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SOFTWAREENGINEERING

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Objectives To introduce software engineering and to

explain its importance.

To set out the answers to key questions aboutsoftware engineering.

To introduce professional issues and to explain

why they are concern to software engineers.

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Roles of people in software people involved in software production

customer / client: wants software

often doesn't know what he/she wants

managers / designers: plan software difficult to predict all problems and issues in

advance

developers: write code to implement software

it is hard to write complex code for large systems

testers: perform quality assurance (QA) it is impossible to test every combination of actions

users: purchase and use software product

users can be inconsistent and can misunderstand theproduct

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Problems with software

today Example: Space shuttle software

cost: $10 Billion, millions of dollars.

time: 3 years quality: first launch of space shuttle was cancelled

because of a synchronization problem .

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The economies of ALL developed nations are

dependent on software.

More and more systems are software controlled.

Software engineering is concerned with theories, methods

and tools for professional software development.

Software engineering expenditure represents a

significant fraction in all developed countries.

Software engineering

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Software costs often dominate system costs.

The costs of software on a PC are often greater

than the hardware cost Software costs more to maintain than it does to

develop. For a long life, maintenance costs may

be several times development costs

Software engineering is concerned with cost-

effective software development

Software costs

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FAQs about software

engineering Scope and necessity of software engineering?

Causes of and solutions for software crisis?

What is software? What is software engineering?

What is the difference between softwareengineering and system engineering?

What is a software process?

What is a software process model?

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FAQs about software

engineering What are the costs of software?

What are software engineering methods?

What is CASE (Computer-Aided SoftwareEngineering)

What are the attributes of good software?

What are the key challenges facing softwareengineering?

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Scope and necessity ofsoftware

engineering Software engineering is an engineering approach for software

development. We can alternatively view it as a systematic

collection of past experience.

The experience is arranged in the form of methodologies andguidelines.

A small program can be written without using software engineering

principles. But if one wants to develop a large software product,

then software engineering principles are essential to achieve a good

quality software cost effectively. Example(1): Suppose you have a friend who asked you to build a small

wall as shown in fig. 1.1. You would be able to do that using your

common sense. You will get building materials like bricks; cement etc.

and you will then build the wall.

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But what would happen if the same friend asked you to build a large

multistoried building as shown in fig. 1.2?

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If you don't have a very good idea about building such ahuge complex. Even if you tried to build a large building, itwould collapse because you would not have the requisiteknowledge about the strength of materials, testing,

planning, architectural design, etc.

Building a small wall and building a large building areentirely different ball games.

So building a large building requires knowledge of civil,architectural and other engineering principles.

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Example(2): Without using software engineering principles it would be difficult to

develop large programs. In industry it is usually needed to develop largeprograms to accommodate multiple functions. A problem with developingsuch large commercial programs is that the complexity and difficulty levelsof the programs increase exponentially with their sizes as shown in fig. 1.3.

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For example, a program of size 1,000 lines of code has

some complexity. But a program with 10,000 LOC is not

just 10 times more difficult to develop, but may as well turn

out to be 100 times more difficult unless softwareengineering principles are used.

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Important Techniques to ReduceProblem Complexity:

Abstraction: The principle of

abstraction (in fig.1.4) implies

that a problem can be

simplified by omitting

irrelevant details.

In other words, the main

purpose of abstraction is to

consider only those aspects of

the problem that are relevant

for certain purpose andsuppress other aspects that are

not relevant for the given

purpose.

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Decomposition:

In this technique, a complexproblem is divided into severalsmaller problems and then thesmaller problems are solved

one by one. However, in this technique any

random decomposition of aproblem into smaller parts willnot help.

The problem has to be

decomposed such that eachcomponent of the decomposedproblem can be solvedindependently and then thesolution of the differentcomponents can be combined

to get the full solution.

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Causes ofand solutions forsoftware crisis

Causes of software crisis: software products are difficultto alter, debug, and enhance; use resources non optimally;often fail to meet the user requirements; are far from beingreliable; frequently crash; and are often delivered late.Among these, the trend of increasing software costs is

probably the most important symptom of the presentsoftware crisis.

solutions for software crisis:

It is believed that the only satisfactory solution to thepresent software crisis can possibly come from a spread ofsoftware engineering practices among the engineers,coupled with further advancements to the software

engineering discipline itself.

