ch1_introduction (2).ppt

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Chapter 1: Introduction to
Software Engineering
Chapter 1 in Software Engineering Book

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Overview
 Learning Objectives.
What is software engineering?
 Why is software engineering important?

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By the end of this chapter, you will...
Understand what software engineering is.
 Understand why software engineering is important.
 Know answers to key questions related to the software
engineering discipline.

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Activity
Think about all the devices and systems
that you encounter in your everyday life
which have software controlling them…
List as many as you can
Virtually all countries
depend on complex
computer-based
systems.

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Why is Software Engineering important?
Complex systems need a disciplined approach for designing,
developing and managing them.

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Software Development Crises
Projects were:
• Late.
• Over budget.
• Unreliable.
• Difficult to maintain.
• Performed poorly.

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Software errors….the cost
Errors in computer software can have
devastating effects.

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Software Crisis
Example 1: 2009,Computer glitch delays flights
Saturday 3rd October 2009-London, England (CNN)
• Dozens of flights from the UK were delayed Saturday after
a glitch in an air traffic control system in Scotland, but the
problem was fixed a few hours later.
• The agency said it reverted to backup equipment as
engineering worked on the system.
• The problem did not create a safety issue but could cause
delays in flights.
• Read more at:
http://edition.cnn.com/2009/WORLD/europe/10/03/uk.fl
ights.delayed

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Software Crisis
Example 2: Ariane 5 Explosion
• European Space Agency spent 10 years and $7
billion to produce Ariane 5.
• Crash after 36.7 seconds.
• Caused by an overflow error. Trying to store a 64-bit
number into a 16-bit space.
• Watch the video:
http://www.youtube.com/watch?v=z-r9cYp3tTE

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Software Crisis
Example 3: 1992, London Ambulance Service
• Considered the largest ambulance service in the
world.
• Overloaded problem.
• It was unable to keep track of the ambulances
and their statuses. Sending multiple units to some
locations and no units to other locations.
• Generates many exceptions messages.
• 46 deaths.

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Therefore…
A well-disciplined approach to software
development and management is
necessary. This is called engineering.

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 The term software engineering first appeared in the 1968 NATO
Software Engineering Conference and was meant to provoke
thought regarding what was then called the “software crisis”..
 “.. An engineering discipline that is concerned with all aspects of
software production from the early stages of system specification
to maintaining the system after it has gone into use.” Sommerville,
pg.7

Software
Programs
Documentation
Data
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What is Software?
System
Documentation
User
Documentation

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Types of Software
• Generic products.
• Stand-alone systems that are marketed and sold to any customer who wishes to buy
them.
• Examples – PC software such as graphics programs, project management tools;
CAD software; software for specific markets such as appointments systems for
dentists.
• The specification of what the software should do is owned by the software developer
and decisions on software change are made by the developer.
• Customized or bespoke products.
• Software that is commissioned by a specific customer to meet their own needs.
• Examples – embedded control systems, air traffic control software, traffic monitoring
systems.
• The specification of what the software should do is owned by the customer for the
software and they make decisions on software changes that are required.

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Software Engineering vs. Computer Science
“Computer science is no more about computers than
astronomy is about telescopes.” Edsger Dijkstra
Computer Science
• Theory.
• Fundamentals.
• Practicalities of software
design, development and
delivery.

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Software Engineering vs. Systems Engineering
Systems Engineering:
 Interdisciplinary engineering field (computer, software, and process eng.).
 Focuses on how complex engineering projects should be designed and managed.
Systems Engineering
• All aspects of computer-
based systems
development: HW + SW +
Process.
• Older than SWE.
• Deals with the design,
development and delivery
of SW.
• Is part of Systems
Engineering.

