Replicated storage systems for the cloud deliver different consistency guarantees to applications that are reading data. Invariably, cloud storage providers redundantly store data on multiple machines so that data remains available in the face of unavoidable failures. Replicating data across datacenters is not uncommon, allowing the data to survive complete site outages. However, the replicas are not always kept perfectly synchronized. Thus, clients that read the same data object from different servers can potentially receive different versions.
In Communications of the ACM
Doug Terry. Replicated Data Consistency Explained Through Baseball, October 2011.