SOFTWARE ENGINEERING
Unit - I
Software & Software Engineering
Introduction to Software Engineering
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Software engineering
• Software is more than just a program code. A program is an
executable code, which serves some computational purpose.
Software is considered to be collection of executable programming
code, associated libraries and documentations. Software, when
made for a specific requirement is called software product.
• Engineering on the other hand, is all about developing products,
using well-defined, scientific principles and methods.
• Software engineering is an engineering branch associated with
development of software product using well-defined scientific
principles, methods and procedures.
• The outcome of software engineering is an efficient and reliable
software product.
What is Software?
The product that software professionals build and then support
over the long term.
Software encompasses:
(1) instructions (computer programs) that when executed
provide desired features, function, and performance;
(2) data structures that enable the programs to adequately
store and manipulate information and
(3) documentation that describes the operation and use of the
programs.
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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 editing, graphics
programs, project management tools; CAD software;
software for specific markets such as appointments
systems for dentists.
• Customized products
– Software that is ordered by a specific customer to
meet their own needs.
– Examples – embedded control systems, air traffic
control software, traffic monitoring systems.
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Why Software is Important?
• The economies of ALL developed nations are
dependent on software.
• More and more systems are software controlled (
transportation, medical, telecommunications,
military, industrial, entertainment,)
• Software engineering is concerned with theories,
methods and tools for professional software
development.
• Expenditure on software represents a
significant fraction 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 Evolution
• The process of developing a software product using software engineering
principles and methods is referred to as software evolution.
• This includes the initial development of software and its maintenance and
updates, till desired software product is developed, which satisfies the
expected requirements.
• Evolution starts from the requirement gathering process. After which
developers create a prototype of the intended software and show it to the
users to get their feedback at the early stage of software product
development.
• The users suggest changes, on which several consecutive updates and
maintenance keep on changing too. This process changes to the original
software, till the desired software is accomplished.
• Even after the user has desired software in hand, the advancing
technology and the changing requirements force the software product to
change accordingly.
• Re-creating software from scratch and to go one-on-one with requirement
is not feasible. The only feasible and economical solution is to update the
existing software so that it matches the latest requirements.
Software Evolution Laws
Lehman has given laws for software evolution. He divided the software into
three different categories:
• S-type (static-type) - This is a software, which works strictly according to
defined specifications and solutions. The solution and the method to
achieve it, both are immediately understood before coding. The s-type
software is least subjected to changes hence this is the simplest of all. For
example, calculator program for mathematical computation.
• P-type (practical-type) - This is a software with a collection
of procedures. This is defined by exactly what procedures can do. In this
software, the specifications can be described but the solution is not
obvious instantly. For example, gaming software.
• E-type (embedded-type) - This software works closely as the requirement
of real-world environment. This software has a high degree of evolution as
there are various changes in laws, taxes etc. in the real world situations.
For example, Online trading software.
Characteristics of good software
• A software product can be judged by what it offers and how well it can be
used. This software must satisfy on the following grounds:
– Operational
– Transitional
– Maintenance
Operational
• This tells us how well software works in operations. It can be measured
on:
– Budget
– Usability
– Efficiency
– Correctness
– Functionality
– Dependability
– Security
– Safety
Characteristics of good software Cont..
Transitional
This aspect is important when the software is moved from one
platform to another:
– Portability
– Interoperability
– Reusability
– Adaptability
Maintenance
This aspect briefs about how well a software has the capabilities to
maintain itself in the ever-changing environment:
– Modularity
– Maintainability
– Flexibility
– Scalability
Features of Software
Characteristics of software :
• Software is developed or engineered, it is not manufactured in the
classical sense which has quality problem.
• Software doesn't "wear out.”
– but it deteriorates (due to change). Hardware has bathtub curve of failure
rate ( high failure rate in the beginning, then drop to steady state, then cumulative effects of dust,
vibration, abuse occurs).
• Although the industry is moving toward component-based
construction(e.g. standard screws and off-the-shelf integrated circuits),
most software continues to be custom-built.
