software engineering

Software Crisis
It was in late 1960’s
•Many software projects failed.
• Many software projects late, over budget, providing
unreliable software that is expensive to maintain.
• Many software projects produced software which did not
satisfy the requirements of the customer.
•Complexities of software projects increased as hardware
capability increased.
•Larger software system is more difficult and expensive to
maintain.
•Demand of new software increased faster than ability to
generate new software.
All the above attributes of what was called a ‘Software
Crisis’. So the term ‘Software Engineering’ first introduced
at a conference in late 1960’s to discuss the software crisis.

Why software engineering?
Once the need for software engineering was identified and
software engineering recognized as a discipline--
• The late 1970’s saw the widespread evolution of
software engineering principles.
• The 1980’s saw the automation of software engineering
and growth of CASE (Computer Aided Software
Engineering).
• The 1990’s have seen increased emphasis on the
‘management’ aspects of projects and the use of
standard quality and ‘process’ models like ISO 9001 and
the Software Engineering Institute’s Software Capability
Maturity Model (CMM).
These models help organizations put their
software development and management
processes in place

What is software Engineering?
• In1969 Fritz Bauer defined software eng. as, ‘the establishment and
use of sound engineering principles in order to obtain, economically,
software that is reliable and works efficiently on real machines’.
• According to Boehm, software engineering involves, ‘the practical
application of scientific knowledge to the design and construction of
computer programs and the associated documentation required
developing, operating and maintaining them’
• IEEE, in its standard 610.12-1990, defines software engineering as:
(i) The application of a systematic, disciplined, quantifiable approach to
the development, operation and maintenance of software; that is, the
application of engineering to software.
(ii) The study of approaches as in (i).
• By combining all the above definition we can define software
engineering as, ‘Software engineering is the technological and
managerial discipline concerned with systematic production and
maintenance of software products that are developed and modified on
time and within cost estimates.’

Goal of software engineering
The primary goals of software engineering
are:
• To improve the quality of the software products.
• To increase the productivity &
• To give job satisfaction to the software
engineers.

Foundation of Software engineering
Software engineering is a technological
discipline distinct from, but based on the
foundation of the following disciplines:
• Computer Science
• Management Science
• Economics
• System Engineering &
• Communication Skills

The relationship of software engineering with other disciplines
• Computer Science gives the scientific foundation to the software as
electrical engineering relies on physics.
• Management Science provides the foundation for software project
management. Since software engineering is labor intensive activity, it
requires both technical and managerial control.
• Economics provides the foundation for resource estimation and cost
control. Since, computing system must be developed and maintained
on time and within cost estimates; thus economics plays an important
role.
• System Engineering is the field concerned with studying complex
systems. Software is often a component of a much larger system. For
example, the software in a factory monitoring system or the flight
software on an airplane; is just the component of more complex
system. System engineering techniques can be applied to study of
such systems
• Good oral, written and interpersonal communication skills are
crucial for the software engineers, because software engineering
activities occur within an organizational context, and a high degree of
communication is required among customers, managers, software
engineers, hardware engineers and other technical workers.

Difference of s/w eng. with traditional engineering
• Software is intangible. It has no mass, no volume, no
color, no odor--- no physical properties. Source code is
merely a static image of computer program, and while
the effects produced by a program are often observable,
the program itself not.
• Software doesn’t degrade with time as hardware does.
Software failures are caused by design and
implementation error, not by degradation over time
• There is always an obscurity in the interface between
software modules. It is difficult to design a software
system so that all the control and interfaces among
modules are explicit, and so that the modules do not
interact to produce unexpected side effects when they
invoked one another.

…. difference
• In classical engineering disciplines, the engineer
is equipped with tools and the mathematical
maturity to specify the properties of the product
separately from those of design.
• The typical software engineering relies much
more on experience and judgment rather than
mathematical formula. While experience and
judgment are necessary, formal analysis are
also essential in the practice of engineering.

The role of software engineer
The evolution of software engineering field has defined the
role of the software engineer. A software engineer
should have the following qualities:
• Should be a good programmer, be well-versed in data
structures and algorithms, and be fluent in one or more
programming languages.
• Should be familiar with several design approaches, be
able to translate vague requirements and desires into
precise specifications and be able to converse with the
use of a system in terms of applications.
• Needs the ability to move among several levels of
abstraction at different stages of the project, from
specific application procedures and requirements, to
abstraction for software systems, to a specific design for
system and finally to the detailed coding level.

The characteristics of software engineer
• Should be able to build and use a model of the
application to guide choices of the many trade-
offs that he or she will face. The model is used
to answer questions about both the behavior of
the system and its performance.
• Needs communication skills and interpersonal
skills. He also needs the ability to schedule work
both of his own and that of others.

