software engineering-i module-i introduction to s/w development lecture-1

Software Engineering-I

Module-I

Introduction to S/W Development Lecture-1

Software Engineering-I Shahbaz Ahmed 2

Reference Books “Software Engineering” by Ian Somerville 7th Edition

“Software Engineering Theory and Practice” by Shari, Lawrance & Pfleeger 2nd Edition

“Fundamentals of Object Oriented Design in UML” by Miler Page-Jones

An Introduction to Software Engineering

Requirements for this Class You are proficient in a programming

language, but you have no experience in analysis or design of a system

You want to learn more about the technical aspects of analysis and design of complex software systems

Objectives To introduce software engineering and to

explain its importance To set out the answers to key questions

about software engineering To introduce ethical and professional

issues and to explain why they are of concern to software engineers

Objectives cont… Appreciate Software Engineering:

• Build complex software systems in the context of frequent change

Understand how to • produce a high quality software system within time• while dealing with complexity and change

Acquire technical knowledge (main emphasis) Acquire managerial knowledge

Understand the Software Lifecycle• Process vs Product• Learn about different software lifecycles• Greenfield Engineering, Interface Engineering,

Reengineering

Acquire Managerial Knowledge

What is an Engineering System? Engineering system = capability for the

development of systems, hardware, or software Components:

• People• Process• Technology• Knowledge

Interfaces:• Internal (among components)• External (to stakeholders and customers)

Primary Relationship Diagram

People

Process

Technology

Knowledge

EngineeringSystem

(Components of an engineering system)

What do these terms mean? People: the people who are part of the engineering system

• Engineers• Infrastructure support personnel• Managers

Process: the processes used by the people or technology to accomplish the functions of the engineering system

Technology: the tools and mechanisms of the engineering system

Knowledge: value-added contextual information necessary to the development and operation of the engineering system

Engineering System Capability

People

Process

Technology

Knowledge

EngineeringSystemPeople

Process

Technology

Knowledge

EngineeringSystem

Resourcesand

Requirements

Systems,Software, &HardwareProducts

….Time….

Why Software Engineering? Software development is hard ! Important to distinguish “easy” systems (one

developer, one user, experimental use only) from “hard” systems (multiple developers, multiple users, products)

Experience with “easy” systems is misleading• One person techniques do not scale up

Analogy with bridge building:• Over a stream = easy, one person job• Over River Severn … ? (the techniques do not scale)

Why Software Engineering ? The problem is complexity Many sources, but size is key:

• UNIX contains 4 million lines of code• Windows 2000 contains 108 lines of code

Software engineering is about managing this complexity.

FAQs about software engineering

What is • software?• software process?• software engineering?• software process model?

What is software engineering? What is the differenceWhat is the difference

• between between software engineering and computer science?• betweenbetween software engineering and system engineering?

FAQs about software engineering What are the costs of software engineering? What are software engineering methods? What is CASE (Computer-Aided Software

Engineering) What are the attributes of good software? What are the key challenges facing software

engineering?

Software engineering (Criticality)

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.

Expenditure on software represents a significant fraction of GNP in all developed countries.

Software costs Software costs often dominate computer

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 systems with a long life, maintenance costs may be several times development costs.

Software engineering is concerned with cost-effective software development.

Introduction Software engineering is an interesting subject.

In order to understand this subject we will need to look at a number of examples and case studies. And we will need to see how we can develop good software and how it could be improved in different scenarios?

Before we move on to software engineering we need to understand what software actually is.

What is software? Computer programs and associated documentation

+ data

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• Specific (custom) - developed for a single customer according to

their specification

Why software is important? Used in

• Business decision-making• Modern scientific investigation and engineering problem solving• Games• Embedded systems

Similarly in many other fields like • education, • office automation, • Internet applications etc, software is being used.

Due to its central importance and massive use in many fields it is contributing a lot in terms of economic activity started by the software products. • Billions and trillions of dollars are being invested in this field

throughout the world every year.

What is software engineering?Software engineering is an engineering discipline

which is concerned with all aspects of software production

Software engineers should • adopt a systematic and organised approach to their work • use appropriate tools and techniques depending on

• the problem to be solved, • the development constraints and • the resources available

• New software can be created by developing new programs, configuring generic software systems or reusing existing software.

Software Engineering as defined by IEEE IEEE 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.”

