GetGroup returns the named group previously created with NewGroup, or nil if there is no such group.

NewGroup create a new instance of Group.

Group 负责缓存值的存储与获取.

Getter loads data for a key.

PeerGetter is the interface that must be implemented by a peer.

PeerPicker is the interface that must be implemented to locate the peer that owns a specific key.

GetterFunc implements Getter with a function.