Introduction to software_engineeringDocument Transcript
Software is (1) instructions (computer programs) that when executed provide desired features,
function, and performance; (2) data structures that enable the programs to adequately manipulate
information; and (3) documents that describe the operation and use of the programs.
1. Software is developed or engineered; it is not manufactured in the classical sense.
2. Software does not wear out
Failure curve for hardware
Increased failure rate due to side effects
Change Actual Curve
Failure curve for software
3. Most software continue to be custom built
Types of Software
1. System Software is a collection of programs written to service other programs. E.g.
compilers, editors and file management utilities.
2. Application Software consists of standalone programs that solve a specific business need
3. Engineering/ Scientific software
4. Embedded software resides within a product or system and is used to implement and
control features and functions for the end user and for the system itself
5. Product-line software are designed to provide a specific capability for use by many different
customers, product line software can focus o a limited and esoteric marketplace.
6. Web-applications can be a little more than a set of linked hypertext files that present
information using text and limited graphics.
7. Artificial intelligence software makes use of non-numerical algorithms to solve complex
problems that are not amenable to computation or straightforward analysis.
8. Legacy software
Nature of Software
1. Software is flexible, or open for changes
2. Software is developed for long term usage
3. Software is complex
4. Software involves communication with the machine
Software Engineering is the establishment and use of sound engineering principles in order to
obtain economically software that is reliable and works efficiently on real machines. It is a
The engineering approach must rest on an organizational commitment to quality. The
foundation of software engineering is the process layer, which defines a framework for effective
delivery of software engineering technology. Methods provide the technical how-to’s for
building software. Software engineering tools provide automated or semi-automated support
for the process and methods.
Software engineering is about solving problems. It can be broken into analyzing (problem) and
Computer Science Customer
Theories Computer Problem
Tools and techniques to
Evolution of Software Engineering
h Unorganized use of past
n experience Systematic use of past
o experience . Formulation of
l Craft scientific basis
Esoteric use of past
Emergence of Software Engineering
Object Oriented Design
Data Flow Oriented Design
Data Structure Oriented Design
Control Flow Oriented Design
Software Engineering Challenges
1. Scale: Rules for small scale do not apply on large scale
2. Quality and productivity: These are the terms that are vaguely defined. Productivity directly
depends upon the people in development. Quality includes N number of parameters.
3. Consistently and repeatability: The methods for software development should be repeatable
across projects leading to consistency in the quality of software produced.
4. Change: Software should accommodate and embrace change. Since as businesses change, they
require that the software supporting it should change.
5. Heterogeneity: Developing techniques for building software that can cope with heterogeneous
platforms and execution environments;
6. Delivery: Developing techniques that lead to faster delivery of software;
7. Trust: Developing techniques that demonstrate that software can be trusted by its users.
What is good software?
Good software includes a few qualities that can be grouped into the following categories.
1. Quality of product
2. Quality of the process
3. Quality in the context of business environment (ROI or Return On Investment)
Essential qualities of a good software include correctness, reliability, robustness, performance, usability,
verifiability, maintainability, repair ability, evolve ability, portability, understandability, interoperability,
productivity, timeliness, visibility and reusability, to name a few.
Measurable Characteristics for Software
1. Functionality: The capability to provide functions which meet stated and implied needs
(suitability) when the software is used, accuracy and security.
2. Reliability: The capability to maintain a specified level of performance
3. Usability: The capability to be understood (understandability), learned (learn ability) and used
4. Efficiency: The capability to provide appropriate performance relative to the amount of
5. Maintainability: The capability to be modified for purposes of making corrections (changeability,
testability), improvements, or adaptation (stability)
6. Portability: The capability to be adopted (adaptability) for different specified environments
(install ability) without applying actions or means other than those provided for this purpose in
There is a rise in software prices as compared to hardware prices. Also the software is difficult to alter,
debug and enhance. This is because of lack of adequate training in software engineering. Large software
has failed and has caused huge losses and was called software runways.
Reasons for software Crisis
1. Lack of communication between developers and users
2. Increase in size of software
3. Increase in cost of developing software
4. Increased complexity of problems
5. Lack of understanding of problem and environment
6. High optimistic estimates
7. Difficult estimation of time
8. Quality parameters not standardized
9. Maintenance problems with code
Software projects utilize a process to organize the execution of tasks to achieve the goals on the
cost, schedule and quality fronts.