What is well engineered software?
If the software system does what the user wants,
and can be made to continue to do what the
user wants, it is well engineered.
• Any well engineered software system should have the
following attributes:
• Be easy to maintain
• Be reliable
• Be efficient
• Provides an appropriate user interface
The development of software must make trade-offs
between these attributes.

Distribution of software effort
The typical life-span for a typical software product is 1 to 3 years in
development and 5 to 15 years in use. The distribution of effort
between development and maintenance has been variously
reported depending on the type of software as 40/60, 30/70 and
10/90.
Maintenance:
• Corrective: Even with the best quality of software, it is likely that
customer will uncover defect in software. Corrective maintenance
changes the software to correct the defects.
• Adaptive: Over time, the original environment (CPU, OS, business
rules, external product character etc.) for which the software was
developed may change. Adaptive maintenance results in
modification to the software to accommodate the change to its
environment.
• Perfective: As the software is used, the customer / user will
recognize additional function that will provide benefit. Perfective
maintenance extends the software beyond its original functional
requirements.

The software product
The objective of software engineering is to produce software
products. Computer software is the product that software
engineers design and built. Software products are
software systems delivered to a customer with the
documentation which describes how to install and use the
system.
• Software products fall into two broad classes:
• Generic products: These are stand alone systems which
are produced by a software development
organizations/firms and sold on the open market to any
customer who is able to buy them.
• Customized products: These are systems which are
commissioned by a particular customer. The software is
developed specially for that customer by some developer.

…. Software products
• Until the 1980’s, the vast majority of software systems
which were sold that were customized and specially
designed systems which run on large computers. They
are expensive because all the development cost had to
be met by a single client.
• After the development of PCs, this situation has
completely changed. The PC market is totally dominated
by software products produces by companies such as
Microsoft. These account for the vast majority of
software sales. These are usually relatively cheap
because their development cost is spread across
hundred or thousands of different customers.

Difference between generic and customized software
• The generic software product specifications are
produced internally by the marketing department
of the product company. They reflect what they
think will sell. They are usually flexible and non-
prescriptive.
• For customized systems are often the basis for
the contract between customer and developer.
They are usually defined in detail and changes
have to be negotiated and carefully costed.

Software product attributes
The attributes of a software product are the
characteristics displayed by the product, once it
is installed and put in use. They are not the
services provided by the product.
Rather, they are concerned with the products
dynamic behavior and the use made of the
product.
Examples of these attributes are therefore,
efficiency, reliability, maintainability,
robustness, portability and so on.
The relative importance of these characteristics
obviously varies from system to system.

… product attributes
Product
characteristics
Description
Maintainability It should be possible to evolve software to meet the
changing needs of the customer.
Dependability Software dependability includes a range of characteristics
including reliability, security and safety. Dependable
software should not cause physical or economic damage in
the event of system failure.
Efficiency Software should not make wasteful use of system
resources such as memory and processor cycle
Usability Software should have an appropriate user interface and
documentation

..product attributes
Optimizing the above attribute is difficult as some
are exclusive.
For example, providing a better user interface may
reduce system efficiency.
The relationship between cost and improvement in
each of the attribute is not linear one.
Small improvement in any of these attributes may
be devoted to optimizing particular attribute.

The software process
• A structured set of activities required to develop a
software system.
• Many different software processes but all involve:
– Specification – defining what the system should do;
– Design and implementation – defining the organization of the
system and implementing the system;
– Validation – checking that it does what the customer wants;
– Evolution – changing the system in response to changing
customer needs.
• A software process model is an abstract representation of a
process. It presents a description of a process from some
particular perspective.

Software process descriptions
• When we describe and discuss processes, we
usually talk about the activities in these processes
such as specifying a data model, designing a user
interface, etc. and the ordering of these activities.
• Process descriptions may also include:
– Products, which are the outcomes of a process
activity;
– Roles, which reflect the responsibilities of the people
involved in the process;
– Pre- and post-conditions, which are statements that
are true before and after a process activity has been
enacted or a product produced.

Plan-driven and agile processes
• Plan-driven processes are processes where
all of the process activities are planned in
advance and progress is measured against
this plan.
• In agile processes, planning is incremental
and it is easier to change the process to
reflect changing customer requirements.
• In practice, most practical processes include
elements of both plan-driven and agile
approaches.

