Web services that enable users in multiple regions to collaborate can increase availability and decrease latency by replicating data across data centers. If such a service spreads its data across multiple cloud providers---for the associated performance, cost, and reliability benefits---it cannot rely on cloud providers to keep the data globally consistent.

Services offered by cloud providers (e.g., for key-value storage and for redirecting clients to nearby data centers) simplify the development of applications deployed in the cloud. However, since any cloud provider’s service offerings are far from complete, many cloud providers host a service marketplace, wherein third parties can advertise new services.

Existing cloud computing platforms leave it up to applications to deal with the complexities associated with data replication and propagation across data centers. In our work, we propose the CSPAN key-value store to instead export a unified view of storage services in several geographically distributed data centers. To minimize the cost incurred by application providers, we combine two principles. First, CSPAN spans the data centers of multiple cloud providers. Second, CSPAN judiciously trades off the lower latencies and the higher storage and data propagation costs based on an application's anticipated workload, latency goals, and consistency requirements.

By offering storage services in several geographically distributed data centers, cloud computing platforms enable applications to offer low latency access to user data. However, application developers are left to deal with the complexities associated with choosing the storage services at which any object is replicated and maintaining consistency across these replicas.