Abstract: In a multiple processing system or cluster, a pair of processes, assuming the role of a primary process and a backup process to the primary process, are replaced on-line by stopping the backup process; creating the replacement backup process; checking to ensure compatibility between the primary and replacement backup processes so that communication between them is possible and information sent by the primary process to the replacement will be correctly received and handled; providing the replacement backup process with that state of the primary process needed in order to take over the function and operation of the primary process; switching roles so that the replacement backup process now takes over the function and operation of the primary, and the primary becomes the backup; and repeating the steps of creating, checking, providing and switching, to conclude with a newly-installed replacement primary process and a replacement backup process, completing the on-line replacement of the process pair.