Distributed object, IDL; RPC vs RMI

Distributed object

A distributed object is an object whose methods can be invoked by processes on other computers. Its state is encapsulated and is accessible only through methods defined in a remote interface, and each remote object has a unique remote object reference (network address, object id, interface) that can be passed as a parameter or result. Examples are Java RMI remote objects and CORBA objects.

IDL

An Interface Definition Language (IDL) is a language-neutral notation that specifies the operations (names, parameter and return types, exceptions) that a remote object or server offers, without implementation. An IDL compiler generates the client stub/proxy and server skeleton in the chosen programming language (CORBA IDL, Sun XDR/rpcgen, Java remote interface).

Example (CORBA IDL):

interface Account {
    void deposit(in float amount);
    float getBalance();
};

Comparison of RPC and RMI architecture

RPC architecture: client program calls client stub, which marshals and sends through the RPC runtime to the server stub, which calls the server procedure. Only stubs exist; binding is to a server (program/procedure number) through a port mapper.

RMI architecture: client object calls a proxy, a remote reference module maps references, and the communication module sends the request; at the server the dispatcher selects the skeleton, which calls the remote object. A registry binds names to remote object references.