A process model specifies a general process, usually as a set of stages in which a project should
be divided; the order in which the stages should be executed and any other constraints and conditions
on the execution of stages. A project’s process may utilize some process model.
The process that deals with the technical and management issues of software development is
called a software process.
The desired Characteristics of a Software Process are:
1. Predictability: It determines how accurately the outcome of following the process can be
predicted. The fundamental basis for quality prediction is that quality of the product is
determined largely by the process followed for developing it. Effective management of
quality control activities depends upon the predictability of the process.
2. Support Testability and Maintainability: One of the most important objectives of software
development should be to reduce the maintenance effort. The process used, should ensure
that maintainability. Both testing and maintenance depend heavily on the quality of design
and code, and these costs can be considerably reduced if the software is designed and
coded to make testing and maintenance easier.
3. Support change: Software changes are driven by business need, people’s mindset etc. Thus
change is prevalent, and a process that can handle change easily is desirable.
4. Early Defect removal: We should attempt to detect errors that occur in a phase, during that
phase itself, to reduce effort and cost or removing them. Error detection and correction
should be a continuous process.
5. Process improvement and feedback: To satisfy the objectives of quality improvement are
cost reduction, the software process must be improved.
In engineering disciplines, products are always constructed from parts or components. Similarly,
software engineering is component based. Until the early 1990’s, components were routines and
libraries. With advances in programming languages, other mechanisms like generic constructs in
languages as ADA and C++ and objects and frameworks in object oriented language also came.
A framework is a collection of related classes that are designed to be used together in
developing applications in a certain domain. Standard template library in C++ is fine grained, code level
component. While swing and Java Beans that provide classes and objects for visual approach to
software development are medium grained components. Large grained components such as database
management systems act as components in the architecture.
A component is a software element that confirms to a component model and can be
independently deployed and composed without modification, according to a composition standard.
Components are standardized, Independent, comparable, deployable and documented.
• Software costs often dominate computer system costs. The costs of software on a PC
are often greater than the hardware costs.
• 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.
• Cost of the software includes the cost of developers, the hardware resources used for
development and the cost of infrastructure required for such development.
Thus following points become important considerations for software engineering
• Attempt to estimate cost/effort
• Define plan
• Prepare schedules
• Involve user
• Identify stages
• Define milestones
• Define deliverables
• Quality assurance steps (testing)
Job of a Software developer
Every developer has to become accurate in three forms of communication
1. Communication with the user. The user is not always well informed. There is also a gap between
the user and the developer in terms of field of work. Thus getting the right information from the
user is very critical task for the developer
2. Communication with the technical specialists. It is difficult to understand the jargon of a
technical specialist and thus the developer has to convey the information correctly to him, in the
way he can understand.
3. Communication with the management. The management personnel have business knowledge
and understanding of the application of the software. But they are not well versed with the
technicalities involved. Judging correctly the importance of the requirement stated by the
management is an important task for the developer.
Software Engineering Code of Ethics
The preamble of the code states that
Computers have a central and growing role in commerce, industry, government, medicine, education,
entertainment, and society at large. Software engineers are those who contribute by direct participation
or by teaching, to the analysis, specification, design, development, certification, maintenance, and
testing of software systems. Because of their roles in developing software systems, software engineers
have significant opportunities to do good or cause harm. To ensure, as much as possible, that their
efforts will be used for good, software engineers must commit themselves to making software
engineering a beneficial and respected profession.
Software engineers shall commit themselves to making the analysis, specification, design, development,
testing and maintenance of software a beneficial and respected profession. In accordance with their
commitment to the health, safety and welfare of the public, software engineers shall adhere to the
following Eight Principles:
The eight principles are as follows:
1. Public: Software engineers shall act consistently with the public interest.
2. Client and Employer: Software engineers shall act in a manner that is in the best interests of their
client and employer, consistent with the public interest.
3. Product: Software engineers shall ensure that their products and related modifications meet the
highest professional standards possible.
4. Judgment: Software engineers shall maintain integrity and independence in their professional
5. Management: Software engineering managers and leaders shall subscribe to and promote an ethical
approach to the management of software development and maintenance.
6. Profession: Software engineers shall advance the integrity and reputation of the profession,
consistent with the public interest.