– Modern reusable components encapsulate data and processing into software parts to
be reused by different programs. E.g. graphical user interface, window, pull-down menus
in library etc.
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Wear vs. Deterioration
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Software Components
• Resuabilty- high quality of software
components
• software components are built using a
programming languages.
– Lowest level- instruction set of the hardware
– Mid level- smarttalk
– Highest level- executable instruction are
automatically generated
• Fourth generation language is called non
procedural language.
Software Applications
• 1. System software: such as compilers, editors, file management
utilities
• 2. Application software: stand-alone programs for specific needs.
• 3. Engineering/scientific software: Characterized by “ number
crunching”algorithms. such as automotive stress analysis, molecular
biology, orbital dynamics etc.
• 4. Embedded software resides within a product or system. (key pad
control of a microwave oven, digital function of dashboard display in
a car)
• 5. Product-line software focus on a limited marketplace to address
mass consumer market. (word processing, graphics, database
management)
• 6. WebApps (Web applications) network centric software. As web 2.0
emerges, more sophisticated computing environments is supported
integrated with remote database and business applications.
• 7. AI software uses non-numerical algorithm to solve complex
problem. Robotics, expert system, pattern recognition game playing
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Risk Management
• Risk is an uncertain event that may have
positive or negative impact on project.
• Risk Management is the process of identifying
and migrating risk.
• Why is it important?
– Risk affect all aspect of your project , your budget
and your schedule.
Reactive and proactive Risk Strategies:
• Reactive:
– Don’t worry! I will think of something
– Fire fighting
• Proactive:
– Identity potential risks
– Assess the likelihood of their happening
– Estimate their impact
– Plan to manage these risks
• Avoid
• Reduce likelihood
• Prevent
• Contingency plan
Difference
Overview of Risk Management:
• Risk Index: Generally risks are categorized into two
factors namely impact of risk events and probability of
occurrence.
• Risk index is the multiplication of impact and
probability of occurrence.
• Risk index can be characterized as high, medium, or
low depending upon the product of impact and
occurrence.
• Risk index is very important and necessary for
prioritization of risk
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• Risk Assessment: Risk assessment is another
important case that integrates risk management
and risk analysis.
• There are many risk assessment methodologies
that focus on different types of risks.
• Risk assessment requires correct explanations of
the target system and all security features.
• It is important that a risk referent levels like
performance, cost, support and schedule must be
defined properly for risk assessment to be useful.
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Software Risk Management:
• Risk Analysis: There are quite different types of risk
analysis that can be used. Basically, risk analysis is used to
identify the high risk elements of a project in software
engineering.
• Also, it provides ways of detailing the impact of risk
mitigation strategies.
• Risk analysis has also been found to be most important in
the software design phase to evaluate criticality of the
system, where risks are analyzed and necessary counter
measures are introduced.
• The main purpose of risk analysis is to understand risks in
better ways and to verify and correct attributes.
• A successful risk analysis includes important elements like
problem definition, problem formulation, data collection
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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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FAQ about software engineering
Question Answer
Whatis software? Computer programs, data structures and associated
documentation. Software products may be developed for a
particular customer or may be developed for a general
market.
Whatare the attributes of good software? Good software should deliver the required functionality and
performance to the user and should be maintainable,
dependable and usable.
Whatis software engineering? Software engineering is an engineering discipline that is
concernedwith all aspects of software production.
What is the difference between software Computer science focuses on theory and fundamentals;
ngineering and computerscience? software engineering is concerned with the practicalities of
developingand delivering useful software.
What is the difference between software System engineering is concerned with all aspects of
ngineering and systemengineering? computer-based systems development including hardware,
software and process engineering. Software engineering is
partof this more generalprocess.
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Essential attributes of good software
Product Description
characteristic
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
expected requirement of a changing business environment.
Dependability Software dependability includes a range of characteristics including reliability,
and security 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.
Software myths
• Myth: means wrong belief or misinformation.
• Software myth: software myth are beliefs about
software and the process used to build it.
• Myths have no. of attributes that causes serious
problem on software.
