High Availability through Output Continuity

Virtual machine (VM) based state machine approaches, i.e. VM replication, provide high availability without source code modifications, unfortunately, existing VM replication approaches suffer from excessive replication overheads. On the other hand, machine state replication is an overly strong (and therefore less efficient) requirement for high availability in the networked client-server system. In this paper, we propose a generic and highly efficient high availability solution through output continuity. Output continuity considers a backup server as a valid replica as long as the responses generated so far are the same between the backup and primary servers, the server state is propagated from the primary server to the backup server if and only if the previous outputs from the backup and primary servers are different.