Geo-replicated storage provides copies of the same data at multiple, geographically distinct locations. Facebook, for example, geo-replicates its data (profiles, friends lists, likes, and so on) to datacenters on the East and West coasts of the U.S. and in Europe. In each datacenter, a tier of separate Web servers accepts browser requests and then handles those requests by reading and writing data from the storage system, as shown in Figure 1.
Geo-replication brings two key benefits to Web services: fault tolerance and low latency. It provides fault tolerance through redundancy: if one datacenter fails, others can continue to provide the service. It provides low latency through proximity: clients can be directed to and served by a nearby datacenter to avoid speed-of-light delays associated with cross-country or round-the-globe communication.