Explain the principles and purposes of clusters

Oracle® Database 2 Day + Real Application Clusters Guide 11g Release 2 (11.2) defines a cluster as:

Multiple interconnected computers or servers that appear as if they are one server to end users and applications.

In Oracle® Clusterware Administration and Deployment Guide 11g Release 2 (11.2), the section, What is Oracle Clusterware?, describes clusters as follows:

Oracle Clusterware enables servers to communicate with each other, so that they appear to function as a collective unit. This combination of servers is commonly known as a cluster. Although the servers are standalone servers, each server has additional processes that communicate with other servers. In this way the separate servers appear as if they are one system to applications and end users.

Oracle Clusterware provides the infrastructure necessary to run Oracle Real Application Clusters (Oracle RAC). Oracle Clusterware also manages resources, such as virtual IP (VIP) addresses, databases, listeners, services, and so on. These resources are generally named ora.host_name.resource_name. Oracle does not support editing these resources except under the explicit direction of Oracle support. Additionally, Oracle Clusterware can help you manage your applications.

A resource is defined as:

A database, application, or process managed by Oracle Clusterware.

In Oracle® Clusterware Administration and Deployment Guide 11g Release 2 (11.2), the section, What is Oracle Clusterware?, says that:

The benefits of using a cluster include:

• Scalability of applications
• Use of less expensive commodity hardware
• Ability to fail over
• Ability to increase capacity over time by adding servers
• Ability to program the startup of applications in a planned order that ensures dependent processes are started
• Ability to monitor processes and restart them if they stop

…

Creating a cluster with Oracle Clusterware provides the ability to:

• Eliminate unplanned downtime due to hardware or software malfunctions
• Reduce or eliminate planned downtime for software maintenance
• Increase throughput for cluster-aware applications by enabling the applications to run on all of the nodes in a cluster
• Increase throughput on demand for cluster-aware applications, by adding servers to a cluster to increase cluster resources
• Reduce the total cost of ownership for the infrastructure by providing a scalable system with low-cost commodity hardware