Brief survey of RMI protocols

Although Java provides an interface for programming with sockets, typical applications require a higher-level protocol that can handle encoding and decoding of messages. Java RMI  [25] is an API for remote method invocation - the invocation of a method in a remote object by a locally resident object. The polymorphism inherent in method calls makes the Java RMI API a more flexible alternative to RPC-based (Remote Procedure Call) APIs [30]. The actual class implementing the method can be dynamically loaded into the running application. The communication between the client and server is implemented by stub class on the client end and a skeleton class on the server end. The stub converts the arguments of the method invocation into a format used for transporting across the network to the remote object (this process is called serialization), and assembled together back again by the skeleton (deserialization) at the remote end. The result object is likewise serialized by the skeleton and deserialized by the stub.

Nexus RMI [6] is an implementation of the Java RMI API that uses Nexus  [17] as the communication medium. Object serialization is achieved by adding public methods to serializable objects so that their private and protected fields can be accessed. Our experiments (see Figure 4, for example) show that Java RMI is at least four times faster than Nexus RMI. However the important feature of Nexus RMI is that it allows interoperability between Java and C++ [6]. Several recent efforts have addressed the problem of improving RMI performance. Philippsen et al. [28] have built drop-in replacements for JDK implementations that significantly improve performance. Thiruvathukal et al. [29] use explicit methods for serialization and deserialization in order to read and write an object's internal state. Manta [22] achieves impressive performance for RMI based on transparent extensions of Java for distributed environments. A native compiler generates efficient serial code and specialized serialization routines for argument classes. As a result these classes avoid run-time inspection.