Date Added: Aug 2009
This paper introduces a class of Paxos algorithms called Vertical Paxos, in which reconfiguration can occur in the middle of reaching agreement on an individual state-machine command. Vertical Paxos algorithms use an auxiliary configuration master that facilitates agreement on reconfiguration. A special case of these algorithms leads to traditional primary-backup protocols. The paper shows how primary-backup systems in current use can be viewed, and shown to be correct, as instances of Vertical Paxos algorithms.