dr. ridha khedri - cas.mcmaster.ca filecas 703 software design dr. r. khedri cas 703 software design...

CAS 703 SoftwareDesign

Dr. R. Khedri

CAS 703 Software Design

Dr. Ridha Khedri

Department of Computing and Software, McMaster UniversityCanada L8S 4L7, Hamilton, Ontario

Dr. R. Khedri CAS 703 Software Design


Dr. R. Khedri



Dr. R. Khedri

Introduction to Software DesignåOverview

During the last half century, the world witnessedgrowth in the scale, complexity, and space distributionof software systems

When a system is complex in its functionality, itsaspects (qualities), and its environment, an obviousneed to reduce that complexity emerges

A top-down strategy in software construction to divideand conquer the complexities faced by the constructors

Roughly speaking, the decomposition of asystem/artifact into “manageable” parts is one of aimsof design



Dr. R. Khedri

Introduction to Software DesignåSoftware Requirements

The requirements specification serves as a startingpoint for the next phase (design)

In the design phase, the architecture of the system isdevised

Requirements analysis and design generally cannot bestrictly separated in time

the requirements specification is very formal and canbe viewed as a high-level design specification of thesystem to be built

Often, a preliminary design is done after an initial setof requirements has been determined



Dr. R. Khedri

Introduction to Software DesignåFunctional Requirements

We look at those requirements that are there becauseof the product’s fundamental reason for existence

The functional requirements specify what the productmust do

They relate to the actions that the product must carryout in order to satisfy the fundamental reasons for itsexistence

The product shall determine which road sections pass through

areas that are predicted to freeze.

It describes an action that the product must take if it isto carry out the work for which it is intended.



Dr. R. Khedri


Functional requirements are:

Specifications of the product’s functionality;

Actions the product must take - check, calculate,record, retrieve;

Derived from the fundamental purpose of the product;

Not a quality - for example, ’fast’ is a quality, andtherefore it is a non-functional requirement.

Think of the functional requirements as the businessrequirements



Dr. R. Khedri


The functional requirements are a specification of thereal work, or business,

:::::::::::independent of the way that

work will be carried out

The functional requirements must fully describe theactions that the intended product can perform

So a requirement for the requirements is that theproduct’s builder be able to use them to construct theproduct desired by your client.

At their discovery stage they are not necessarilyrigorous nor complete

The writing activity formalises each requirement



Dr. R. Khedri


The product shall show the weather for the next twenty-four

hours.

The product shall show all roads predicted to freeze.

Anything has the potential to be ambiguous

Continually minimise that risk by clarifying the context

Shut off the pumps if the water level is above 100 meters for 4

seconds



Dr. R. Khedri


Shut off the pumps if the water level is above 100 meters for 4

seconds.

There are several reasonable interpretations

1 “Shut off the pumps if the:::::mean water level over the

past 4 seconds was above 100 meters”.[( ∫ TT−4WL(t)dt

)÷ 4 > 100

]2 “Shut off the pumps if the

:::::::median water level over the

past 4 seconds was above 100 meters”.

(Max[t−4,t](WL(t)) + Min[t−4,t](WL(t)))÷ 2 > 100

3 “Shut off the pumps if the:::::::::minimum water level over

the past 4 seconds was above 100 meters”.

Min[t−4,t](WL(t)) > 100. . . . . . . . .Dr. R. Khedri CAS 703 Software Design


Dr. R. Khedri

Introduction to Software DesignåNon-functional Requirements

Look and Feel Requirements

The product shall have the same layout as the district maps that

the engineering department uses now.

Usability Requirements

Ease of use

The product shall be easy for 11-year-old children to use.

Ease of learning

The product shall be easy for an engineer to learn.



Dr. R. Khedri


Performance RequirementsSpeed requirements

Any interface between a user and the automated product musthave a maximum response time of two seconds.

Safety critical requirements

The product shall not emit noxious gases.

Precision requirements

All monetary amounts must be accurate to two decimal places.

Reliability and availability requirements

The product shall be available for use 24 hours per day, 365 daysper year.



Dr. R. Khedri


Performance Requirements (Continued ...)Capacity requirements

The product must be able to cater for 300 simultaneous userswithin the period from 9:00 a.m. to 11:00 a.m. Maximum loadingat other periods will be 150.

Operational RequirementsExpected physical environment

The product is to be used by a worker, standing up, outside incold, rainy conditions.

Expected technological environmentPartner applications

We must be able to interface with any HTML (Hyper-TextMark-up Language) browser.



Dr. R. Khedri


Maintainability and Portability Requirements

How easy must it be to maintain this product?

New MIS (Management Information System) reports must beavailable within one working week of the date the requirementsare agreed.

Are there special conditions that apply to themaintenance of this product?

The maintenance releases will be offered to end-users once a year.

Portability requirements

The product is expected to run under Windows 95 and UNIX.



Dr. R. Khedri


Security Requirements

Is the product confidential?

Only direct managers can see the personnel records of their staff.

File integrity requirementsAudit requirements + Prevention, detection andrecovery policies

Cultural and Political Requirements

Are there any special factors about the product thatwould make it unacceptable for some political reason?

