Lecture 1 Slides 6

8/16/2019 Lecture 1 Slides 6

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ECNG-3023 Introduction to

Software Engineering(September 2009)

Dr. Sastry MKSDepartment of Electrical &

Computer Engg

University of West Indies

St. Augustine Campus

Trinidad & Tobago

The IEEE Computer Society defines software

engineering as:

• The application of a systematic,

disciplined, quantifiable approach to the

development, operation and maintenance of

software; that is, the application of

engineering to software.

• The study of approaches as above.

What is Software Engineering?

Problem solving (Analysis)

We typically solve a problem by analysing it,

that is by breaking it down into pieces.

Problem

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Once we have analysed the problem, we must

construct our solution from components that

address the problem's various aspects

Solution

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Problem solving (Synthesis)

• Invisibility

– No physical presence

• Flexibility – After all it's software

• Complexity

– no physical constraints, yet many possible paths

of solution flow depending on the logic for

decision making.

How are Software Projects different?

The

ProductWhat the customers see

Compilers, Operating Systems, Utilities

DBMSCASE

Configuration Management

Code

Data Definitions

Databases

Specifications

Plans

Standards & ProceduresManuals

Simulators

Development Systems

Automatic test Equipment

The Software Iceberg

What is CASE???

Computer Aided Software Engineering

Find out different CASE Tools!!!


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• Unfulfilled demand

• Increasing complexity

• Greater customer expectations

• Rapid obsolescence

• Increasing competition

• Shorter product cycle times

• Producing more with less

• Evolving methods and tools

Key Issues Facing Software Developers

• A fault occurs when a human makes a mistake,

called an error in performing some software

activity.

• A failure is a departure from the systems

required behaviour It can be discovered before

or after system delivery or during operation and

maintenance. Since requirements documents

can contain faults, failures can exist even

though the system is performing as specified!

Terminology – “Bugs”

• Late delivery? No delivery at all?

• Not delivering what was agreed to

or what was announced?

• Over budget?

• Not meeting revenue expectations?

• Quality below expectations?

What Does Project Failure Mean??

• Not understanding what the product

must do

• Uncontrolled changes

• Optimistic thinking

• Insufficient resources

• Lack of disciplined development

• Confusion about what needs to be done

• Ineffective teamwork

• Failure to consider business needs

Some reasons why SW Projects Fail

• Price

• Mature market

• Lack of essential capabilities• Difficult to use

• Unreliable

• Poor developer reputations

• Poor product support

Some reasons why Products Fail

• Good understanding of the problem to solve

• Good planning and execution

• Extraordinary effort and commitment by

individuals• Luck

• Satisfy an unfulfilled need

• Novelty

• Value

• Marketing strategy

Reasons for Success


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• Quality of the product?

• Quality of the process?• Quality in the context of the

Business Environment?

What is a good SW?

• Correctness

• Reliability• Efficiency

• Integrity

• Usability

• Maintainability

• Testability

• Flexibility• Portability

• Reusability

• Interoperability

Quality of SW Product

• Correctness

– extent to which program

fulfils its specification

• Reliability

– systems ability not tofail

• Efficiency

– use of resources, e.g. processor time, storage

• Integrity

– protection of programfrom unauthorisedaccess

• Usability

– ease of software

• Maintainability

– effort required to locateand fix a fault in

program within itsoperating environment

• Testability

– ease of testing program,

to ensure it is error-free

and meets its

specification

• Flexibility

– Ease of makingchanging required by

changes in the operatingenvironment


• Portability

– Effort required to transfer a programfrom one environment to another

• Reusability

– Ease of reusing software in adifferent context

• Interoperability

– Effort required to couple system toanother system


• many activities can affect ultimate product quality

• By modelling a process, we can examine it and look

for improvements

– Where and when are we likely to find a particular kind of

fault?

– How can we find faults earlier in the development process?

– How can we build in fault tolerance so that we canminimize the likelihood that a fault will become afailure?

– Are there alternative activities that can make our processmore effective or efficient at assuring quality?

Quality of SW Process

• A view in terms of the products and

services being provided by the business

in which the software is embedded.

– i.e., we look at the business value.

– can be as simple as Return On Investment

(ROI) or some more elaborate measure.

Quality from Business Point of View


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CustomerSponsors systemdevelopment

DeveloperBuilds

system

UserUses

system

$$$,

needs

Contractual

obligation

Needs

Software system

Who does SWE?Systems Approach

• Elements of a system: – Activities and Objects

– Relationships and the System Boundary

• Nested systems and system interrelationships

Activities and Objects

• Distinguish between activities and objects:

– activity is something that happens in a system.

• Usually described as event initiated by trigger

• Transforms one thing to another by changing a characteristic,

e.g.

• data element moved from one location to another

• data element is changed from one value to another

• data element is combined with other data to supply input for yet

another activity

– object or entity is element involved in the activity.