Software Engineering is defined by Ian Somerville as• “A field concerned with all aspects of software

production’ Software engineering is not just concerned with the technical processes of software development but also with activities such as software project management and with the development of tools, methods and theories to support software production”.

What is the difference between software engineering and computer science? Computer science is concerned with theory and

fundamentals; software engineering is concerned with the practicalities of developing and delivering useful software.

Computer science theories are still insufficient to act as a complete underpinning for software engineering (unlike e.g. physics and electrical engineering).

This sums up software Engineering as• ”SE is the process of utilizing our knowledge of

computer science in effective production of software systems.”

What is the difference between software engineering and system engineering?

Software engineering is part of System engineering System engineering is concerned with all aspects of

computer-based systems development including • hardware, • software and • process engineering

System engineers are involved in system specification, architectural design, integration and deployment

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

Software Crisis in 1960 which lead to SE Lets imagine a person who use to live in a village and who have constructed a hut for

him to live. Definitely he should have face some problems in the beginning but was managed to build a hurt for him. Now if you ask him to construct another hut, he may be able to construct one more easily and in a better way. This new hut may be better than the first one and he may construct it in a relatively less time. But if you ask him to construct concrete and iron houses then ?

Evolution is the name of the game in IT industry In most of the cases that software which was tried to be build

using those old tools and techniques were not complete. Most of the times it was delivered too late. Most of the projects were over-budgeted. And in most of the case systems build using these techniques

were not reliable – meaning that they were not be able to do what they were expected to do.

This lead to realization that only coding is not enough

What is a software process model?A simplified representation of a software process,presented from a specific perspective

Examples of process perspectives: Workflow perspective represents inputs, outputs and dependencies Data-flow perspective represents data transformation activities Role/action perspective represents the roles/activities of the

people involved in software process Generic process models

• Waterfall• Evolutionary development• Formal transformation• Integration from reusable components

The costs of software engineering?

Roughly 60% of costs are development costs, 40% are testing costs. For custom software, evolution costs often exceed development costs

Costs vary depending on the type of system being developed and the requirements of system attributes such as performance and system reliability

Distribution of costs depends on the development model that is used

Product development costs

Specification Development System testing

25 50 75 1000

What is CASE ? (Computer-Aided Software Engineering)

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

Software systems which are intended to provide automated support for software process activities, such as requirements analysis, system modelling, debugging and testing

What are the attributes of good software? The software should deliver the required functionality and

performance to the user and should be maintainable, dependable and acceptable.

Maintainability• Software must evolve to meet changing needs;

Dependability• Software must be trustworthy;

Efficiency• Software should not make wasteful use of system resources;

Acceptability• Software must accepted by the users for which it was designed. This means it

must be understandable, usable and compatible with other systems.

What are the key challenges facing software engineering? Heterogeneity, delivery and trust. Heterogeneity

• Developing techniques for building software that can cope with heterogeneous platforms and execution environments;

Delivery• Developing techniques that lead to faster delivery of

software; Trust

• Developing techniques that demonstrate that software can be trusted by its users.

What are the key challenges facing software engineering?

Software engineering in the 21st century faces three key challenges:

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

Professional and ethical responsibility of s/w engineers Software engineering involves wider

responsibilities than simply the application of technical skills.

Software engineers must behave in an honest and ethically responsible way if they are to be respected as professionals.

Ethical behaviour is more than simply upholding the law.

Issues of professional 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 of

competence. They should not knowingly accept work which is outwith their competence.

Issues of professional 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 the intellectual property of employers and clients is protected.

Computer misuse • Software engineers should not use their technical skills

to misuse other people’s computers. Computer misuse ranges from relatively trivial (game playing on an employer’s machine, say) to extremely serious (dissemination of viruses).

Some General Discussion that will

demonize Software Engineering

in your hearts

Can you develop this?

Requirements

Software

Limitations of Non-engineered Software

Software Production has a Poor Track Record Example: Space Shuttle Software Cost: $10 Billion, millions of dollars more than planned Time: 3 years late Quality: First launch of Columbia was cancelled because of a

synchronization problem with the Shuttle's 5 onboard computers. • Error was traced back to a change made 2 years earlier when a

programmer changed a delay factor in an interrupt handler from 50 to 80 milliseconds.

• The likelihood of the error was small enough, that the error caused no harm during thousands of hours of testing.

Substantial errors still exist.• Astronauts are supplied with a book of known software problems

"Program Notes and Waivers".

Quality of today’s software…. The average software product released on the market is not error free.