7. Colleagues: Software engineers shall be fair to, and supportive of, their colleagues.
8. Self: Software engineers shall participate in lifelong learning regarding the practice of their profession
and shall promote an ethical approach to the practice of the profession.
Software Metrics are quantifiable measures that could be used in Software engineering to calculate
1. The complexity of the software which in turn helps in other ways
2. Cost estimation based on the calculations
3. Allocation of appropriate resources as per the complexity of project
4. Correct scheduling of tasks as per the complexity
5. Evaluation of Quality and reliability of system
6. Improving quality of the system
7. Help in management of risks
They help in the overall development of the system in a more organized and methodical way. Metrics
are important to manage the project successfully. There are three types of software metrics:
1. Product metrics measure the software
2. Process metrics quantify characteristics of the process used
3. Project metrics quantifies the whole project such as amount of project completion
There are different scales for measuring metrics such as:
1. Nominal which categorizes in groups and is thus a qualitative and not a quantitative measure
2. Ordered scale and tell the sequence
3. Interval can be incremental or regression analysis
4. Ratio can tell exact quantitative differences which can be ordered and grouped
Examples: Measures are of different types
1. Size metrics
2. Function points
a. ‘O’ notation
b. Algorithm/ logic
a. Cyclomatic complexity
5. Process measurement
a. Time taken for process activities to be
completed E.g. Calendar time or effort to complete an activity or process.
b. Resources required for processes or activities E.g. Total effort in person-days.
c. Number of occurrences of a particular event E.g. Number of defects discovered.
Issue Software Engineering Engineering
Based on computer science,
Based on science, mathematics, and empirical
Foundations information science, and discrete
Compilers and computers are
cheap, so software engineering In some projects, construction and
and consulting are often more manufacturing costs can be high, so engineering
Cost than half of the cost of a project. may only be 15% of the cost of a project. Major
Minor software engineering engineering cost overruns may not affect the
cost-overruns can adversely total project cost.
affect the total project cost.
Replication (copying CDs or Radically new or one-of-a-kind systems can
downloading files) is trivial. require significant development effort to create a
Most development effort goes new design or change an existing design. Other
into building new (unproven) or kinds of systems may require less development
changing old designs and adding effort, but more attention to issues such as
Engineers generally try to apply known and
Software engineers often apply
tested principles, and limit the use of untested
Innovation new and untested elements in
innovations to only those necessary to create a
product that meets its requirements.
Software engineers emphasize
Some engineers solve long-ranged problems
Duration projects that will live for years
(bridges and dams) that endure for centuries.
Engineers in some disciplines, such as civil
Management Few software engineers manage
engineering, manage construction,
manufacturing, or maintenance crews.
Blame Software engineers must blame Engineers in some fields can often blame
construction, manufacturing, or maintenance
themselves for project problems.
crews for project problems.
611,900 software engineers 1,157,020 total non-software engineers
Software engineering is about 50 Engineering as a whole is thousands of years
years old. old.
Software engineers are typically In many jurisdictions it is illegal to call yourself
Title self-appointed. A computer an engineer without specific formal education
Regulations science degree is common but and/or accreditation by governmental or
not at all a formal requirement.[ engineering association bodies.
Some engineering disciplines are based on a
Methods for formally verifying closed system theory and can in theory prove
correctness are developed in formal correctness of a design. In practice, a lack
Analysis computer science, but they are of computing power or input data can make such
Methodology rarely used by software proofs of correctness intractable, leading many
engineers. The issue remains engineers to use a pragmatic mix of analytical
controversial. approximations and empirical test data to ensure
that a product will meet its requirements.
Engineers have nominally refined synthesis
techniques over the ages to provide exactly this.
However, this has not prevented some notable
SE struggles to synthesize (build engineering failures, such as the collapse of
to order) a result according to Galloping Gertie (the original Tacoma Narrows
requirements. Bridge), the sinking of the Titanic, and the
Pentium FDIV bug. In addition, new
technologies inevitably result in new challenges
that cannot be met using existing techniques.
