3.4 The Remote Reference Layer

The remote reference layer deals with the lower-level transport interface. This layer is also responsible for carrying out a specific remote reference protocol which is independent of the client stubs and server skeletons.

Each remote object implementation chooses its own remote reference subclass that operates on its behalf. Various invocation protocols can be carried out at this layer. Examples are:

• Unicast point-to-point invocation.
• Invocation to replicated object groups.
• Support for a specific replication strategy.
• Support for a persistent reference to the remote object (enabling activation of the remote object).
• Reconnection strategies (if remote object becomes inaccessible).

The remote reference layer has two cooperating components: the client-side and the server-side components. The client-side component contains information specific to the remote server (or servers, if the remote reference is to a replicated object) and communicates via the transport to the server-side component. During each method invocation, the client and server-side components perform the specific remote reference semantics. For example, if a remote object is part of a replicated object, the client-side component can forward the invocation to each replica rather than just a single remote object.

In a corresponding manner, the server-side component implements the specific remote reference semantics prior to delivering a remote method invocation to the skeleton. This component, for example, would handle ensuring atomic multicast delivery by communicating with other servers in a replica group (note that multicast delivery is not part of the JDK 1.1 release of RMI).

The remote reference layer transmits data to the transport layer via the abstraction of a stream-oriented connection. The transport takes care of the implementation details of connections. Although connections present a streams-based interface, a connectionless transport can be implemented beneath the abstraction.