Clustering

Clustering is a technology that automatically allows one physical server to take over the tasks and responsibilities of another physical server that has failed. The obvious goal behind this, given that all computer hardware and software will eventually fail, is to ensure that users running mission-critical applications will have little or no downtime when such a failure occurs. Downtime can be very expensive, and our goal as DBA is to help reduce it as much as possible.

More specifically, clustering refers to a group of two or more servers (generally called nodes) that work together and represent themselves as a single virtual server to a network. In other words, when a client connects to clustered SQL Servers, it thinks there is only a single SQL Server, not more than one. When one of the nodes fails, its responsibilities are taken over by another server in the cluster, and the end-user notices little, if any differences before, during, and after the failover.