Software Development Life cycle (SDLC)
• A life cycle model prescribes the different activities that
need to be carried out to develop a software product and
sequencing of these activities.
• Also referred to as Systems Development Life cycle.
• Every software product starts with a request for the
product by the customer.- Production conception.
• The software life cycle can be considered as the
business process for software development and
therefore is often referred to as a Software process.
(SLCM).
• Process models – More detailed and precise life cycle
activities.

• Different stages in a life cycle model: After Product
conception. The stages are: (Life cycle phase)
 Feasibility study stage
 Requirements analysis and specification.
 Design
 Coding
 Testing and
 Maintenance.
• A SDLC is a series of identifiable stages that a software
product undergoes during its lifetime.
• A SDLC is a descriptive and diagrammatic
representation of the software life cycle.
• A life cycle model maps the different activities performed
on a software product from its beginning to retirement
into a set of life cycle phases.

• Why use a life cycle model?
• Encourages development of software in a systematic
and disciplined manner
• S.D organisations have realized that adherence to a
suitable well-defined life cycle model helps to produce
good quality products and that too without time and cost
overruns.
• Why document a life cycle model?
• A documented life cycle model, besides preventing
misinterpretations that occur when the life cycle model is
no adequately documented, also helps to identify
inconsistencies, redundancies, and omissions in the
development process

1. The classical waterfall model
Basic life cycle model- Theoretical way of
developing software.

Classical Waterfall model
phases
• There are separate identified phases in the
waterfall model:
– Requirements analysis and definition
– System and software design
– Implementation and unit testing
– Integration and system testing
– Operation and maintenance
• The main drawback of the waterfall model is the
difficulty of accommodating change after the
process is underway. In principle, a phase has to
be complete before moving onto the next phase.

Classical Waterfall model
problems
• Inflexible partitioning of the project into distinct stages
makes it difficult to respond to changing customer
requirements.
– Therefore, this model is only appropriate when the
requirements are well-understood and changes will be
fairly limited during the design process.
– Few business systems have stable requirements.
• The waterfall model is mostly used for large systems
engineering projects where a system is developed at
several sites.
– In those circumstances, the plan-driven nature of the
waterfall model helps coordinate the work.

2. Structured Evolutionary Prototyping
Model
• Developers build a prototype during the
requirements phase.
• Prototype is evaluated by end users.
• Users give corrective feedback.
• Developers further refine the prototype.
• When the user is satisfied, the prototype
code is brought up to the standards
needed for a final product.

Structured Evolutionary Prototyping Steps
• A preliminary project plan is developed.
• An partial high-level paper model is created.
• The model is source for a partial requirements
specification.
• A prototype is built with basic and critical attributes
• The designer builds
– the database
– user interface
– algorithmic functions
• The designer demonstrates the prototype, the user
evaluates for problems and suggests improvements.
• This loop continues until the user is satisfied.

Structured Evolutionary Prototyping Strengths
• Customers can “see” the system requirements
as they are being gathered.
• Developers learn from customers.
• A more accurate end product.
• Unexpected requirements accommodated.
• Allows for flexible design and development.
• Steady, visible signs of progress produced.
• Interaction with the prototype stimulates
awareness of additional needed functionality.

Structured Evolutionary Prototyping Weaknesses
• Tendency to abandon structured program
development for “code-and-fix” development.
• Bad reputation for “quick-and-dirty” methods
• Overall maintainability may be overlooked.
• The customer may want the prototype delivered.
• Process may continue forever (scope creep).

When to use
Structured Evolutionary Prototyping
• Requirements are unstable or have to be
clarified.
• As the requirements clarification stage of a
waterfall model.
• Develop user interfaces.
• Short-lived demonstrations.
• New, original development.
• With the analysis and design portions of object-
oriented development.

35
 Code-and-Fix Life-Cycle Model
The easiest way to develop software
The most expensive way for maintenance
(i.e., maintenance nightmare)
• No design
• No specifications

36
Code-and-Fix Life-Cycle Model (Cont.)
• The product is implemented without
requirements or specifications, or any
attempt at design.
• The developers simply throw code together
and rework it as many times as necessary to
satisfy the client.
• It is used in small project and is totally
unsatisfactory for products of any
reasonable size.

3. Spiral Model
Since end-user requirements are hard to
obtain/define, it is natural to develop software
in an experimental way: e.g.
1. Build some software
2. See if it meets customer requirements
3. If no go to 1 else stop.

This loop approach gives rise to structured
iterative lifecycle models.
In 1988 Boehm developed the spiral model as
an iterative model which includes risk analysis
and risk management.
Key idea: on each iteration identify and solve
the sub-problems with the highest risk.

