A computer cluster is a group of computers working together as one unit. A computer cluster can, in many ways, be viewed as a single system, even though it is built from many computers. All cluster members are closely connected, each one of them being prepared to take any actions needed at any time. This sounds simple but, actually, it requires complex software products and solutions for a computer cluster to operate in this way. A cluster software stack makes sure that all the magic happens. It introduces a bundle of combined software products to form a complete high-availability computer cluster solution. It is a software stack and its goal is to provide a highly available cluster service.
Generally speaking, the cluster software stack must provide two significant features: