It's important to distinguish between fault tolerance and high availability a term it's often confused with, analysts say. Both use backup resources like mirrored servers and disks as a fundamental approach for recovering from a failure. But while the goal of high-availability configurations is to help firms recover quickly from a crash, that's not good enough for fault tolerance or for continuous availability, which seeks to eliminate the recovery time completely. The emphasis is on application takeover and not merely fail-over to a new server. All main hardware elements such as CPUs, memory and disks are duplicated.
All redundant components execute all processes in lockstep.
Replicated components perform the same instructions at the same time, so even if one were to fail, the application would keep running without change. As a result, administrators can add capacity, run routine maintenance or remove failed components without taking the whole system down. You are talking about two systems running the same load at the same time. Compaq's NonStop Himalaya servers provide one example. The systems are composed of a cluster of processors each with its own memory and copy of the operating system working in tandem but completely independent of one another.