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What is software? Computer programs and associated documentation

Software products may be developed for a particular

customer or may be developed for a general market

Software products may be

Generic - developed to be sold to a range of different

customers

Custom - developed for a single customer according to

their specification

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What is software engineering? Software engineering: the profession, practiced by

developers, concerned with creating and maintainingsoftware applications by applying technologies and

practices from computer science, project management, andother fields.

Software engineering is an engineering discipline which isconcerned with all aspects of software production.

Software engineers should adopt a systematic andorganised approach to their work and use appropriate toolsand techniques depending on the problem to be solved.

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What is a software process? A set of activities whose goal is the development or

evolution of software.

Generic activities in all software processes are:

Specification - what the system should do and its

development constraints

Development - production of the software system

Validation - checking that the software is what the

customer wants

Evolution - changing the software in response to

changing demands

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What is a software process model?

A simplified representation of a software process,

presented from a specific perspective.

Generic process models Waterfall

Evolutionary development

Formal transformation

Integration from reusable components

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What are the costs ofsoftware? Roughly 60% of costs are development costs, 40%

are testing costs. For custom software, evolution costsoften exceed development costs.

Costs vary depending on the type of system beingdeveloped and the requirements of system attributessuch as performance and system reliability.

Distribution of costs depends on the developmentmodel that is used.

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What are software engineering

methods? Structured approaches to software development which include system

models, notations, rules, design advice and process guidance

Model descriptions

Descriptions of graphical models which should be produced Rules

Constraints applied to system models

Recommendations

Advice on good design practice

Process guidance

What activities to follow

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What is CASE (Computer-Aided

Software Engineering) Software systems which are intended to provide automated

support for software process activities. CASE systems are

often used for method support.

Upper-CASE Tools to support the early process activities of

requirements and design.

Lower-CASE

Tools to support later activities such as programming,debugging and testing.

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What are the attributes ofgood

software? The software should deliver the required

functionality and performance to the user and

should be maintainable and usable.

Maintainability

Software must evolve to meet changing needs.

Efficiency

Software should not make wasteful use of

system resources.

Usability

Software must be usable by the users for which

it was designed.

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What are the key challenges facing

software engineering? Coping with legacy systems, coping with increasing

diversity and coping with demands for reduced delivery

times.

Legacy systems Old, valuable systems must be maintained and updated.

Heterogeneity

Systems are distributed and include a mix of hardware

and software. Delivery

There is increasing pressure for faster delivery of

software.

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P

rofessional Responsibility Software engineering involves wider responsibilities than

simply the application of technical skills.

Software engineers must behave in an honest and

responsible way if they are to be respected as professionals.

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Issues OfProfessional Responsibility Confidentiality

Engineers should normally respect the

confidentiality of their employers or clients

irrespective of whether or not a formal

confidentiality agreement has been signed.

Competence

Engineers should not misrepresent their level ofcompetence. They should not intentionally accept

work which is outside their competence.

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Issues ofprofessional responsibility Intellectual property rights

Engineers should be aware of local laws governing the

use of intellectual property such as patents, copyright,

etc. They should be careful to ensure that theintellectual property of employers and clients is

protected.

Computer misuse

Software engineers should not use their technical skillsto misuse other peoples computers.

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Key Points Software engineering is an engineering discipline which is

concerned with all aspects of software production.

Software products consist of developed programs andassociated documentation. Essential product attributes aremaintainability, efficiency and usability.

The software process consists of activities which areinvolved in developing software products. Basic activitiesare software specification, development, validation andevolution.

Methods are organised ways of producing software. Theyinclude suggestions for the process to be followed, thenotations to be used, rules governing the systemdescriptions which are produced and design guidelines.

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Key points CASE tools are software systems which are designed to

support routine activities in the software process such asediting design diagrams, checking diagram consistency andkeeping track of program tests which have been run.

Software engineers have responsibilities to the engineeringprofession and society. They should not simply beconcerned with technical issues.