B. Computer Sci. (SE) (Hons.)

CSEB233: Fundamentals of

Software Engineering

Software Design

Objectives

Explain set of design principles, concepts, and practices

Describe four design models required for a complete design specification: data design architectural design user interface design component-level design

Use design support tools and evaluation Explain the use of design specification document

What is Design?

Design creates a representation or model of the software

But unlike the analysis model, the design model provides details about:

software architecture data structures interfaces, and components that are necessary to implement the system.

Why is it important? The model can be assessed for quality and improved before

code is generated Tests are conducted, and more users are involved.

Analysis Models Design Model

Analysis Model

use-cases - text use-case diagrams activity diagrams swim lane diagrams

data flow diagrams control-flow diagrams processing narratives

f low-oriented elements

behavioralelements

class-basedelements

scenario-basedelements

class diagrams analysis packages CRC models collaboration diagrams

state diagrams sequence diagrams

Data/ Class Design

Architec tural Design

Interface Design

Component- Level Design

Design Model

The four design models

Each of the elements of the analysis/requirements model provides information that is necessary to create the design models required for a complete design specification.

Characteristics of Good Design

Must implement all of the explicit requirements contained in the analysis model

Must accommodate all the implicit requirements desired by the customer

Must be a readable, understandable guide for those who generate code and for those who test and subsequently support the software

Should provide a complete picture of the software, addressing the data, functional, and behavioral domains from an implementation perspective

(McGlaughlin, 1991)

Design Principles

The design process should not suffer from ‘tunnel vision’

The design should be traceable to the analysis model The design should not reinvent the wheel

Reinventing the wheel means to duplicate a basic method that has previously been created or optimized by others

The design should ‘minimize the intellectual distance’ between the software and the problem as it exists in the real world

The design should exhibit uniformity and integration(Davis,1995 )

Design Principles

The design should be structured to accommodate change

The design should be structured to degrade gently, even when irregular data, events, or operating conditions are encountered

The design should be assessed for quality as it is being created, not after

The design should be reviewed to minimize conceptual (semantic) errors

Design Concepts

Fundamental design concepts that span both traditional and OO software development include:

Abstraction Architecture Patterns Separation of concerns Modularity Information hiding

Functional Independence Refinement Aspects Refactoring OO Design concepts Design classes

Abstraction

Designers should work to derive both procedural and data abstractions that serve the problem

Procedural abstraction – sequence of instructions that have a specific and limited function

Data abstractions – a named collection of data that describes a data object

Abstraction

door

manufacturer model number type swing direction weight

door

manufacturer model number type swing direction weight

Implemented as a data structure

door

details of enter algorithm

door

details of enter algorithm

Implemented with a "knowledge" of the object that is associated with “enter”

Abstraction

Data abstraction refers to, providing only essential features by hiding its background details

b1 is an object calling input and output member functions.

But that code is invisible to the object b1

Architecture

Is concerned with: describing the fundamental organization of the system, identifying its various components and their

relationships to each other, and the environment in order to meet the system's quality

goals It also describes the overall structure of the

software: organization of program modules, the manner in which these modules interact, and the structure of data used by the components

Patterns

Online pattern searching: http://inventors.about.com/gi/dynamic/offsite.

htm?site=http%3A%2F%Fpatents.uspto.gov%2Fpatft%2Findex.html

Under Quick Search, try to search pattern no. 7669112.

You will get ‘automated spell analysis’ pattern.

Separation of Concerns

It is a rule of thumb to define how modules should be separated to each other: different or unrelated concerns should be restricted to

different modules Any complex problem can be easily handled if it is

sub-divided into pieces that can be solved independently

Why? So that a problem takes less time and effort to solve

Manifested in other design concepts: modularity, aspects, functional independence and refinement

Modularity

When software is divided into components (modules)

A design is modularized: To ease the planning for implementation

(coding) To define and deliver software increments To easily accommodate changes To efficiently test and debug program, and To conduct long-term maintenance without

serious side effects

Modularity : Example

Without Module

With Module

vs.

Information Hiding

Suggests that modules should be specified and designed so that information (data structures and algorithm) contain within a module is inaccessible to other modules that have no need for such information

Implies that effective modularity can be achieved by defining a set of independent modules that communicate with one another only necessary information

Information Hiding

Serves as an effective criterion for dividing any piece of equipment, software or hardware, into modules of functionality

Provides flexibility This flexibility allows a programmer to modify functionality

of a computer program during normal evolution as the computer program is changed to better fit the needs of users

When a computer program is well designed decomposing the source code solution into modules using the principle of information hiding, evolutionary changes are much easier because the changes typically are local rather than global changes

Information Hiding: Example

Suppose we have a Time class that counts the time of day:

The Display() member function prints the current time onscreen. This member function is accessible to all It is therefore declared public

In contrast, the data member ticks is declared private. External users could not access it

Regardless of how Display() extracts the current timestamp from ticks, users can be sure that it will “do the right thing” and display the correct time onscreen.

Functional Independence

Achieved by developing independent modules – each module address a specific subset of requirements

Is assessed using cohesion and coupling. Cohesion is an indication of the relative functional

strength of a module a cohesive module should (ideally) do just one thing

Coupling is an indication of the relative inter-dependence among modules depends on the interface complexity between modules,

the point at which entry or reference is made to a module, and what data pass across the interface.

