It seems similar to the primary-backup instance of the Vertical Paxos family. In the primary-backup Vertical Paxos, one can tolerate f faults with f+1 replicas as long as a reliable external reconfiguration master is there to replace failed replicas and make sure everyone agrees on the configuration. Here the external reconfiguration master would be ZooKeeper and the primary-backup protocol the ISR protocol.

http://research.microsoft.com/en-us/um/people/lamport/pubs/v...