Oracle Clusterware is the cross platform cluster software required to run the Real Application Clusters

Oracle RAC

In database computing, Oracle Real Application Clusters — an option for the Oracle Database software produced by Oracle Corporation and introduced in 2001 with Oracle9i — provides software for clustering and high availability in Oracle database environments...

 (RAC) option for Oracle Database

Oracle Database

The Oracle Database is an object-relational database management system produced and marketed by Oracle Corporation....

. 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

Veritas Cluster Server

Veritas Cluster Server is a High-availability cluster software, for Unix, Linux and Microsoft Windows computer systems, created by Veritas Software...

 with the exception of Linux

Linux

Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

 and Windows

Microsoft Windows

Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

.

Oracle Clusterware Components

Oracle Clusterware is the software, which enables the nodes to communicate with each other, and forms the cluster and makes the nodes act as single logical server. Oracle Clusterware is run by Cluster Ready Services (CRS) using two key components. They are Oracle Cluster Registry (OCR), which records and maintains the cluster and node membership information. The other component is voting disk, which acts a tiebreaker during communication failures. Consistent heartbeat information from all the nodes is sent to voting disk when the cluster is running.
CRS service has four components namely OPROCd, CRS Daemon (crsd), Oracle Cluster Synchronization Service Daemon (OCSSD) and Event Volume Manager Daemon (evmd) and each handles a variety of functions. Failure or death of the CRS daemon can cause the node failure and it automatically reboots the nodes to avoid the data corruption because of the possible communication failure between the nodes. The CRS daemon runs as the super user ‘root’ in the UNIX

Unix

Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

 platforms and runs as a service in the windows platforms.

The following functionalities are covered by Oracle Cluster Ready Services

• CRS is installed and run from a different oracle home known as ORA CRS HOME, which is independent from ORACLE HOME.

crsd

• CRSd manages the resources like starting and stopping the services and failing-over the application resources. It spawns separate processes to manage application resources.
• CRS daemon has two modes of running. During startup and after a shutdown. During planned clusterware start it is started as ‘reboot’ mode. It is started as ‘restart’ mode after unplanned shutdown.
• In reboot mode it ‘auto’ starts all the resources under its management. In restart mode it prevails the previous state and brings back the resources to it previous state before shutdown
• Manages the Oracle Cluster Registry and stores the current known state in the Oracle Cluster Registry
• Runs as ‘root’ on Unix and ‘LocalSystem’ on windows and automatically restarts in case of failure.
• CRS requires the public interface, private interface and the Virtual IP (VIP) for the operation. All these interfaces should be up and running, should be able to ping each other before starting CRS Installation. Without the above network infrastructure CRS cannot be installed.

ocssd

Oracle Cluster Synchronization Services Daemon (ocssd) provides synchronization services between nodes. OCSSD provides the access to the node membership. It also enables 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 split-brain situation. This is also required in a single instance configuration if Automatic Storage Management

Automatic Storage Management

Automatic Storage Management is a feature provided by Oracle Corporation within the Oracle Database from release Oracle 10g onwards. ASM aims to simplify the management of database files...

 (ASM) is used. Automatic Storage management was a new feature in oracle database 10g. Ocssd runs as ‘oracle’ user. The following functionalities are covered by CSS daemon

• CSS 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.
• ’Group services’ use vendor clusterware group services when vendor clusterware is available. But it is capable of working independently if there is no vendor clusterware group services available
• ‘Lock services’ is another service from the CSS daemon. Lock services provide the basic cluster wide serialization locking functions. It uses FIFO mechanism to manage locking
• Node Services is the third service produced by the CSSD. It uses OCR to store the data and updates the information during reconfiguration. It also manages the OCR data, which is static otherwise.

evmd

The third component in OCS is called Event Management Logger. Event Management logger also runs as daemon process ‘evmd’. The daemon process ‘evmd’ spawns a permanent child process called ‘evmlogger’ and generates the events when things happen. EVMD child process ‘evmlogger’ spawns new children processes on demand and scans the callout directory to invoke callouts. It will restart automatically on failures and death of the evmd process does not halt the instance. Evmd runs as ‘oracle’ user.

oprocd

Oprocd provides the server fencing solution for the Oracle Clusterware. It is the process monitor for the oracle clusterware and it uses the hang check timer or watchdog

Watchdog timer

A watchdog timer is a computer hardware or software timer that triggers a system reset or other corrective action if the main program, due to some fault condition, such as a hang, neglects to regularly service the watchdog A watchdog timer (or computer operating properly (COP) timer) is a computer...

 timer (depending on the implementation) for the cluster integrity. Oprocd is locked in the memory and runs as a real time

Real-time computing

In computer science, real-time computing , or reactive computing, is the study of hardware and software systems that are subject to a "real-time constraint"— e.g. operational deadlines from event to system response. Real-time programs must guarantee response within strict time constraints...

process. This sleeps for a fixed time and runs as ‘root’ user. Failure of the Oprocd process causes the node to restart.

External links

• Oracle Real Application Clusters Administrator's Guide
• Oracle Database 10g Real Application Clusters Handbook - Oracle Press
• Using srvctl to Manage your 10g RAC Database - includes description of Oracle Clusterware components.