QuickTime™ and aCinepak decompressor

are needed to see this picture.

Software Engineering: A Problem Solving Activity Analysis: Understand the nature of the problem and break the

problem into pieces Synthesis: Put the pieces together into a large structure

For problem solving we use Techniques (methods):

• Formal procedures for producing results using some well-defined notation

Methodologies: • Collection of techniques applied across software development and

unified by a philosophical approach Tools:

• Instrument or automated systems to accomplish a technique

Software Engineering-I Shahbaz Ahmed 43 20

Software Engineering: DefinitionSoftware Engineering is a collection of techniques,

methodologies and tools that help with the production of

a high quality software system with a given budget before a given deadline while change occurs.

Scientist vs Engineer Computer Scientist

• Proves theorems about algorithms, designs languages, defines knowledge representation schemes

• Has infinite time… Engineer

• Develops a solution for an application-specific problem for a client• Uses computers & languages, tools, techniques and methods

Software Engineer• Works in multiple application domains• Has only 3 months...• …while changes occurs in requirements and available technology

Factors affecting the quality of a software system Complexity:

• The system is so complex that no single programmer can understand it anymore

• The introduction of one bug fix causes another bug

Change: • The “Entropy” of a software system increases with each change: Each

implemented change erodes the structure of the system which makes the next change even more expensive (“Second Law of Software Dynamics”).

• As time goes on, the cost to implement a change will be too high, and the system will then be unable to support its intended task. This is true of all systems, independent of their application domain or technological base.

Why are software systems so complex? The problem domain is difficult The development process is very difficult to

manage Software offers extreme flexibility Software is a discrete system

• Continuous systems have no hidden surprises (Parnas)

• Discrete systems have!

Dealing with Complexity1. Abstraction 2. Decomposition3. Hierarchy

What is this?

1. Abstraction Inherent human limitation to deal with

complexity• The 7 +- 2 phenomena

Chunking: Group collection of objects Ignore unessential details: => Models

Models are used to provide abstractions System Model:

• Object Model: What is the structure of the system? What are the objects and how are they related?

• Functional model: What are the functions of the system? How is data flowing through the system?

• Dynamic model: How does the system react to external events? How is the event flow in the system ?

Task Model:• PERT Chart: What are the dependencies between the tasks?• Schedule: How can this be done within the time limit?• Org Chart: What are the roles in the project or organization?

Issues Model:• What are the open and closed issues? What constraints were posed

by the client? What resolutions were made?

Interdependencies of the Models

System Model (Structure, Functionality, Dynamic Behavior)

Issue Model(Proposals,Arguments,Resolutions)

Task Model(Organization,ActivitiesSchedule)

Example of an Issue: Galileo vs the Church What is the center of the Universe?

• Church: The earth is the center of the universe. Why? Aristotle says so.

• Galileo: The sun is the center of the universe. Why? Copernicus says so. Also, the Jupiter’s moons rotate round Jupiter, not around Earth.

Issue-ModelingIssue:

What is the Center of the

Universe?

Proposal1: The earth!

Proposal2:The sun!

Pro: Copernicus

says so.

Pro: Aristotlesays so.

Pro: Change will disturb

the people.

Con: Jupiter’s moons rotate

around Jupiter, not around Earth.

Resolution (1615):The church

decides proposal 1is right

Resolution (1998): The church declares

proposal 1 was wrong

2. Decomposition A technique used to master complexity (“divide and conquer”) Functional decomposition

• The system is decomposed into modules• Each module is a major processing step (function) in the application

domain• Modules can be decomposed into smaller modules

Object-oriented decomposition• The system is decomposed into classes (“objects”) • Each class is a major abstraction in the application domain• Classes can be decomposed into smaller classes

Functional DecompositionTop Level functions

Level 1 functions

Level 2 functions

Machine Instructions

System Function

Load R10 Add R1, R10

Read Input Transform ProduceOutput

Transform ProduceOutputRead Input

Functional Decomposition Functionality is spread all over the system Maintainer must understand the whole system to make

a single change to the system Consequence:

• Codes are hard to understand• Code that is complex and impossible to maintain• User interface is often awkward and non-intuitive

Example: Microsoft Powerpoint’s Autoshapes

Autoshape

Functional Decomposition: Autoshape

DrawRectangle

DrawOval

DrawCircle

DrawChangeMouseclick

ChangeRectangle

ChangeOval

ChangeCircle

What is This?

