1. CS 240, Prof. Sarwar Slide 1 CS 240: Software Project Fall 2003 Sections 1 & 2 Dr. Badrul M. Sarwar San Jose State University Lecture #23

2. CS 240, Prof. Sarwar Slide 2 Distributed object architectures l There is no distinction in a distributed object architectures between clients and servers l Each distributable entity is an object that provides services to other objects and receives services from other objects l Object communication is through a middleware system called an object request broker (software bus) l However, more complex to design than C/S systems

3. CS 240, Prof. Sarwar Slide 3 Distributed object architecture

4. CS 240, Prof. Sarwar Slide 4 Advantages of distributed object architecture l It allows the system designer to delay decisions on where and how services should be provided l It is a very open system architecture that allows new resources to be added to it as required l The system is flexible and scaleable l It is possible to reconfigure the system dynamically with objects migrating across the network as required

5. CS 240, Prof. Sarwar Slide 5 Uses of distributed object architecture l Dist. Obj. Arch. can be used in two ways: 1. As a logical model that allows you to structure and organise the system. In this case, you think about how to provide application functionality solely in terms of services and combinations of services 2. As a flexible approach to the implementation of client-server systems. The logical model of the system is a client-server model but both clients and servers are realised as distributed objects communicating through a software bus l makes it possible to change the system easily, e.g., from a two-tier to a multi- tier system

6. CS 240, Prof. Sarwar Slide 6 CORBA l CORBA is an international standard for an Object Request Broker - middleware to manage communications between distributed objects l Several implementation of CORBA are available l DCOM is an alternative approach by Microsoft to object request brokers l CORBA has been defined by the Object Management Group

7. CS 240, Prof. Sarwar Slide 7 Application structure l Application objects l Standard objects, defined by the OMG, for a specific domain e.g. insurance l Fundamental CORBA services such as directories and security management l Horizontal (i.e. cutting across applications) facilities such as user interface facilities. Horizontal facilities include UI facilities, system management facilities etc.

8. CS 240, Prof. Sarwar Slide 8 CORBA application structure

9. CS 240, Prof. Sarwar Slide 9 CORBA standards l CORBA standard has four major elements: 1. An object model for application objects 1. A CORBA object is an encapsulation of state with a well-defined, language-neutral interface defined in an IDL (interface definition language) 2. An object request broker that manages requests for object services 3. A set of general object services of use to many distributed applications 4. A set of common components built on top of these services (these may be vertical—domain specific components or horizontal—general purpose components)

10. CS 240, Prof. Sarwar Slide 10 CORBA objects l CORBA objects are comparable, in principle, to objects in C++ and Java l They MUST have a separate interface definition that is expressed using a common language (IDL) similar to C++ l There is a mapping from this IDL to programming languages (C++, Java, etc.) l Therefore, objects written in different languages can communicate with each other

11. CS 240, Prof. Sarwar Slide 11 Object request broker (ORB) l The ORB handles object communications. It knows of all objects in the system and their interfaces l Using an ORB, the calling object binds an IDL stub that defines the interface of the called object l Calling this stub results in calls to the ORB which then calls the required object through a published IDL skeleton that links the interface to the service implementation

12. CS 240, Prof. Sarwar Slide 12 ORB-based object communications

13. CS 240, Prof. Sarwar Slide 13 Inter-ORB communications l ORBs are not usually separate programs but are a set of objects in a library that are linked with an application when it is developed l ORBs handle communications between objects executing on the same machine l Several ORBS may be available and each computer in a distributed system will have its own ORB l Inter-ORB communications are used for distributed object calls  IIOP—Internet InterOrb Protocol was defined in the later version of CORBA

14. CS 240, Prof. Sarwar Slide 14 Inter-ORB communications

15. CS 240, Prof. Sarwar Slide 15 CORBA services l Naming and trading services  These allow objects to discover and refer to other objects on the network l Notification services  These allow objects to notify other objects that an event has occurred l Transaction services  These support atomic transactions and rollback on failure