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Question Answer
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.
What are the attributes of good software? Good software should deliver the required functionality and
performance to the user and should be maintainable,
dependable and usable.
What is software engineering? Software engineering is an engineering discipline that is
concerned with all aspects of software production.
What are the fundamental software
engineering activities?
Software specification, software development, software
validation and software evolution.
What is the difference between software
engineering and computer science?
Computer science focuses on theory and fundamentals;
software engineering is concerned with the practicalities of
developing and delivering useful software.
What is the difference between software
engineering and system engineering?
System engineering is concerned with all aspects of
computer-based systems development including hardware,
software and process engineering. Software engineering is
part of this more general process.
Frequently asked questions about software
engineering

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Frequently asked questions about software
engineering
Question Answer
What are the key challenges facing software
engineering?
Coping with increasing diversity, demands for reduced delivery
times and developing trustworthy software.
What are the costs of software engineering? Roughly 60% of software costs are development costs, 40% are
testing costs. For custom software, evolution costs often
exceed development costs.
What are the best software engineering
techniques and methods?
While all software projects have to be professionally managed
and developed, different techniques are appropriate for
different types of system. For example, games should always be
developed using a series of prototypes whereas safety critical
control systems require a complete and analyzable
specification to be developed. You can’t, therefore, say that
one method is better than another.
What differences has the web made to
software engineering?
The web has led to the availability of software services and the
possibility of developing highly distributed service-based
systems. Web-based systems development has led to
important advances in programming languages and software
reuse.

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What is a Software Process?
 Activities and results that produce a software product:
SW Process Activity What is going on there?
Specification
What does the customer need?
What are the constraints?
Development Design & programming.
Validation Checking whether it meets requirements.
Evolution Modifications (e.g. customer/market).

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What is a Software Process Model?
 Description of the software process that represents one view, such
as the activities, data or roles of people involved.
Examples of views Focus on…
Workflow
Activities = human actions.
What is input, output, and dependencies.
Dataflow
Activities = transformations of information.
How the input is transformed into output.
Role/Action
What is the role of people involved in each step of
the process?

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Software Process Models
Waterfall approach Iterative development
Component-Based
Software Engineering CBSE
assembled form existing
components

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The Cost of Software Engineering
 Depends on:
 The process used, and
 The type of software being developed.
 Each generic approach has a different profile of cost distribution.
 Roughly 60% of costs are development costs, 40% are testing costs.
 For custom software, evolution costs often exceed development
costs.

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Cost distribution
Custom software development (Bespoke)
Software Model
Cost units
Cost distribution
Software development activity
Waterfall Model
0 25 50 75 100
Specification Design Development Integration and testing
Iterative Development
0 25 50 75 100
Specification Iterative Development System testing
Component-based Software Engineering
0 25 50 75 100
Specification Development Integration and testing
Development and evolution costs for long-lifetime systems
0 100 200 300 400
System development System evolution

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Cost distribution
Generic software development
Product development costs
0 25 50 75 100
Specification Development System testing

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What is CASE?
 Computer Aided Software Engineering.
 Programs that support:
 Requirements analysis.
 System modeling.
 Debugging.
 Testing.

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Attributes of good software
 Functional attributes (performance; what the system does).
 Non-functional attributes (quality; how the system does it).
Product Characteristic Description
Maintainability Evolution qualities such as Testability, extensibility.
Dependability Reliability, security, safety.
Efficiency Response time, processing time, memory utilization.
Usability Easy to learn how to use the system by target users.
Efficient to use the system by users to accomplish a task.
Satisfying to use by intended users.

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Activity
 What are the key attributes for..
Interactive game Banking system
Cardiac monitor in an ICU
unit
Players, score, scenes,
theme.
Client accounts, stocks
bonds, money transfers.
heart rate, temperature,
blood pressure.

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Challenges facing software engineering
Challenge Why? Software needs to ..
Heterogeneity
Different computers, different
platforms, different support systems.
Cope with this variability.
Delivery
Businesses are more responsive
 supporting software needs to
evolve as rapidly.
Be delivered in shorter time
without compromising quality.
Trust
Software is a part of many aspects of
our lives (work, study, leisure).
Demonstrate that it can be
trusted by users.

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References
 PRESS&SUN-BULLETIN, The Binghamton Press Co., Binghamton, NY, October 1,1999.
 “Software Hell: Is there a way out?”, BUSINESS WEEK, December 6, 1999.
 IEEE Standards Collection: Software Engineering, IEEE standard 610.12-1990, IEEE 1993.
 Sommerville, Ian “Software Engineering”, 9th edition, Addison-Wesley.

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