The product shall not use icons that could be considered offencive

in any of our market countries.



Dr. R. Khedri


Legal RequirementsDoes the product fall under the jurisdiction of any law?

Personal information must be implemented so as to comply withthe Data Protection Act.

Are there any standards with which we must comply?

The product must comply with the appropriate MilSpec (MilitarySpecification) standards.

Open IssuesIssues that have been raised and do not yet have aconclusion

Our investigation into whether or not the new version of theprocessor will be suitable for our application is not yet complete.



Dr. R. Khedri


Off-the-Shelf Solutions

Is there a ready-made product that could be bought?

Can ready-made components be used for this product?

Is there something that we could copy?

Another electricity company has built a customer service system.

Their hardware is different from ours but we could buy their

specification and cut our analysis effort by approximately 60%.



Dr. R. Khedri


New Problems

What problems could the new product cause in thecurrent environment?

Any change to the scheduling system will affect the work of theengineers in the divisions and the truck drivers.

Will any of our existing users be adversely affected bythe new development?What limitations exist in the anticipatedimplementation environment that may inhibit the newproduct?

The planned new server is not powerful enough to cope with ourprojected growth pattern.

Will the new product create other problems?



Dr. R. Khedri

Introduction to Software DesignåProcess Requirements

Tasks

What steps have to be taken to deliver the product?Development phases

Cutover

What special requirements do we have to get theexisting data, and procedures to work for the newproduct?What data has to be modified/translated for the newproduct?

Risks

What risks do you face when you develop this product?What contingency plans are you making?



Dr. R. Khedri

Introduction to Software DesignåProcess Requirements

Costs

User Documentation

The plan for building the user documentation

Waiting Room

requirements that will not be part of the agreed product

These requirements might be included in futureversions of the product



Dr. R. Khedri

Introduction to Software DesignåWhat is Design?

What is Design?

One cannot define design as would define amathematical activity such as “rewriting“,“derivation”, etc.

The nature of design is not easy to grasp in a cleardefinition

Looking for a definition of design may not help yougrasp what it is

Design can be seen in any human artefact and in nature

The single word “design” encompasses an awful lot ofobjectives and subjective aspects



Dr. R. Khedri


They can be

aesthetic (subjective)

functional (Objective)

many other aspects of an object or a process (Objectiveor subjective), which usually requires considerable

research,thought,modeling,interactive adjustment,and re-design



Dr. R. Khedri


Depending on the designed entity, either the objectiveor the subjective aspects might takeover

When the subjective aspects take over, design is thenviewed as a more rigorous form of art, or art with aclearly defined purpose

When the objective aspects take over, design is thenviewed as a simple mathematical transformation of themathematical entities expressing the functionalrequirements into a design which is a collection ofmathematical entities with their mathematicalconnectorsSo, design involves both mathematics and art astransformation means (and may be other skills?!!!)



Dr. R. Khedri


Example

Design as a mathematical activity Construction of adeterministic finite automaton (DFA) that accepts all binarystrings with an even number of 0’s and the number of 1’s isa multiple of 3.Design Process: Let L(M3) be the language that containsonly all binary strings with an even number of 0’s and thenumber of 1’s is a multiple of 3.

L(M3)

= 〈 From the textual description of the languagesas given by the question. 〉

{x | x ∈ {0, 1}∗ ∧ x contains even number of 0’s

∧ x contains a number of 1’s that is multiple of 3}= 〈 Definition of set intersection 〉{x | x ∈ {0, 1}∗ ∧ x contains even number of 0’s}∩ {x | x ∈ {0, 1}∗ ∧ x ∧ contains a number of 1’s that is multiple of 3}



Dr. R. Khedri


Example

From the above derivation, we can notice that L(M3) is theintersection of two languages:

1 L(M1) = {x | x ∈ {0, 1}∗ ∧x contains even number of 0’s}

2 L(M2) = {x | x ∈ {0, 1}∗ ∧ x ∧contains a number of 1’s that is multiple of 3}

L(M1) and L(M2) are regular since they are accepted by themachine M1 and M2, respectively.Since L(M3) = L(M1) ∩ L(M2), then M3 can be obtainedfrom M1 and M2 using the product construction.



Dr. R. Khedri


Example

Continued ...Product construction:

1 Q3 = {α, β} × {a, b, c} ={(α, a), (α, b), (α, c), (β, a), (β, b), (β, c)}

2 Σ = {0, 1}3 s3 = (s1, s2) = (α, a)

4 F3 = F1 × F2 = {α} × {a} = {(α, a)}5 δ3 is given by the following table.

0 1

(α, a) (β, a) (α, b)

(α, b) (β, b) (α, c)

(α, c) (β, c) (α, a)

(β, a) (α, a) (β, b)

(β, b) (α, b) (β, c)

(β, c) (α, c) (β, a)



Dr. R. Khedri


Example

Continued ...The graphical representation of M3.

1

( , a)α

( , b)

( , c)

( , b)

( , c)

α

α β

( , a)β

01

0

β

1

01

0

1

0

1

0

Figure: Machine M3 (The product of M1 and M2)



Dr. R. Khedri


Example

Design as an artistic activity

Figure: Game Interface



Dr. R. Khedri


Example

Design as an artistic activity (Continued...)

