K. Ingram (with thanks to A. Seddon) Staffordshire UNIVERSITY School of Computing Introduction to Software System Design

K. Ingram (with thanks to A. Seddon)

StaffordshireUNIVERSITY

Schoolof

Computing Introduction to Software System Design



Schoolof

Computing

Information systems design

Software is an example of an artifact. An artifact is:

a product of human art & workmanship(Oxford dictionary)

Artifacts need to be:– designed (in order to meet some specific need)– fabricated (according to the plans created by

the design)



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The Waterfall Approach

1. Requirements Analysis2. Solution Design3. Software Creation4. Software Testing5. System Hand-over6. Maintainance



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Requirements Analysis

Requirements Analysis is primarily concerned with

what the end user wantsNeed knowledge of the problem domain the solution space(often part of feasibility study)



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Systems Analysis

This task specifieswhat the chosen solution should doTwo forms of requirement emerge

from this: functional requirements non-functional requirements



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Software Design

Design is concerned withhow the requirement is to be met by

the eventual system

The designer’s task is to solve the problems posed by requirements elicitation and systems analysis



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A major difficulty

The fundamental problem is that designers are obliged to use current information to predict a future state that will not come

about unless their predictions are correct. The final outcome of designing has to be

assumed before the means of achieving it can be explored: designers have to work

backwards in time from an assumed effect upon the world to the beginning of a chain of events that will bring the effect about

(J C Jones, Design Methods: Seeds of Human Futures, 1970)



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The design process

Purpose: to produce a plan which can be used as a blueprint for the implementation, which describes:– system structure (how elements are

related)– how elements fit together

For software design an added difficulty is the use of static forms to model dynamic properties of our solution



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The design plan should...

describe the process aspects of the design describe the product aspects of the design

Software is distinguished from artifacts such as bridges, clocks etc because of this dual nature of design.



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How designers work

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How designers work

Experienced designers working in familiar domains will make extensive use of experience (reuse of domain knowledge)

Work in an opportunistic manner, adapting strategy to problem and evolving solution

If no domain knowledge then use of a design method (method knowledge) is the substitute



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Aspects of Software Design

Information Structure

Interfaces

Processes

Controls

Security



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Processes

What the software must do Use the Data Flow Diagram

processes as a basis Create “mini-specifications” for

each action required by the system Event-driven systems: need a mini-

spec for the response to each event



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Information Structure

How is the information to be stored? Use Logical Data Structure as the

basis Example:

– Logical Data = “customer information”,– Physical Data = Customer contact table,

Customer Purchasing History, Customer Query History.

Use Normalisation for database design



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Interfaces

How will the inputs and outputs look?

Create “sketches” of screen layouts and printouts.

Identify “action points” to link to your process design e.g. command buttons, combo boxes

Consider the “look and feel”



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Controls

How does the user know the system is running correctly

Examples:– Can get summary statistics– Cannot accidentally repeat

procedures



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Security

Keeping the system safe from interference, keeping it up and running.



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Privacy

Keeping information in the system private – not for unauthorised eyes.



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V & V

VerificationAre we building the system right?

ValidationAre we building the right system?



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V & V - some difficulties

Expressing requirements in terms that allow them to be related to the design model

User requirements are likely to change during development

Requirements describe a process and this is difficult to describe analytically



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Design Guidelines

Many software development methodologies include the design stage



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Design as problem solving

Key factor is the use of abstraction to:– help model solution of essential aspects;– separate logical and physical aspects– help make decisions about choices

The ultimate criteria should be that offitness of purpose

Documents

K. Ingram (with thanks to A. Seddon) Staffordshire UNIVERSITY School of Computing Introduction to Software System Design