A single zone configuration typically consists of one zone group, containing one zone and one or more RGW instances, where you may load balance gateway client requests between instances. In a single zone configuration, typically, multiple gateway instances point to a single Ceph storage cluster. With the Jewel release, Ceph supports several multi-site configuration options for the Ceph Object Gateway:

• Zone: In a zone, one or more Ceph Object Gateways are logically grouped.
• Zone group: A zone group is a container of multiple zones. In a multi-site configuration there should be a master zone group.  All the changes to configurations are handled by the master zone group.
• Realm: A realm can have multiple zone groups. It allows separation of the zone groups themselves between clusters. There can be multiple realms for having different configurations in the same cluster.
• Period: Every realm has a corresponding current period. Each period is a container of an epoch and an unique id. A period holds the current state of configuration of the zone groups and object storage strategies. Each period's commit operation, as well as any configuration change for a non-master zone, will increment the period’s epoch. 

Following is the diagrammatic representation  of zone configuration: