Zones can be added to or removed from a running system as new datacenters are brought into service and old ones are turned off, respec- tively. Zones are also the unit of physical isolation: there may be one or more zones in a datacenter, for example, if different applications’ data must be partitioned across different sets of servers in the same datacenter. Figure 1 illustrates the servers in a Spanner universe. A zone has one zonemaster and between one hundred and several thousand spanservers. The former assigns data to spanservers; the latter serve data to clients. The per-zone location proxies are used by clients to locate the spanservers assigned to serve their.
