At a very high level, RAC architecture consists of these components:
Physical nodes or hosts
Physical interconnects and interconnect protocols
Oracle instances and cache fusion
Shared disk system
Clustered file system, raw devices, network file system, Automatic
Workload Management Services - Virtual IP configuration
A complete Oracle instance consists of disk files, shared memory
structures, and background processes. The shared memory area is
further subdivided into numerous caches and pools which are used to
transfer data, programs, and instructions from processes to and from
the disks and users.
Each of the instances in the cluster configuration communicates with
other instances by using the clusterware. Clusterware
is the middleware that glues all the clustered instances and projects
a single database system image.
Cluster platforms depend on the cluster management program to track
the cluster node status.
Clusterware allows clustering of servers so that they act as a single
system. Clusterware is
supported and required on every operating system that is certified for
Online Install Guides for Clusterware and RAC
must be installed before installing the Oracle database software.
Below are the URLs of two excellent click–by-click Oracle 11g
RAC install guides.
Thanks to Jeff Hunter and Tim Hall for all their hard work to provide
these excellent resources.
Jeff's guide prints out to be over 130 pages!
Jeff Hunter's install guide:
Tim Hall's install guide:
Oracle Clusterware was released with Oracle 10g and was known as CRS
. The product was renamed
to Oracle Clusterware in version Oracle 10g R2.
Proving that the rename of CRS to Clusterware was more
marketing than anything, the reference to the clusterware home
directory still uses the name CRS_HOME.
Oracle Clusterware Explained
"In the event of a system failure, clustering ensures high
availability to users.
A redundant hardware component, such as additional nodes,
interconnects, and disks, allow the cluster to provide high
redundant hardware architectures avoid single points-of-failure
and provide exceptional fault resilience.
In Real Application Cluster environments, Oracle Clusterware monitors
and manages Real Application Cluster (RAC) databases.
When a node in a cluster is started, all instances, services
and listeners are started automatically.
If an instance fails, the clusterware will restart the instance
so the service is often restored before the administrator notices it
"In this sense, Oracle Clusterware is the basis for Oracle Real
Therefore, there needs to be one incarnation of the Oracle
Clusterware on every node of the cluster that an Oracle RAC Database
Instance is supposed to run on." (Quoted from Oracle)
provides node monitoring for the other nodes in the system via a
heartbeat signal sent over the cluster interconnect.
All nodes in a RAC cluster keep track of which nodes are
The shared storage provides concurrent access by all the cluster nodes
to the storage array. The storage array is presented in the form of
(LUNS) to the cluster host or node and the file system is mounted on
all nodes. Thus, when the same file system is mounted and used on all
nodes in the cluster, it is called a cluster file system.
Oracle 10g and 11g provides a flexible and high performing shared
storage methodology that is known as Automatic Storage Management
(ASM). ASM may used in lieu of the cluster file system.