Chapter 1- Introduction To SE
Lecture 1
Software engineering
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.
Software products
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.
Customized 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.
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Product specification
Generic products
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 products
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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Frequently asked questions about software
engineering
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 Software specification, software development, software
engineering activities? validation and software evolution.
What is the difference between software Computer science focuses on theory and fundamentals;
engineering and computer science? software engineering is concerned with the practicalities
of developing and delivering useful software.
What is the difference between software System engineering is concerned with all aspects of
engineering and system engineering? computer-based systems development including
hardware, software and process engineering. Software
engineering is part of this more general process.
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Frequently asked questions about software
engineering
Question Answer
What are the key challenges facing Coping with increasing diversity, demands for reduced
software engineering? delivery times and developing trustworthy software.
What are the costs of software Roughly 60% of software costs are development costs,
engineering? 40% are testing costs. For custom software, evolution
costs often exceed development costs.
What are the best software engineering While all software projects have to be professionally
techniques and methods? 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 The web has led to the availability of software services
software engineering? 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 Software Engineering
Relationship between computer science and software
engineering
Essential attributes of good software
Product characteristic Description
Maintainability Software should be written in such a way so that it can evolve to
meet the changing needs of customers. This is a critical attribute
because software change is an inevitable requirement of a
changing business environment.
Dependability and Software dependability includes a range of characteristics
security including reliability, security and safety. Dependable software
should not cause physical or economic damage in the event of
system failure. Malicious users should not be able to access or
damage the system.
Efficiency Software should not make wasteful use of system resources such
as memory and processor cycles. Efficiency therefore includes
responsiveness, processing time, memory utilisation, etc.
Acceptability Software must be acceptable to the type of users for which it is
designed. This means that it must be understandable, usable and
compatible with other systems that they use.
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What Is Good Software?
McCall’s quality model
Software engineering
Software engineering is an engineering discipline that is
concerned with all aspects of software production from
the early stages of system specification through to
maintaining the system after it has gone into use.
Engineering discipline
Using appropriate theories and methods to solve problems
bearing in mind organizational and financial constraints.
All aspects of software production
Not just technical process of development. Also project
management and the development of tools, methods etc. to
support software production.
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What is Software Engineering
Method: refers to a formal procedure; a formal “recipe”
for accomplishing a goal that is typically independent of
the tools used
Tool: an instrument or automated system for
accomplishing something in a better way
Procedure: a combination of tools and techniques to
produce a product
Paradigm: philosophy or approach for building a product
(e.g., OO vs structured approaches)
Who Does Software Engineering?
Customer: the company, organization, or person who
pays for the software system
Developer: the company, organization, or person who is
building the software system
User: the person or people who will actually use the
system
Importance of software engineering
More and more, individuals and society rely on advanced
software systems. We need to be able to produce
reliable and trustworthy systems economically and
quickly.
It is usually cheaper, in the long run, to use software
engineering methods and techniques for software
systems rather than just write the programs as if it was a
personal programming project. For most types of
system, the majority of costs are the costs of changing
the software after it has gone into use.
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Software process activities
Software specification, where customers and engineers
define the software that is to be produced and the
constraints on its operation.
Software development, where the software is designed
and programmed.
Software validation, where the software is checked to
ensure that it is what the customer requires.
Software evolution, where the software is modified to
reflect changing customer and market requirements.
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1.6 Engineering Approach
Requirement analysis and definition
System design
Program design
Writing the programs
Unit testing
Integration testing
System testing
System delivery
Maintenance
Members of the Development Team
Requirement analysts: work with the customers to
identify and document the requirements
Designers: generate a system-level description of what
the system us supposed to do
Programmers: write lines of code to implement the
design
Testers: catch faults
Trainers: show users how to use the system
Maintenance team: fix faults that show up later
Librarians: prepare and store documents such as
software requirements
Configuration management team: maintain
correspondence among various artifacts
Members of the Development Team
Typical roles played by the members of a development
team
Application types
Stand-alone applications
These are application systems that run on a local computer,
such as a PC. They include all necessary functionality and do
not need to be connected to a network.
Interactive transaction-based applications
Applications that execute on a remote computer and are
accessed by users from their own PCs or terminals. These
include web applications such as e-commerce applications.
Embedded control systems
These are software control systems that control and manage
hardware devices. Numerically, there are probably more
embedded systems than any other type of system.
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Application types
Batch processing systems
These are business systems that are designed to process data
in large batches. They process large numbers of individual
inputs to create corresponding outputs.
Entertainment systems
These are systems that are primarily for personal use and which
are intended to entertain the user.
Systems for modeling and simulation
These are systems that are developed by scientists and
engineers to model physical processes or situations, which
include many, separate, interacting objects.
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Application types
Data collection systems
These are systems that collect data from their environment using
a set of sensors and send that data to other systems for
processing.
Systems of systems
These are systems that are composed of a number of other
software systems.
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How Has SE Changed?