Functional Independence

This class lacks cohesion

Functional Independence

The CashRegister involves two concepts: cash register and coin

Solution: Make two classes:

Functional Independence

Coupling:

High and Low Coupling between Classes

Refinement

open

walk to door;reach for knob;

open door;walk through;

close door.

repeat until door opensturn knob clockwise;if knob doesn't turn, then

take key out;find correct key;insert in lock;

endif

pull/push doormove out of way;end repeat

Refactoring

“Refactoring is the process of changing a software system in such a way that it does not alter the external behavior of the code [design] yet improves its internal structure”

(Fowler, 1999) When software is refactored, the existing design is

examined for redundancy unused design elements inefficient or unnecessary algorithms poorly constructed or inappropriate data structures or any other design failure that can be corrected to yield a

better design.

Design Model Elements

Data/Class Design Class diagrams transformed into the design class

realization and the data structures required to implement the software

Architectural Design Provides high-level overview of the system with detailed

descriptions to be given by other design elements Defines the relationship between major structural

elements of the software, the architectural styles and design patterns that can be used to achieve the requirements of the system and the constraints that affect the way in which the architecture can be implemented

Design Models

Interface Design Describes how the software communicates

with systems that interoperate with it, and with human who use it

Component-Level Design Defines the data structures, algorithms,

interface characteristics, and communication mechanisms allocated to each software component or module

Data/Class Design Elements

Example: High-level DFD Source: http://yourdon.com/strucanalysis/wiki/index.php?title=Chapter_9

Data/Class Design Elements

Example: ERD Source: http://www.svgopen.org/2003/papers/SvgInterfaceElectricalSwitching/index.html

Data/Class Design Elements

Example: Class Diagram Source: http://www.agiledata.org/essays/objectOrientation101.html

Architectural Design Elements

The architectural model is derived from three sources: information about the application domain for

the software to be built specific requirements model elements such

as data flow diagrams or analysis classes, their relationships and collaborations for the problem at hand, and

the availability of architectural patterns and styles

Architectural Design Elements

Example: Architecture diagram Source: http://blog.tmcnet.com/blog/tom-keating/2004/10/index.asp?page=7

Architectural Design Elements

Example: Architectural styles

Interface Design Elements

External interfaces to other systems, devices, networks or other producers or consumers of information

ControlPanel

LCDdisplay LEDindicators keyPadCharacteristics speaker wirelessInterface

readKeyStroke() decodeKey() displayStatus() lightLEDs() sendControlMsg()

Figure 9.6 UML interface representation for ControlPanel

KeyPad

readKeystroke() decodeKey()

<<interface>>

WirelessPDA

KeyPad

MobilePhone

Interface Design Elements

The user interface (UI)

Component Design Elements

UML Component Diagram Source: http://edn.embarcadero.com/article/31863#component-and-deployment-diagrans

Design Support Tools

All aspects of the software engineering can be supported by software tools:

from project management software through tools for business and functional analysis, system design, code storage, compilers, translation tools, test software, and so on.

However, tools that are concerned with analysis and design, and with using design information to create parts (or all) of the software product, are most frequently thought of as CASE tools.

List of CASE tools: http://www.unl.csi.cuny.edu/faqs/software-enginering/tools.ht

ml

CASE Tool: Example

IBM Rational Software Architect Source:

http://www.ibm.com/developerworks/rational/library/10/whats-new-in-rational-software-architect-8/index.html

CASE Tool: Example

Database Modeling: Enterprise Architect by Sparx Systems Pty Ltd.

Source: http://www.sparxsystems.com/images/screenshots/platforms/databasemodeling_tn.jpg

Design Evaluation

“A good software design minimizes the time required to create, modify, and maintain the software while achieving run-time performance”

(Shore and Chromatic, 2007) Design quality is people-sensitive.

For instance, design quality is dependent upon the programmers writing and maintaining the code.

Design quality is change-specific. There are two general ways to make designs of higher quality

with respect to this aspect: generalizing from the tools like design patterns, and using tests and refactoring as change-enablers.

(Elssamadisy, 2007)

Design Evaluation

Modification and maintenance time are more important than creation time because

time spent in maintenance is much more than creation time of a software, and

in iterative development, modification happens during the initial creation of the software

Design quality is unpredictable Because quality is really dependant on the team

developing the software. As the team changes, or evolves, then the design quality also evolves

You really only know how good a design is by its standing the test of time and modifications

Design Evaluation

Points to ponder: To maintain the quality of the design, we

must maintain the theory of the design as the programming team evolves,

That design quality is tied to the people who are building and maintaining the software

(Shore and Chromatic, 2007)

Design Specification

IEEE Standard for Information Technology -Systems Design - Software Design Descriptions (IEEE 1016-2009) an improved version of the 1998 version

This standard specifies an organizational structure for a software design description (SDD) An SDD is a document used to specify system

architecture and application design in a software related project

Provides a generic template for an SDD

SDD: Examples

Refer to http://www.cs.uofs.edu/~dmartin/exsesrm.ht

m#_Toc514249734

Refer to SDDSample.doc

Summary

This module has: Introduced set of design principles, concepts,

and practices Described four design models required for a

complete design specification: data design, architectural design, user interface design, and component-level design

Introduced design support tools and evaluation Introduced design specification document

(SDD)

THE END

Copyright © 2013Mohd. Sharifuddin Ahmad,

PhD

College of Information Technology