We begin with the Emerald System for distributed mobile objects [9]. Because objects are mobile and can be located anywhere in the system, Emerald has a uniform object name space. All objects must be uniquely named so clients can find them, wherever they reside in the system.
Emerald has a very simple consistency policy: only one copy of each object exists in the system, and access is synchronized by monitors. No replicas exist, so no updates are propagated among clients. When a client accesses an object, one of three things happens: (1) if the object resides on the client's node, it is accessed directly, (2) a remote object can be moved to the client's node, then accessed locally, or (3) a remote object can be accessed by a remote method invocation.
The limitations of the Emerald approach to distributed shared state derive from the one copy per object policy. One copy of an object means many network transmissions when multiple clients want to access it. Also, if a client becomes disconnected from the system then it cannot access remote objects.