Wasserman's Seven Key Factors (continued)
The key factors that have changed the software
development
Software engineering ethics
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
but involves following a set of principles that are morally
correct.
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).
Ethical dilemmas
Disagreement in principle with the policies of senior
management.
Your employer acts in an unethical way and releases a
safety-critical system without finishing the testing of the
system.
Participation in the development of military weapons
systems or nuclear systems.
Case studies
A personal insulin pump
An embedded system in an insulin pump used by diabetics to
maintain blood glucose control.
A mental health case patient management system
A system used to maintain records of people receiving care for
mental health problems.
A wilderness weather station
A data collection system that collects data about weather
conditions in remote areas.
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Insulin pump control system
Collects data from a blood sugar sensor and calculates
the amount of insulin required to be injected.
Calculation based on the rate of change of blood sugar
levels.
Sends signals to a micro-pump to deliver the correct
dose of insulin.
Safety-critical system as low blood sugars can lead to
brain malfunctioning, coma and death; high-blood sugar
levels have long-term consequences such as eye and
kidney damage.
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Real Time Example
Ariane-5 rocket, from the European Space Agency
June 4, 1996: functioned well for 40 seconds, then
veered off course and was destroyed.
Explosion was because of software problems (
The cause of the failure was a software error in the inertial reference
system. Specifically a 64 bit floating point number relating to the horizontal
velocity of the rocket with respect to the platform was converted to a 16 bit
signed integer)
Contained four satellites: cost was $500 million
Reused code from Ariane-4 rocket
Insulin pump hardware architecture
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Activity model of the insulin pump
Chapter 1 Introduction 31
Essential high-level requirements
The system shall be available to deliver insulin when
required.
The system shall perform reliably and deliver the correct
amount of insulin to counteract the current level of blood
sugar.
The system must therefore be designed and
implemented to ensure that the system always meets
these requirements.
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A patient information system for mental health
care
A patient information system to support mental health
care is a medical information system that maintains
information about patients suffering from mental health
problems and the treatments that they have received.
Most mental health patients do not require dedicated
hospital treatment but need to attend specialist clinics
regularly where they can meet a doctor who has detailed
knowledge of their problems.
To make it easier for patients to attend, these clinics are
not just run in hospitals. They may also be held in local
medical practices or community centres.
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MHC-PMS
The MHC-PMS (Mental Health Care-Patient
Management System) is an information system that is
intended for use in clinics.
It makes use of a centralized database of patient
information but has also been designed to run on a PC,
so that it may be accessed and used from sites that do
not have secure network connectivity.
When the local systems have secure network access,
they use patient information in the database but they can
download and use local copies of patient records when
they are disconnected.
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MHC-PMS goals
To generate management information that allows health
service managers to assess performance against local
and government targets.
To provide medical staff with timely information to
support the treatment of patients.
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The organization of the MHC-PMS
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MHC-PMS key features
Individual care management
Clinicians can create records for patients, edit the information in
the system, view patient history, etc. The system supports data
summaries so that doctors can quickly learn about the key
problems and treatments that have been prescribed.
Patient monitoring
The system monitors the records of patients that are involved in
treatment and issues warnings if possible problems are detected.
Administrative reporting
The system generates monthly management reports showing the
number of patients treated at each clinic, the number of patients
who have entered and left the care system, number of patients
sectioned, the drugs prescribed and their costs, etc.
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MHC-PMS concerns
Privacy
It is essential that patient information is confidential and is never
disclosed to anyone apart from authorised medical staff and the
patient themselves.
Safety
Some mental illnesses cause patients to become suicidal or a
danger to other people. Wherever possible, the system should
warn medical staff about potentially suicidal or dangerous
patients.
The system must be available when needed otherwise safety
may be compromised and it may be impossible to prescribe the
correct medication to patients.
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Wilderness weather station
The government of a country with large areas of
wilderness decides to deploy several hundred weather
stations in remote areas.
Weather stations collect data from a set of instruments
that measure temperature and pressure, sunshine,
rainfall, wind speed and wind direction.
The weather station includes a number of instruments that
measure weather parameters such as the wind speed and
direction, the ground and air temperatures, the barometric
pressure and the rainfall over a 24-hour period. Each of these
instruments is controlled by a software system that takes
parameter readings periodically and manages the data collected
from the instruments.
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The weather station’s environment
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Weather information system
The weather station system
This is responsible for collecting weather data, carrying out some
initial data processing and transmitting it to the data management
system.
The data management and archiving system
This system collects the data from all of the wilderness weather
stations, carries out data processing and analysis and archives the
data.
The station maintenance system
This system can communicate by satellite with all wilderness
weather stations to monitor the health of these systems and provide
reports of problems.
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Additional software functionality
Monitor the instruments, power and communication
hardware and report faults to the management system.
Manage the system power, ensuring that batteries are
charged whenever the environmental conditions permit
but also that generators are shut down in potentially
damaging weather conditions, such as high wind.
Support dynamic reconfiguration where parts of the
software are replaced with new versions and where
backup instruments are switched into the system in the
event of system failure.
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