System Architecture III Distributed Objects presents of real time - software considerations; buffering example cd controller; continuous operation; example - routers and other network computing; example - transaction monitor; software reuse - application packages; reuse - objects basic definitions; objects distributed objects; object request broker (orb); interface definition language; distributed objects and the system life cycle
• Low level language (e.g., C) where programmer has close link to machine
Resource considerations may dictate software design and implementation:
• Inter-process communication may be too slow (e.g., C fork).
• May implement special buffering, etc., to control timings
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3 4 1
5
Input block
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Output block 6 7
Circular buffer
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Many systems must operate continuously
• Software update while operating
• Hardware monitoring and repair
• Remote operation
• Alternative power supplies, networks, etc.
These functions must be designed into the fundamental architecture.
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• 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
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messages
Transaction monitor
processes
A transaction monitor: monitors transactions, routes them across services, balances the load, restarts transactions after failure.
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• Package supports a standard application (e.g., payroll, user interface to Internet information, mathematical algorithms)
• Functionality can be enhanced by:
=> extensibility at defined interfaces
=> configuration parameters (e.g., table driven)
=> custom written source code extensions
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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.
=> Inexperienced Java programmers write inelegant and
=> Java experts can write complex systems quickly
buggy programs
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• 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.
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• 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)
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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
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Objects on separate computers interact through method calls and instance data.
Major systems:
• Microsoft family: OLE, COM, DCOM, Active X ...
• CORBA (Common Object Request Broker Architecture)
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• Reusable code: components
• Different languages and operating environments
• Future changes can be localized
• Architecture can be extensible
• Standard tools used for client/server interactions
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-- very simple Interface Definition Language
A research project to explore extensibility:
-- powerful tools for extensions
-- interoperability, Cornell and CNRI
http://www.cs.cornell.edu/cdlrg/fedora.html
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Objects Java Other Cobol C++ C
IDL IDL IDL Interface IDL IDL
Object Request Broker
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Naming context
Define a class
interface
See next slide
}
Define a class
interface
{ 16
interface
{
Define a method
Define a method
[
}
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Client Server
a
a
Object X Object Y
Invoke a on object X Invoke a on object Y
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• Static and dynamic method invocations
An ORB lets objects make requests to and receive response from other objects located locally or remotely.
• High-level language bindings
• Self-describing system
• Local/remote transparency
Internet Inter-ORB Protocol (IIOP)
• Inter-ORB protocols
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Object implementation Implementation repository Client
Static skeletons
Dynamic invocation
Interface repository
Dynamic invocation ORB interface Client IDL stubs
Object Request Broker
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• 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 21
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.
• Code can be reused in different environments.
Production environments changes with time.
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