CS 501: Software EngineeringFall 2000

Lecture 16

System Architecture III

Distributed Objects

2

Administration

•

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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

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Buffering Example: CD Controller

Input block Output

block

12

345

67

Circular buffer

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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.

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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

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Example: Transaction Monitor

messages

A transaction monitor: monitors transactions, routes them across services, balances the load, restarts transactions after failure.

Transaction monitor processes

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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

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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

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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.

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Reuse: Objects - Characteristics

• Encapsulation. An object has a public interface that defines how other objects or applications can interact with it.

methodspublic 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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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.

StraightforwardAllows type checking

• Dynamic binding or late binding: The link is established at run time.

Flexible and extensibleComplex

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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 ...

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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

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Example: Fedora IDL

A research project to explore extensibility:

-- very simple Interface Definition Language

-- powerful tools for extensions

-- interoperability, Cornell and CNRI

http://www.cs.cornell.edu/cdlrg/fedora.html

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Object Request Broker (ORB)

ObjectsC C++ Java OtherCobol

IDL

Client Server

IDL IDL IDL IDL

Object Request Broker

Interface

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Interface Definition Language

module <identifier>{ <type declarations>; <constant declarations>; <exception declarations>;

interface <identifier> [:<inheritance>] {

See next slide }

interface <identifier> [:<inheritance>] { ..... }{

Naming context

Define a class

Define a class

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Interface Definition Language (continued)

interface <identifier> [:<inheritance>] {

<type declarations>; <constant declarations>; <exception declarations>;

[<op_type] <identifier>(<parameters>) [raises exception] [context]; .... [<op_type] <identifier>(<parameters>) [raises exception] [context]; .... }

Define a class

Define a method

Define a method

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ORB: Programmer's View

Object Request Broker

Invoke a on

object X

Invoke a on

object Y

Object X

a

Object Y

a

Client Server

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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)

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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

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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

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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.