“With explicitly delineated levels of service, the infrastructure providers can effectively externalize the difference in the cost it takes to provide service at a given level to clients. Exposing cost in this way motivates the clients to choose the level of service with the lowest cost that still meets their needs. For example, Google + can decide to put data critical to enforcing user privacy in a high-availability, globally consistent datastore (e.g., a globally replicated SQL-like system like Spanner [Cor12]), while putting optional data (data that isn’t critical, but that enhances the user experience) in a cheaper, less reliable, less fresh, and eventually consistent datastore (e.g., a NoSQL store with best-effort replication like Bigtable).”