CS 501: Software Engineering Fall 2000 Lecture 16 System Architecture III Distributed Objects

2 Administration

3 Real-Time: Software Considerations Resource considerations may dictate software design and implementation: Low level language (e.g., C) where programmer has close link to machine Inter-process communication may be too slow (e.g., C fork). May implement special buffering, etc., to control timings

4 Buffering Example: CD Controller Input block Output block Circular buffer

5 Continuous Operation Many systems must operate continuously Software update while operating Hardware monitoring and repair Alternative power supplies, networks, etc. Remote operation These functions must be designed into the fundamental architecture.

6 Example: Routers and Other Network Computing Interoperation with third party devices Support for several versions of protocols Restart after total failure Defensive programming -- must survive => erroneous or malicious messages => extreme loads Time outs, dropped packets, etc. Evolution of network systems

7 Example: Transaction Monitor messages A transaction monitor: monitors transactions, routes them across services, balances the load, restarts transactions after failure. Transaction monitor processes

8 Software Reuse: Application Packages Package supports a standard application (e.g., payroll, user interface to Internet information, mathematical algorithms) Functionality can be enhanced by: => configuration parameters (e.g., table driven) => extensibility at defined interfaces => custom written source code extensions

9 Reuse: Object Object Oriented Languages Example: Java is a relatively straightforward language with a very rich set of class hierarchies. Java programs derive much of their functionality from standard classes Learning and understanding the classes is difficult. => Java experts can write complex systems quickly => Inexperienced Java programmers write inelegant and buggy programs

10 Reuse: Objects - Basic Definitions An object is a piece of code that owns attributes and provides services through methods. The methods operate on instance data owned by the object. A class is a collection of like objects.

11 Reuse: Objects - Characteristics Encapsulation. An object has a public interface that defines how other objects or applications can interact with it. methods public instance data Inheritance. Subclasses can be derived from parent classes. They inherit or override the parents' methods and instance data. Polymorphism. The effect of a method can vary depending on the class that implements it (e.g., display_object)

12 Reuse: Objects - Object Binding Binding is the linking of the software interface between two objects. Static binding: The interface is determined at compile or build time. Straightforward Allows type checking Dynamic binding or late binding: The link is established at run time. Flexible and extensible Complex

13 Reuse: Objects - Distributed Objects Objects on separate computers interact through method calls and instance data. Major systems: CORBA (Common Object Request Broker Architecture) Microsoft family: OLE, COM, DCOM, Active X...

14 Desirable Properties of Distributed Objects Different languages and operating environments Reusable code: components Architecture can be extensible Future changes can be localized Standard tools used for client/server interactions

15 Example: Fedora IDL A research project to explore extensibility: -- very simple Interface Definition Language -- powerful tools for extensions -- interoperability, Cornell and CNRI

16 Object Request Broker (ORB) Objects C C++ Java Other Cobol IDL ClientServer IDL Object Request Broker Interface

17 Interface Definition Language module { ; interface [: ] { See next slide } interface [: ] {..... } { Naming context Define a class

18 Interface Definition Language (continued) interface [: ] { ; [ ( ) [raises exception] [context];.... [ ( ) [raises exception] [context];.... } Define a class Define a method

19 ORB: Programmer's View Object Request Broker Invoke a on object X Invoke a on object Y Object X a Object Y a ClientServer

20 Object Request Broker (ORB) An ORB lets objects make requests to and receive response from other objects located locally or remotely. Static and dynamic method invocations High-level language bindings Self-describing system Local/remote transparency Inter-ORB protocols Internet Inter-ORB Protocol (IIOP)

21 ORB: System View Object Request Broker Interface repository Dynamic invocation Client IDL stubs ORB interface Implementation repository Static skeletons Dynamic invocation Object adapter Client Object implementation

22 CORBA Services Naming service Event service Concurrency control service Transaction service Relationship service Externalization service Query service Life cycle service Persistence service Licensing service Properties service Security service Time service

23 Distributed Objects and the System Life-Cycle All large systems change with time. Dynamic binding of objects combined with polymorphism permits the addition of extra object types, incremental changes, etc. to be localized. Development environments change with time. Language bindings and IIOP permit changes. Production environments changes with time. Code can be reused in different environments.