A stoppable state machine is one whose execution can be terminated by a
special stopping command. Stoppable state machines can be used to imple-
ment recon¯guration in a replicated state machine; a recon¯gurable state
machine is implemented by a sequence of stoppable state machines, each
running in a ¯xed con¯guration. Stoppable Paxos, a variant of the ordinary
Paxos algorithm, implements a replicated stoppable state machine.