Figure: Game Interface



Dr. R. Khedri


In the literature, you find some unsuccessful attempts todefine the term design:

In some literature, you find that design is “makingthings better for people”

But, for instance, we all could say that we too “makethings better for people”!!!!!!!

“Design is that area of human experience, skill andknowledge which is concerned with man’s ability tomould his environment to suit his material and spiritualneeds.”[Archer, B (1973) The Need for Design Education.Royal College of Art]



Dr. R. Khedri


Design could be viewed as an activity that translates anidea/goal/wish into a blueprint for an artefact or aprocess that is fit for its environment

We will focus on examples of design that would giveeach one of us his/her own understanding of design(which enhances creativity)

We however agree that the obtained artefact or processis suitable for (fit for use in) the intended environment



Dr. R. Khedri


What is in design more than the translation of an idea to anartefact or process?

Design can be seen as an activity of reconciliationbetween conflicting needs or constraints put by theenvironment

Design is a creative process based around the ”buildingup” of ideas

Avoiding negative judgments of contributors whendesigning eliminates the fear of failure and encouragesmaximum input and participation

Encourage outside the box thinking in this processsince this can often lead to creative solutions



Dr. R. Khedri


Although design is subject to personal taste, designthinkers share a common set of values that driveinnovation

These values are mainly creativity, fertile imagination,“whole brain thinking”, teamwork, end-user focus,curiosity

The design thinking process should evolve through thefollowing stages: define, research, ideate, prototype,choose, implement, and learn



Dr. R. Khedri


A bad software design may result in

a poor quality software product which does not meetuser requirement,

A system that is not adaptive to any furtherrequirement changes

A system that is not reusable

A system that has unpredictable behavior

A system that has poor performance

Software production itself may be very ineffective interms of time and cost



Dr. R. Khedri


A good software design

Reduces risks in software production

Coordinates development teams to work togetherorderly

Makes the system traceable for implementation andtesting

Allows the software product to have better overallsystem-qualities (quality attributes)



Dr. R. Khedri

Introduction to Software DesignåWhere does software design sit in SDLC?

Software Development Life Cycle (SDLC)

8. System Testing

5. Coding

4. Detailed Design

9. Production Operation & Maintenance

6. Unit Testing

7. Integration & Testing

Cost

2. Product Requirements Spec.

3. Architecture Design

1. Pre−requirements

Figure: Software Process V-model



Dr. R. Khedri

Introduction to Software DesignåWhat is meant by Software Architecture?

Software architecture can be seen as a blueprintguideline for developing a large and complex softwaresystem based on the software requirement analysisspecification

The architecture highlights early design decisions thathave a decisive impact on the ultimate success ofsoftware products

Must be included in a high level design called ”softwarearchitecture”

The specializations of software elementsConnection typesSet of constraints (space, time, budget, etc.) on howthey can be plannedSet of desired quality attributes (such as performance)



Dr. R. Khedri


However, the proposed architecture can addresspartially or totally some of the desired quality attributes

Families of architectures that have “similar” choices onelement types and connections are very often calledarchitecture style

Each architecture style represents

The way the elements are arrangedThe connections and interactions among the elementsThe control transfer among the elements in the systemThe semantic constraints and behaviors concerningdata transfer of the architecture styleThe quality attributes trade-off



Dr. R. Khedri


A partial list of quality attributes: performance,reliability, portability, usability, security, maintainability,adaptability, usability, etc.

Quality attributes are closely related to architecturestyles

Each architecture style contributes some qualityfeatures

An architecture style represents tradeoffs among manyconflicting quality attributes



Dr. R. Khedri


BA A controls B

Sub−system

Component/Element

Connector

Figure: Rough representation of a software system architecture



Dr. R. Khedri


IEEE Std 1471 defines the system architecture as ”thefundamental organization of a system embodied in itscomponents, their relationships to each other, and tothe environment, and the principle guiding its designand evolution”

The basis of a software architecture design is thesoftware requirement specification document

The requirement process covers information domainmodeling, data modeling, function modeling,behavioral modeling, and user interface modeling

Software requirements has two aspects (functional,non-functional)



Dr. R. Khedri


Role of the software architect

System static partitioning and decomposition intosub-systems and communications between sub-systems

Establishing dynamic control relationships betweendifferent sub-systems in data flow controlling, orcontrol flow orchestration, or message dispatching

Considering and evaluating all alternative architecturestyles for the problem domain

Analysis of the trade-off on quality attributes duringthe selection of architecture styles



Dr. R. Khedri


Map a requirement specification to the softwarearchitecture and guarantee that the softwarearchitecture satisfies all functional and non-functionalrequirements

There are many kind of traceabilities

The architect does backward traceability

Might get involved in the forward traceability

If the requirements cannot be practically materialized,system analysts and software architects can go back tostakeholders in order to amend the requirements



Dr. R. Khedri


dr. ridha khedri - cas.mcmaster.ca filecas 703 software design dr. r. khedri cas 703 software design...

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