Efficient Inter-cloud Replication for High-Availability Services*

Amazon´s recent service disruption and investigations into the underlying causes of similar major outages indicate that cloud outages in future cannot be ruled out with certainty. This paper investigates the idea of tolerating outages by inter-cloud replication, i.e., through service replication on multiple, fail-independent clouds. A challenge in realizing this idea is to minimize performance degradation that inevitably arises when replicas on multiple clouds have to be kept in a mutually consistent state over the Internet. It is addressed by developing a new order protocol that makes the most use of the high bandwidth communication within a cloud and uses the Internet communication to minimum necessary. The protocol also deals with cloud outages and widely differing rates with which service requests can arrive at replicas in different clouds. Experiments performed confirm that the protocol reduces the ordering latencies considerably and also improves throughput.