HA cluster

http://en.wikipedia.org/wiki/High-availability_cluster

The most common size for an HA cluster is a two-node cluster, since that is the minimum required to provide redundancy, but many clusters consist of many more, sometimes dozens of nodes. Such configurations can sometimes be categorized into one of the following models:

• Active/active — Traffic intended for the failed node is either passed onto an existing node or load balanced across the remaining nodes. This is usually only possible when the nodes utilize a homogeneous software configuration.

• Active/passive — Provides a fully redundant instance of each node, which is only brought online when its associated primary node fails. This configuration typically requires the most extra hardware.

• N+1 — 

• N+M — 

• N-to-1 — 

• N-to-N —