• Usually related in some way: arranged in a table or grouped as

records with pre-defined format

Relationships and the System

Boundary• A system is defined as a collection of things:

– a set of entities,

– a set of activities,

– a description of the relationships among entities and

activities,

– and a definition of the boundary of the system.

• Boundary states what is included and what is not

Examples: the human respiratory system, a paycheck production system

Nested systems

• It is possible for one system to exist within

another system, e.g. sensor system

– One can have various functions of the sensors nes tedwithin each other.

• Boundary of system is important to see what is:

– within the system

– outside of the system

– what crosses the boundary of the system

An analogy of software

engineering

• Determining andanalysing

requirements• Producing and

documenting thedesign

• Detailedspecifications

• Identifying anddesigning components

• Requirements analysisand definition

• System design

• Program design


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Analogy ...

•Building components

• Testing components

• Integratingcomponents

• Making finalmodifications

• Continuingmaintenance

•Writing programs

• Unit testing

• Integration testing

• System testing

• System delivery

• Maintenance

7 Factors Altering SW Engineering

1.Criticality of time to market for commercial products

Business must ready their new products and services before their competitors do

2.Shifts in the economics of computing

Lower hardware costs and greater development andmaintenance costs

3.Availability of powerful desktop computing

More development power in hands of users, thereforesoftware engineers are likely to be building morecomplex systems than before

4.Extensive networks available

Makes it easier for users to find information withoutspecial applications e.g. searching WWW is quick, easyand effective

Key Factors affecting SWE

5. Availability and adoption of object-orientedtechnology

Makes available reusable modules for immediate andspeedy inclusion in new applications

6. Graphical User Interfaces (GUIs)

Helps to put a friendly face (appearance) oncomplicated applications

7. Unpredictability of the waterfall method

Developing subsystems in parallel requiresdevelopment process very different from waterfall

model

Key Factors that changed SWE

• Abstraction

• Analysis and Design methods and Notations

• User Interface Prototyping• Software Architecture

• Software Process

• Reuse

• Measurement

• Tools and Integrated Environments

Discipline of SWE

Abstraction

• Abstraction is a description of the problem at somelevel of generalisation that allows us to concentrateon the key aspects of the problem without getting

involved in the difficulties of the details.

• Typically involves identifying classes of objectsthat allow us to group items together so we:

– Can deal with fewer things, and

– Concentrate on the commonalities of items in each class


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Analysis and Design Methods and Notations

These offer a

• Communication medium

– Communication and documentation of decisions among

development team

• Ability to build models

• Ability to check models for completeness and

consistency

• Reuse requirements and design components from

previous projects

User interface Prototyping

• Prototyping is building a small version of a system,

usually with limited functionality• Prototypes can be used to:

– Help the user/customer identify the key requirements of asystem

– Demonstrate feasibility of a design or approach

• Usually an iterative process:

– Build prototype

– Evaluate it with user and customer feedback

– Consider how changes might improve product or design

– Build another prototype OR iteration ends whendeveloper and customer are satisfied with solution

Software architecture

• Importance of overall architecture of system is:

– ease of implementation and testing

– speed and effectiveness of maintenance and change.

• The quality of the architecture can make or break asystem.

• System’s architecture describes system in terms of:

– set of architectural units, and

– map of how units relate to each other

Software architecture (continued)

Five ways of decomposing a system

• Modular: based upon assigning functions to

modules

• Data-oriented: based upon external data structures

• Event-oriented: based upon events that the system

must handle

• Outside-in: based upon user inputs to the system

• Object-Oriented: based upon identifying classes of

objects and their interrelationships

Software processReuse

• We take advantage of commonalities acrossapplications by reusing items from previousdevelopment.

• Reuse pertains not only to code but to design, testdata, requirements etc.


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Measurement

• Measurement is a driving force in softwareengineering research:

– improving our processes, resources and methods so thatwe produce and maintain better products.

– But sometimes we express measurements generally, withno quantitative description.

• We would like to quantify:

– where we can and what we can,

– describe our actions and outcomes in a commonmathematical language that allows us to compare

progress across disparate projects .

Tools and integrated environments

Issues in tool integration

• Platform: the ability of tools to interoperate on a

heterogeneous network • Presentation: commonality of the user interface

• Process: linkage between the tools and thedevelopment process

• Data: the way the tools share data

• Control: the ability of one tool to notify and initiateaction in another.

Computers are increasingly being introduced into safety-critical systems and as a consequence, have been involvedin accidents.

Some of the most widely cited software-related accidents insafety-critical systems involved a computerized radiationtherapy machine called the Therac-25.

The Therac-25 is a medical linear accelerator manufactured by AECL to treat Cancer Patients with radiation therapy.

Between June 1985 and January 1987, six known accidents

involving massive overdoses by the Therac-25 --- withresultant deaths and serious injuries.

Therac-25 (A SW Disaster) Therac-25 routines(from IEEE ComputerJuly 1993)

End of Lesson I

Thank you

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