• There are 3 types of myths:
– Management myths
– Customer Myths
– Software Engineers’ Myths or Developer myths
Management Myths [Pressman]
• We already have standards and procedures for building
software; isn’t that enough?
– How widely used is it?
– How relevant to the team?
– How useful to the project?
• If we’re behind schedule, we’ll just add more programmers to
catch up
– “Adding people to a late project makes it later”
– Interference
• They think they have latest computer.
• A good manager can manage any project.
Customer Myths [Pressman]
• s/w myths believed by customer who can internal
or external.
• Customer always think that s/w is development is
an easy process
• A general statement of work is sufficient to kick
off the project,
• Requirements can change, and that’s OK because
software is so flexible
– Most software project failures can be traced to
inadequacy of requirement specifications
Software Engineers’ Myths [Pressman]
• If I miss something now. I Fix it later
• Once the program is written, I’m done
– Between 60-80% of effort expended after delivery
• Until the program is written, quality is uncertain
– Formal design reviews
– Formal code reviews
– Test-first approaches
– Prototyping to verify design and structure
– Prototyping to validate requirements
• The only deliverable is the program itself
– Lots of documentation: installation guides, usage guides,
maintenance guides, API definitions and examples
Software Engineers’ Myths [Pressman]
• Documentation is Software-Engineering busy
work
– Focus is on quality, not quantity
– Documentation can be hard for engineers to
write, just as C++ may be difficult for poets.
– Conserve energy: documented code can serve as
a basis for useful documentation
The Essence of Practice
(Software Problems)
• How does the practice of software engineering
fit in the process activities mentioned above?
Namely, communication, planning, modeling,
construction and deployment.
• George Polya outlines the essence of problem
solving, suggests:
1. Understand the problem(communication and
analysis).
2. Plan a solution (modeling and software design).
3. Carry out the plan (code generation).
4. Examine the resultforaccuracy (testing and
quality assurance).
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Understand the Problem
• Who has a stake in the solution to the
problem? That is, who are the stakeholders?
• What are the unknowns? Whatdata,
functions, and features are required to
properly solve the problem?
• Can the problembe compartmentalized? Is it
possible to representsmallerproblems that
may be easierto understand?
• Can the problembe represented graphically?
Can an analysis model be created?
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Professional Visibility
1. Confidentiality: You should normally respect the confidentiality of
your employers or clients irrespective of whether a formal
confidentiality agreement has been signed.
2. Competence: You should not misrepresent your level of
competence. You should not be knowingly accept work that is
outside your computer.
3. Intellectual property rights: You should be aware of local laws
governing the use of intellectual property such as patents and
copyrights. You should be careful to ensure that the intellectual
property of employers and clients is protected.
4. Computer misuse. You should not use your technical skills to
misuse other people computers. Computer misuse ranges from
relatively trivial to extremely serious.
Software reuse
• In most engineering disciplines, systems are
designed by composing existing components
that have been used in other systems
• Software engineering has been more focused
on original development but it is now
recognised that to achieve better software,
more quickly and at lower cost, we need to
adopt a design process that is based on
systematic reuse
Benefits of reuse
• Increased reliability
– Components exercised in working systems
• Reduced process risk
– Less uncertainty in development costs
• Effective use of specialists
– Reuse components instead of people
• Standards compliance
– Embed standards in reusable components
• Accelerated development
– Avoid original development and hence speed-up
production
Process Visibility
1. Maintainability: Software should be written in such a way
that is may evolve to meet the changing needs of
customers. This is a critical visibility because software
change is an inevitable consequence of changing business
environment.
2. Dependability: Software dependability has a range of
characteristics, including reliability , security & safety.
Dependable software should not cause physical or
economic damage in the event of system failure.
3. Efficiency: Software should not make wasteful use of
system resources such as memory & processor cycles.
Efficiency therefore includes responsiveness, processing
time, memory utilization etc.
4. Usability: Software must be usable, without undue effort,
by the type of user for whom it is designed. This means that
it should have an appropriate user interface & adequate
documentation.
SDLC
