The process of constructing a system out of a number of similar computers in order to achieve a high reliability. There are a number of ways of arranging for these replica computers to operate. One way is to execute them in parallel and continually check their answers; if one disagrees then it must be malfunctioning. Once this happens the malfunctioning computer is stopped and often replaced by a spare computer. A cheaper way of using hardware replication is to keep one computer running and monitor its operations; when it malfunctions work is immediately transferred to a spare computer, a technique known as hot swapping. See also data replication and fault-tolerant system.