Model of an Eskimo

EskimoSize

Dress()Smile()Sleep()

ShoeSizeColorType

Wear()

* CoatSizeColorType

Wear()

Iterative Modeling then leads to ....

EskimoSize

CaveLightingEnter()Leave()

lives in

but is it the right model?

Entrance*

OutsideTemperature

LightSeasonHunt()

Organize()

moves around

WindholeDiameter

MainEntranceSize

Alternative Model: The Head of an Indian

IndianHair

MouthNrOfTeethsSizeopen()speak()

*EarSizelisten()

FaceNosesmile()close_eye()

Class Identification Class identification is crucial to object-oriented modeling Basic assumption:

1. We can find the classes for a new software system: We call this Greenfield Engineering

2. We can identify the classes in an existing system: We call this Reengineering

3. We can create a class-based interface to any system: We call this Interface Engineering

Why can we do this? Philosophy, science, experimental evidence What are the limitations? Depending on the purpose of the system

different objects might be found• How can we identify the purpose of a system?

What is this Thing?

Modeling a Briefcase

BriefCase

Capacity: IntegerWeight: Integer

Open()Close()Carry()

A new Use for a Briefcase

BriefCase

Capacity: IntegerWeight: Integer

Open()Close()Carry()

SitOnIt()

Questions

Why did we model the thing as “Briefcase”? Why did we not model it as a chair? What do we do if the SitOnIt() operation is the most

frequently used operation? The briefcase is only used for sitting on it. It is never

opened nor closed. • Is it a “Chair”or a “Briefcase”?

How long shall we live with our modeling mistake?

3. Hierarchy We got abstractions and decomposition

• This leads us to chunks (classes, objects) which we view with object model

Another way to deal with complexity is to provide simple relationships between the chunks

One of the most important relationships is hierarchy 2 important hierarchies

• "Part of" hierarchy• "Is-kind-of" hierarchy

Part of HierarchyComputer

I/O Devices CPU Memory

Cache ALU Program Counter

Is-Kind-of Hierarchy (Taxonomy)

Muscle Cell Blood Cell Nerve Cell

Striate Smooth Red White Cortical Pyramidal

So where are we right now? Three ways to deal with complexity:

• Abstraction • Decomposition• Hierarchy

Object-oriented decomposition is a good methodology• Unfortunately, depending on the purpose of the system, different objects

can be found How can we do it right?

• Many different possibilities• Our current approach: Start with a description of the functionality (Use

case model), then proceed to the object model• This leads us to the software lifecycle

Software Lifecycle Activities

Subsystems

Structured By

class...class...class...

SourceCode

Implemented By

Solution Domain Objects

Realized By

SystemDesign

ObjectDesign

Implemen-tation Testing

Application

Domain Objects

Expressed in Terms Of

Test Cases

Verified By

class.... ?

RequirementsElicitation

Use CaseModel

Analysis

...and their models

Software Lifecycle Definition Software lifecycle:

• Set of activities and their relationships to each other to support the development of a software system

Typical Lifecycle questions:• Which activities should I select for the software

project?• What are the dependencies between activities? • How should I schedule the activities?

Reusability A good software design solves a specific problem but is general

enough to address future problems (for example, changing requirements)

Experts do not solve every problem from first principles• They reuse solutions that have worked for them in the past

Goal for the software engineer:• Design the software to be reusable across application domains and

designs How?

• Use design patterns and frameworks whenever possible

And oh yes one more thing Always remember the law of diminishing

returns.• “Your tea will get sweater up to a certain level

only and the excess sugar will produce no effect”

• Same is the case with software engineering Moral

• Efficient resource Utilization

Key points

Software engineering is an engineering discipline that is concerned with all aspects of software production.

Software products consist of developed programs and associated documentation. Essential product attributes are maintainability, dependability, efficiency and usability.

The software process consists of activities that are involved in developing software products. Basic activities are software specification, development, validation and evolution.

Methods are organised ways of producing software. They include suggestions for the process to be followed, the notations to be used, rules governing the system descriptions which are produced and design guidelines.

Key points CASE tools are software systems which are designed

to support routine activities in the software process such as editing design diagrams, checking diagram consistency and keeping track of program tests which have been run.

Software engineers have responsibilities to the engineering profession and society. They should not simply be concerned with technical issues.

Professional societies publish codes of conduct which set out the standards of behaviour expected of their members.

software engineering-i module-i introduction to s/w development lecture-1

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