What is cluster ?

A server cluster is a group of independent servers 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. You can use server clusters to ensure that users have constant access to important server-based resources.

1. High-availability clusters (Failover Clusters)

are implemented primarily for the purpose of improving the availability of services that the cluster provides.

They operate by having redundant nodes, which are then used to provide service when system components fail.

2. Load-balancing clusters

Load-balancing is when multiple computers are linked together to share computational workload or function as a single virtual computer.

Logically, from the user side, they are multiple machines, but function as a single virtual machine.

Requests initiated from the user are managed by, and distributed among, all the standalone computers to form a cluster.

This results in balanced computational work among different machines, improving the performance of the cluster system.

3. Compute clusters

Often clusters are used primarily for computational purposes, rather than handling IO-oriented operations such as web service or databases.

For instance, a cluster might support computational simulations of weather or vehicle crashes.

4. Grid computing

Grids are usually computer clusters, but more focused on throughput like a computing utility rather than running fewer, tightly-coupled jobs.

Often, grids will incorporate heterogeneous collections of computers, possibly distributed geographically, sometimes administered by unrelated organizations.