Oracle Cluster Repository
(OCR). When the status of a resource changes, CRS generates an event.
CRS monitors resources, such as instances and listeners. In the event of the failure of a resource, CRS will attempt to automatically restart the component.
2.Oracle Cluster Synchronization Service Daemon (OCSSD)
This component manages the cluster configuration by controlling which nodes are members of the cluster. When a node joins or leaves the cluster, CSS notifies the other nodes of the change in configuration.
If this process fails, then the cluster will be restarted.
3.Event Volume Manager Daemon (evmd)
The EVM component publishes events created by Oracle Clusterware. It will restart Automatically on failures and death of the evmd process does not halt the instance
4.Process Monitor Daemon (oprocd)
Oprocd provides the I/O Fencing solution for the Oracle Clusterware. It is the process monitor for the oracle clusterware and it uses the hang check timer or watchdog timer (depending on the implementation) for the cluster integrity.