Object middleware is used to call an operation (implemented by a method) in an object instance (instantiated from a class) that resides in another process. Instead of client and server, there is client and object. This type of middleware is often called an Object Request Broker (ORB).

Concept (Top)

The concept is similiar to the concept used for a remote procedure call. The main difference between a remote procedure call and a remote object call is that there is only one instance of a procedure but there can be many instances of an object. This means that in order to call an operation in a remote object, the caller must first obtain a reference to the object. For example, using procedure calls we might have:

   acct = 12345;
   name = call getName(acct);
   bal = call getBalance(acct);
     ... display
     ... get action (debit by 100)
   call debit(acct,100);

Using remote objects we might have:

   Account accountRef = accountSet.getAccount(12345);
   String name = accountRef.getName();
   Money bal = accountRef.getBalance();
     ... display
     ... get action (debit by 100)
   accountRef.debit(100);

There are four ways to get an object reference:

• A special root object reference is returned to the client when it firsts connects to the middleware. This object can then be used to locate other objects.

• The client calls a naming service. The naming service looks up the name in a directory, gets the location of the object, and returns an object reference.

• The client requests a new remote object be instantiated from a class and gets back a reference to the object.

• An operation on one remote object gets back a reference to another remote object.

Enabling a Call (Top)

The process that is used to enable calls to operation is remote objects is basically the same as the process use to enable remote procedure calls. That is, a client stub and server skeleton are used. However, most object middleware implements two ways to use stubs and skeletons:

• Compiled Runtime Environment
  The stubs and skeletons are generated from an IDL and linked into the client and server.

• Interpretative Runtime Environment
  An interface definition is generated from an IDL and stored in a repository. At runtime, the client queries the repository to obtain the interface definition.

Advantages (Top)

Object middleware has the following advantages over procedure middleware:

• It is more flexible.

• It is a natural fit with object-oriented languages.

• Because and object encapsulates both attributes and behavior, it makes you think about state held in the server on behalf of the client.

Technologies (Top)

There are three major distributed object technologies:

• Java Remote Method Invokation (RMI)

• Common Object Request Broker (CORBA)

• Distributed Component Object Model (DCOM)