Recovery for computer switchover in a real-time system

In real-time systems which demand uninterrupted servicing of incoming messages, it is customary to employ duplexed central computers, so that one may take the place of the other when needed. Questions arise concerning: 1. The economic justification of the second computer. 2. Keeping the operating personnel constantly alert and prepared for sudden switchover. 3. Loss of information and reduction in service when such switch-over occurs.