Consistency and Complexity Tradeoffs for Highly-Available Multi-Cloud Store

Cloud-based storage services have established themselves as a paradigm of choice for supporting bulk storage needs of modern networked services and applications. Although individual storage service providers can be trusted to do their best to reliably store the user data, exclusive reliance on any single provider or storage service leaves the users inherently at risk of being locked out of their data due to outages, connectivity problems, and unforeseen alterations of the service contracts. An emerging multi-cloud storage paradigm addresses these concerns by replicating data across multiple cloud storage services, potentially operated by distinct providers. In this paper, we study the impact of the storage interfaces and consistency semantics exposed by individual clouds on the complexity of the reliable multi-cloud storage implementation. Our results establish several inherent space and time tradeoffs associated with emulating reliable objects over a collection of unreliable storage services with varied interfaces and consistency guarantees.