Server Cluster

What Is a Server Cluster ?

A server cluster is a group of independent servers running Windows Server 2003/2008/2008r2/2012, Enterprise Edition, Data center Edition and working together as a single system to provide high availability of services for clients.
When a failure occurs on one computer in a cluster, resources are redirected and the workload is redistributed to another computer in the cluster.
Server clusters are designed so that the servers in the cluster work together to protect data, keep applications and services running after failure on one of the servers, and maintain consistency of the cluster configuration over time.
Server clusters are designed for applications that have long-running in-memory state or frequently updated data. Typical uses for server clusters include file servers, print servers, database servers, and messaging servers.
A cluster consists of two or more computers working together to provide a higher level of availability, reliability, and scalability than can be obtained by using a single computer.

Server clusters require network technologies that use IP-based protocols and depend on the following basic elements of network infrastructure: 1. The Active Directory directory service (AD DS),
2. A name resolution service : Windows Internet Name Service (WINS) / Domain Name System (DNS) or both.

3. or You can also use IP broadcast name resolution, but IP broadcast name resolution increases network traffic, and is ineffective in routed networks.

## Types of Server Clusters :

There are three types of server clusters, based on how the cluster systems, called nodes, are connected to the devices that store the cluster configuration and state data.

1. Single quorum device cluster / standard quorum cluster :
This is most widely used cluster type. In this type of cluster there are multiple nodes with one or more cluster disk arrays, also called the cluster storage, and a connection device, that is, a bus. Each disk in the array is owned and managed by only one server at a time. The disk array also contains the quorum resource.

Fig. Single Quorum Device Cluster

2. Majority node set cluster
In this, each node maintains its own copy of the cluster configuration data. The quorum resource keeps configuration data consistent across the nodes.

Advantage of majority node:

i. majority node set clusters can be used for geographically dispersed clusters.
ii. a quorum disk can be taken offline for maintenance and the cluster as a whole will continue to operate.

Fig. Majority Node Set Cluster

The major difference between majority node set clusters and single quorum device clusters is that single quorum device clusters can operate with just one node, but majority node set clusters need to have a majority of the cluster nodes available for the server cluster to operate.

3. Local Quorum Cluster

it has a single node and is often used for testing. also called a single node cluster.