On the Consistency of Heterogeneous Composite Objects

Several recent cloud-backed storage systems advocates the composition of a number of cloud services for improving performance and fault tolerance (e.g., [1, 3, 4]). An interesting aspect of these compositions is that the consistency guarantees they provide depend on the consistency of such base services, which are normally different. In this short paper we discuss two ways in which these services can be composed and the implications in terms of the consistency of the composed object. Although these techniques were devised (or observed) when solving practical problems in dealing with the eventual consistency guarantees of current cloud storage services (e.g., Amazon S3 [6], Windows Azure Blob Storage [7]), we believe they might be of general interest, and deserve the  attention of the community. In particular, we want to discuss some initial ideas about the theoretical underpinnings of object compositions in which base objects provide different consistency guarantees.