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Software Design

Reference: Software Engineering, by Ian Sommerville, Ch. 12 & 13, 5th edition and Ch. 10, 6th edition

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Topics• What is Design?• Design Description • The Design Process

– Architectural Design

• Design Strategies– Functional– OO

• Design Quality– Component Cohesion– Component Coupling

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What is Design?

• Where informal ideas are transformed to detailed implementation descriptions

• It is a creative process

• There is no design “cookbook”

• It is learned by experience and study of existing systems

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Design DescriptionThree main design notations:

• Graphical notations– Display relationships between components– Relate the design to the real-world system

• Program Description Languages (PDLs)– Pseudocode

• Informal text– For anything that can’t be described formally

(e.g., design rationale, non-functional considerations)

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The Design Process

• Architectural Design– Subsystems and their relationships are

identified and documented

• Abstract Specification– Document an abstract specification of the

services provided by and constraints on each subsystem

• Interface Design– Document each subsystem’s interface

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The Design Process (con’t)

• Component Design– Break subsystems into components and

document their interfaces

• Data Structure Design– Specify the data structures used in the

system implementation

• Algorithm Design– Specify the implementation algorithms

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Architectural DesignHow a system is decomposed into subsystems

that provide some related set of services• Different “flavors” of architectural design

models:– Structural models– Control models– Others

• There is no one “correct” architectural design model.

• Designs may be hybrids of several models or even self-defined.

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Structural Model Examples

• Simple block diagram

• Domain-independent architectures– Repository Model– Client-Server Model– Event-driven Model– Many others ...

• Domain-specific architectures

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Simple Block Diagram

• Presents an overview of the system structure (subsystems) and their interconnections

• Good overview, but may not present enough information (e.g., connection to databases)

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Alarmcontroller

Voicesynthesizer

Movementsensors

Siren

Doorsensors

Telephonecaller

Externalcontrol centre

Intruder Alarm System

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Visionsystem

Objectidentification

system

Armcontroller

Grippercontroller

Packagingselectionsystem

Packingsystem

Conveyorcontroller

A Packing Robot Control System

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Repository Model

• Systems which use large amounts of data are organized around a shared database or repository

• Suited to applications where data is generated by one subsystem and used by others

• Example: a management information system

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Student Information Repository

CourseScheduleGenerator

TranscriptGenerator

GraduationCheckoutSystem

StudentRegistration

System

Grade ReportGenerator

A Student Information System

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Projectrepository

Designtranslator

Programeditor

Designeditor

Codegenerator

Designanalyser

Reportgenerator

A CASE Toolset

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Client-Server Model

• A distributed system model which shows how data and processing is distributed across a range of processors

• Servers offer services to other subsystems

• Clients call on the services offered by the servers

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Film and picture library

Catalogueserver

Catalogue

Videoserver

Film clipfiles

Pictureserver

Digitizedphotographs

Hypertextserver

Hypertextweb

Client 1 Client 2 Client 3 Client 4

Wide-bandwidth network

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Control Models

Are concerned with the control flow between subsystems.

Examples:– Centralized control: One subsystem has

overall responsibility for control and starts and stops other subsystems

– Event-driven: System is driven by externally generated events where the timing of the event is out of the control of the subsystem(s) that processes the event (e.g., spreadsheets, GUIs)

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Design Strategies

• Functional DesignSystem is designed from a functional

viewpoint, starting with a high-level view and refining this into a more detailed design. The system state is centralized and shared between the functions.

• Object-oriented DesignSystem is viewed as a collection of objects

rather than functions. The system state is decentralized. Each object manages its own information.

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Design Quality

What is “good” design? No general agreement, but ...

• Should correctly implement specification• Must be understandable

– Good naming conventions– Good internal and external documentation– Minimize complex algorithms

• Must be able to adapt to modification or addition of new functionality– High component cohesion– Low component coupling

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Component Cohesion

• A measure of the closeness of the relationships between the component’s components

• Component should implement a single logical function/task (functional) or implement a single logical entity (OO)

• We want strong cohesion

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Component Cohesion (con’t)7 levels of cohesion (Constantine &

Yourdan), weakest to strongest:• Coincidental Cohesion

– The parts of a component are not related but simply bundled into a single component

• Logical Association– Components that perform similar functions

such as input, error handling and so on are put together in a single component

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Component Cohesion (con’t)• Temporal Association

– All of the components that are activated at a single time, such as start up or shut down, are brought together

• Procedural Cohesion– The elements in a component make up a

single control sequence

• Communicational Cohesion– All of the elements of a component operate

on the same input data or produce the same output data

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Component Cohesion (con’t)

• Sequential Cohesion– The output from one element in the component

serves as input for some other element

• Functional Cohesion– Each part of the component is necessary for

the execution of a single function/task

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Component Cohesion (con’t)Cohesion applies to both functional and

OO design approaches:

• Cohesive Function– Performs a single task

• Cohesive Object– A single entity is represented and all the

operations on that entity are included with the object

So, which promotes strong cohesion better -- functional or OO design?

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Component Coupling

• A measure of the strength of the interconnections between components in a design

• Want components to be as independent as possible

• We want low coupling

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Component Coupling (con’t)

• Functional Design– No/little global data– No hard-coded constants– Nothing that causes one function to require

knowledge of another’s implementation

• OO Design– Inheritance by nature causes coupling between

base and derived classes– Multiple inheritance greatly increases coupling

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How Components Are Coupled• References from one component to another, such as invocation

• Amount of data passed from one component to another

• Amount of control one component has over another

• Degree of complexity of interface, e.g., one entry point vs. mutual entry points

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Goal is to Minimize Coupling• Enables us to change portion of system while disrupting rest of system little as possible

• Very low coupling might allow pull-out, plug-in replacement of only one component

• Loose coupling may require changing or replacing a few components

• High coupling may require widespread perturbations in system

• Low coupling reduces the number of components needing revision

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Types of Coupling

• Content: one component directly modifies data or control flow of another

• Common: Organizing data into a common store

• Control: Control flags passed as parameters between components

• Stamp: Data structures passed• Data: Only primitive data passed