Software EngineeringLecture 10Software Design: Architecture, Interface, Procedural

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The Design ActivitiesArchitectural DesignInterface DesignProcedural DesignDatabase Design

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Architectural DesignDesign Concepts

◦Abstraction◦Modularity◦Control Hierarchy

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AbstractionAbstraction permits one to

concentrate on a problem at some level of generalization without regard to irrelevant low level details.

Use of abstraction also permits one to work with concepts and terms that are familiar, in the problem environment without having to transform them to an unfamiliar structure. 4

Modularity A module is defined as a sub-program that is

invoked by another module. The statement are collectively referred to by

a descriptive name called the module name. A module must return to its caller i.e. have a

single entry and exit. The module should be relatively small in

size. It should be easy to read, modify and use. A module should preferably have a single

function.

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Advantages of Modularity Allow large program to be written by several or

different people. Encourage creation of commonly used routines

to be placed in library and/or be used by other programs.

Simplify overlay procedure of loading large program into main storage.

Provide more check point to measure progress. Simplify design, making program easy to modify

and reduce maintenance costs. Provide a framework for more complete testing,

easier to test. Produces well-designed and more readable

program.6

Disadvantages of ModularityExecution time, compilation/loading

time and storage size requirements may be (though not always) increase.

Intermodule communication problems may be increased, due to the larger number of interfaces between parts of the software.

Demands more initial design time, since greater time would need to be spent in the architectural phase of design.

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Modularity and Software Cost

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optimal number of modules

cost of software

number of modules

moduleintegration

cost

module development cost

Control HierarchyControl Hierarchy, represents the

hierarchical organization of program components (modules) and implies a hierarchy of control.◦ Fan-Out, which is a measure of the number of modules

that are directly controlled by another module.◦ Fan-in, which indicates how many modules directly

control a given module.◦ Depth: Number of levels of control.◦ Width: Overall span of control.◦ Superordinate: i.e. a module that controls another

module.◦ Subordinate: i.e. a module that is controlled by another

module.

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Control Hierarchy

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M

A B C

D E F G H

Depth = 2 Fan-In (A) = 1

Width = 5 Fan-Out (A) = 3

Effective Modular DesignFunctional Independence

◦Is achieved by developing modules with “single minded” function and an “aversion” to excessive interaction with other modules.

Independence is measured by two qualitative criteria:◦Cohesion: a measure of relative

functional strength of a module◦Coupling: a measure of relative

interdependence among modules11

Cohesion

Low Cohesion (undesirable)◦Coincidental Cohesion◦Logical Cohesion◦Temporal Cohesion

Moderate◦Procedural Cohesion◦Communicational Cohesion

High (desirable)◦One distinct task in a module (High

Cohesion)12

Coupling

Low (desirable)◦Data Coupling◦Stamp Coupling

Moderate◦Control Coupling

High (undesirable)◦External Coupling◦Common Coupling◦Content Coupling

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User Interface Design

ease ofdevelop-ment

ease ofuse

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A good UI is not easy to create, since the UI development effort is inversely proportional to the ease of use finally obtained in the software

Three Golden Rules (Theo Mandel)

Place the user in controlReduce the user’s memory loadMake the interface consistent

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Place the user in controlMinimize unnecessary /

undesired actionsProvide flexible interactionProvide undoAllow customized interactionHide technical internals from the

casual userDesign direct interaction with

screen objects16

Reduce the user’s memory load

Reduce demand on Short-term Memory

Establish meaningful defaultsDefine intuitive short-cutsUse real-world metaphorDisclose information in a

progressive fashion

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Make the interface consistent

Allow the user to put the current task into a meaningful context

Maintain consistency across a family of applications

If past interactive models have created user expectations, do not making unnecessary changes

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Interface Design Consideration (1)• Interface Consistency

• An inconsistent interface is often the source of frustration to the user.

• Predictability• Predictability is related to the user

model and the consistency of the interface, but emphasizes the principle of least surprise.

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Interface Design Consideration (2)• Error Handling

• A good UI minimizes the chances of an error occurring, and provides a way to recover from any error that does occur.

• Ease of Use• A UI should have on-line help and

user guidance, which are accessible when user has trouble.

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Interface Design Consideration (3)• Clarity

• Errors can be minimized if the UI communication between the UI and the user is as clear as possible.

• Feedback• Feedback to the user is important in

reducing frustration and provides reassurance that the user task is actually being carried out.

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User classificationNovices

No syntactic knowledge, little semantic knowledge of the computer usage in general

Knowledgeable, intermittent usersReasonable semantic knowledge of the application, but low recall of syntactic knowledge

Knowledgeable, frequent usersGood semantic and syntactic knowledge

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PrototypingPrototyping is one of the best known method

used to design the software interface.The prototype is evaluated by the user, who

provides the designer with direct comments about the efficacy of the interface.

If formal evaluation techniques are used (e.g. questionnaires, rating sheets), the designer may extract information from these data.

Design modification are made based on user input and the next level of prototype is created.

The evaluation cycle continues until no further modification to the interface design are necessary.

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Procedural DesignGraphical Design Notation: Flow-chartTabular Design Notation: Decision TableProgram Design Language (PDL)

At the component level, the software engineer must represent data structures, interfaces, and algorithms in sufficient detail to guide the generation of programming language source-code.

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Flow-Chart Boxes to indicate processing

steps. Diamond to indicate logical

conditions. Arrows to indicate flow of control. Two boxes connected by a line of

control will indicate a Sequence.

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Flow-Chart Example

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Decision TableProvides a notation that

translates actions and conditions (described in a processing narrative) into a tabular form. The upper left-hand section contains a list

of all conditions. The lower left-hand section lists all actions

that are possible based on the conditions. The right-hand sections form a matrix that

indicates condition combinations and the corresponding actions that will occur for a specific combination

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Decision Table Example

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Program Design Language Program Design Language (PDL)

is also called structured English, or Pseudocode.

PDL have the following characteristics: A fixed syntax of keywords that provide for all

structured constructs, data declaration, and modularity characteristics

A free syntax of natural language that describes processing features

Data declaration facilities that should include both simple (scalar, array) and complex (linked list or tree) data structures.

Subprogram definition and calling techniques that support various methods of interface description

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ReferencesPressman, Chapter 13-16

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