Spiral model
• Spiral with many loops.
• Each loop of the spiral is called the phase of a
software process.
• Over each loop, one or more features of the
product are elaborated and analysed and risks
at that point of time are identified and resolved
through prototyping. Based on this, the identified
features are implemented.

4 Quadrants of spiral model
• 1st Quadrant:
– The objectives are investigated, elaborated and analysed.
– Risks are also identified.
– Alternative Solutions are proposed
• 2nd Quadrant:
• Alternative solutions are evaluated to select best.
• 3rd Quadrant:
• Developing and verifying the next level of the product
• 4th Quadrant:
• Reviewing the results of the stages traversed so far with the
customer.
• Planning the next iteration around the spiral.
:

Spiral Model
Determine objectiv es,
alternatives, & constr aints
Evaluate alter natives,
identify& resolv e r isks
Develop & ver ify
next level productPlan next phase
Requirements
Development
Integration
plan
plan
plan
Requirements
Design
validation
validation
Software
System
Product
Risk
analysis
Risk
analysis
Prototype1
Prototype2
Prototype3
Risk
analysis
Concept of
operation
Requirements
Design
Code
Unit Test
Integration &Test
Acceptance
Detailed
Design
P1
P2
Test
Project P1
Project P2

Cycle 1, Quadrant IV: Determine Objectives, Alternatives
and Constraints
Project
Start

Cycle 1, Quadrant I: Evaluate Alternatives, Identify,
resolve risks
Build
Prototype

Cycle 1, Quadrant II: Develop & Verify Product
Concept of Operation
Activity

Cycle 1, Quadrant III: Prepare for Next Activity
Requirements and
Life cycle Planning

Cycle 2, Quadrant IV: Software Requirements Activity
Start
of Round 2

Spiral Model Strengths
• Provides early indication of insurmountable
risks, without much cost.
• Users see the system early because of rapid
prototyping tools.
• Critical high-risk functions are developed first.
• The design does not have to be perfect.
• Users can be closely tied to all lifecycle steps.
• Early and frequent feedback from users.
• Cumulative costs assessed frequently.

Spiral Model Weaknesses
• Time spent for evaluating risks too large for small or low-
risk projects.
• Time spent planning, resetting objectives, doing risk
analysis and prototyping may be excessive.
• The model is complex.
• Risk assessment expertise is required.
• Spiral may continue indefinitely.
• Developers must be reassigned during non-development
phase activities.
• May be hard to define objective, verifiable milestones
that indicate readiness to proceed through the next
iteration.

4. Evolutionary Model
• Successive versions model.
• Incremental model.
• A simple working system is built, which
subsequently undergoes many functionality
improvements and additions until the desired
system is realized .
• Also sometimes referred to as design a little,
build a little, test a little, deploy a little model.
• That is once the requirements have been
specified, the design, build, test and deployment
activities are interleaved.

• The software requirements is first broken down into
several modules(functional units) that can be
incrementally constructed and delivered.
• The development team first develop the core modules of
the system.
• The Core modules are those that don’t need services
from the other modules.
• The initial product Skelton is refined into increasing
levels of capability by adding new functionalities in the
successive versions.

Iterative Model
• An iterative lifecycle model does not attempt to start with
a full specification of requirements.
• Instead, development begins by specifying and
implementing just part of the software, which can then be
reviewed in order to identify further requirements.
• This process is then repeated, producing a new
version of the software for each cycle of the model.
• Consider an iterative lifecycle model which consists of
repeating the following four phases in sequence:

• A Requirements phase, in which the requirements for
the software are gathered and analysed. Iteration should
eventually result in a requirements phase that produces
a complete and final specification of requirements.
• A Design phase, in which a software solution to meet
the requirements is designed. This may be a new
design, or an extension of an earlier design.
• An Implementation and Test phase, when the software
is coded, integrated and tested.
• A Review phase, in which the software is evaluated, the
current requirements are reviewed, and changes and
additions to requirements proposed.

Iterative Waterfall model
• The iterative waterfall model is classical waterfall model
with necessary changes so that it becomes applicable to
practical software development projects.
• The main change to the classical waterfall model is in
the form of providing feedback paths from every phase
to its preceding phase.
• The feedback paths allows for correction of errors
committed during a phase, as and when these are
detected in a later phase.
• The principle of detecting errors as close to their points
of introduction as possible is known as phase
containment of errors.

Comparison of Life-Cycle Models
• Different life-cycle models have been
presented
– Each with its own strengths and weaknesses
• Criteria for deciding on a model include:
– The organization
– Its management
– The skills of the employees
– The nature of the product
• Best suggestion
– “Mix-and-match” life-cycle model

software engineering

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