Oracle RAC. Introduction to Oracle RAC.
Nov 29, 2019.
Prakash.

Overview of Oracle RAC
A cluster comprises multiple interconnected computers or servers that appear as if they are one server to end users and applications.
Oracle RAC enables you to cluster an Oracle database. Oracle RAC uses Oracle Clusterware for the infrastructure to bind multiple
servers so they operate as a single system.

Oracle Clusterware is a portable cluster management solution that is integrated with Oracle Database. Oracle Clusterware is also a required
component for using Oracle RAC. In addition, Oracle Clusterware enables both noncluster Oracle databases and Oracle RAC databases
to use the Oracle high-availability infrastructure. Oracle Clusterware enables you to create a clustered pool of storage to be used
by any combination of noncluster and Oracle RAC databases.

Oracle Clusterware is the only clusterware that you need for most platforms on which Oracle RAC operates.
You can also use clusterware from other vendors if the clusterware is certified for Oracle RAC.

A cluster consists of multiple interconnected servers that appear to end users and applications as if they are one single server.

Typical Oracle RAC Architecture.

Oracle RAC databases differ architecturally from noncluster Oracle databases in that each Oracle RAC database instance also has:
At least one additional thread of redo for each instance.
An instance-specific undo tablespace.
The combined processing power of the multiple servers can provide greater throughput and Oracle RAC scalability than is available from a single server.

Simply put Real Application Cluser (RAC) is,
Multiple instances accessing the same database

One instance per node
Physical or logical access to each database file
Software-controlled data access.

Clusterware.
Clusteware is software which will bind all the Nodes in a single system.

How the RAC software changed from version to version.
Oracle RAC Software.

9i                       Cluster manager
10gR1                  CRS S/W
10gR2                 Clusterware
11gR1                   Clusterware
11gR2                  Grid Infrastructure Services
12cR1                  Grid Infrastructure Services
12cR2                 Grid Infrastructure Services

Criteria for RAC deployment..

* Oracle defines the database as being the structures that reside in files permanently on disk,including the datafiles,the controlfiles  and online redologs.
* In the case of RAC  cluster ,the database must be located on shared physical disk storage that all servers in the cluster must be able to access equally.
* The servers communicate with the shared storage via a storage network.
* The servers in the cluster are bound together using cluster management software called Oracle Clusterware/Grid Infrastructure Services.
* This enables the servers to appear as though they are single server. Servers in the cluster communicate
   with each other using a dedicated private network known as the Cluster Interconnect.
* Each server is also connected to a public network that allows the cluster to communicate with the  outside world,including clients and application servers.
* In RAC cluster ,each instance can execute transactions concurrently because access is coordinated to ensure data consistency and integrity.
* Instances exchange messages  with each other across the interconnect to manage the state of individual Oracle data blocks and locks.

RAC Installation Notes.
* At the time installation , we need to select on which nodes we have to install Grid setup . Only one note needs the Grid Setup. Node 1.
* OrainstRoot.sh ,root.sh will be executes manually on all the nodes.
* we have to set oratab and initab in all the nodes
* All the nodes have to access the shared storage to get the data from physical datafiles.
* ./runInstaller need to be run on only one node.

Benefits of Real Application Cluster.
* Lower Overall Cost of Ownership
* Expanded Scalability
* High Availability
* Buffer Cache Management
* Row Locking
* Recovery Manager, Online Backups, and Archiving.

Archivelog Notes.
* In Real Application Clusters, each instance automatically archives its own redo log files, or one or more instances can archive
   the redo log files for some or all of the instances in the cluster database.
* If you operate your database in NOARCHIVELOG mode, then you can only make offline backups. If you cannot afford data loss,
   then Oracle strongly recommends that you use ARCHIVELOG mode.

Source: https://www.ktexperts.com/introduction-to-oracle-rac-environment/