August 1999
WW221 : WORLD WIDE WEB FUNDAMENTALS

QUESTION 4

Total Marks: 20 Marks

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(a) There are three fundamentals capabilities of Java which make it particularly suited to distributed/network applications. What are they? [3]
  • It allows clients to dynamically gain an intelligence they did not originally have by downloading classes.
  • It simplifies software upgrade issues in large distributed systems. Upgrading client software becomes a lot easier, because it only involves downloading Java code over the network to local machine and replacing it with the old code.
  • A single application can run on multiple platforms.

 
(b) Describe four ways in which object technology can provide benefits to the World Wide Web. [6]
  • It can turn static Web documents into active compound documents that can interact with users just like applications on the desktop.
  • It can provide a seamless integration of various information media like video, audio, text and images.
  • It supports secure, long-lived transactions that execute over a long period of time, as they travel from server to server.
  • It supports roaming agents.
  • It enables server and network load balancing by building hierarchies of interoperating proxy cache servers.
  • It brings some order to the content on the Web, by providing interfaces to object-oriented and Relational databases.

 
(c) What is the purpose of CORBA? CORBA is composed of five major components. What are they? [6]
CORBA is a set of specifications for providing the interoperability and portability to distributed object oriented applications.

Object Request Broker (ORB): The ORB is the object bus. IT provides the middleware the mediates the interactions between client applications needing services and server applications capable of providing them.

Interface Definition Language (IDL): IDL provides architecture and implementation independence to CORBA-compliant applications. It is a declarative language in which object interfaces are defined and advertised. The interface definition is independent of the actual object implementation.

Interface Repository: It is an on-line databases of object definitions' that can be queries at run-time for dynamic method calls, for locating potentially reusable software components and for type-checking of method signatures.

Object Adapter: It provides the run-time environment for the server application and handles incoming client calls.

CORBA Services : These augment the functionality of the ORB. CORBA defines services for persistence, transactions, congruency, database queries, licensing, etc.

 
(d) Java RMI and CORBA have similar purposes. Describe three differences between them. [5]
  • CORBA is much wider in scope than RMI; RMI equates to the base ORB.
  • Java RMI is not CORBA-compliant, and uses Java to define interfaces, rather than COBRA's IDL (1).
  • CORBA assumes a heterogeneous environment; Java RMI works only when all objects are Java-Based.
  • Because of the language-mapping involved, CORBA has a weaker object/type system than Java, so in RMI subclasses persist through interfaces, which they do not in CORBA.