Research Software engineering is often Traditional engineering nominally separates
during busy with researching the these activities. A project is supposed to apply
Projects unknown (e.g. to derive an research results in known or new clever ways to
algorithm) right in the middle of build the desired result. However, ground-
a project. breaking engineering projects such as Project
Apollo often include a lot of research into the
Some engineering disciplines have thousands of
years of best practice experience handed over
Software engineering has just
from generation to generation via a field's
Codified Best recently started to codify and
literature, standards, rules and regulations.
Practice teach best practice in the form of
Newer disciplines such as electronic engineering
and computer engineering have codified their
own best practices as they have developed.
Software Development Life Cycle
The software Life Cycle encompasses all activities required to define, develop, test deliver, operate and
maintain a software product. Planning the software development process involves several important
considerations. The first is to define a product life cycle model. The SDLC activities are:
1. Feasibility: Determine if the software has significant contribution to business. It includes market
analysis if similar software is in demand in market. Software is evaluated on the basis of cost,
schedule and quality.
2. Requirements are determined such as functional properties desired by users, system
requirements such as availability, performance and safety, establishing set of objectives the
system should meet, characteristics that the system should not exhibit. Such requirements are
obtained by various methods such as interviews, communication with stake holders, scenario
discussions, use cases and ethnography. These requirements are then verified and tested.
3. Project Planning, cost analysis, scheduling, quality assurance plans are made
4. Designing i.e. Architectural design, Interface design and detailed design
5. Implementation includes writing the code for the project as per the designs and plan
6. Testing ensures accuracy and reliability
7. Delivery, Installation, Training, Help Desk
8. Maintenance, software configuration management.
Another view of SDLC emphasizes the milestone, documents and reviews throughout product
development. It is difficult for project managers to assess progress or anticipate problems. Establishing
milestones, improves product visibility. The following are the milestones for the project.
1. Feasibility report
2. System definition, project plan
3. Software requirement specification, Preliminary user manual
4. Architectural design document
5. Detailed design document
6. Software verification plan
7. Product Schedule
8. Software test plan, Acceptance test
9. Software quality assurance plan
10. User manual
11. Source code
12. Test results
13. Defect report
Every software engineering organization should describe a unique set of framework activities for the
software process it adopts. Various models have been developed over the years to accommodate these
Models for Software Development
1. Waterfall Model suggests a systematic sequential approach to software development that
begins with customer specification of requirements. The principle stages of the model map onto
fundamental development activities.
a. Requirement Analysis and definition
b. System and software design
c. Implementation and unit testing
d. Integration and system testing
e. Operation and maintenance
The verification at each stage ensures that the output is consistent with its input and overall
requirement of the system. These outputs are often called work products and can be listed as:
a. Requirements documents
b. Project plan
c. Design documents
d. Test plan and test reports
e. Final code
f. Software manuals (e.g. user, installation, etc)
1. Simple method with clear steps
2. Easy to administer as it is systematic
3. Verification at each stage ensures early detection of errors / misunderstanding
4. Documentation helps in future maintenance and revisions
1. Unchanging requirements are not realistic
2. Document driven process
3. Change adaptability is very slow
2. Incremental Model: Customer identifies the services to be provided. The delivery increments are
then defined, with each increment providing a sub-set of the system functionality. Once the
system increments for the services have been identified, the requirements to be delivered in the
first increment are defined in detail and the increment is developed.
Define outline Design System
to increments increment
Validate Integrate Validate
Increment Increment System Final System
3. Prototyping: First a working prototype of the software is developed instead of developing the
actual software. The developers use this prototype to refine the requirements and prepare final
specification document. After the finalization of SRS document, the prototype is discarded and
actual system is then developed using the waterfall approach.
Quick Design / Prototype
Customer Evaluation Requirements as per
Implementation and Unit Testing
Integration and System Testing
Operation and Maintenance
There are 2 types of prototypes; throw-away, where the initial prototype is discarded after the
requirements are defined and the evolutionary prototype, where the system is built further on
the prototype itself. A prototype is built for those requirements which are critical for the project
and are not understood well. If the number of requirements which need clarification is more,
then a working prototype is built for them. The importance or criticality of such requirements is
also a significant factor in determining the need of a prototype.
Advantages of Prototyping
1. Users are actively involved in the development
2. It provides a better system to users, as users have natural tendency to change their mind in
specifying requirements and this method of developing systems supports this user
3. Since in this methodology a working model of the system is provided, the users get a better
understanding of the system being developed.
4. Errors can be detected much earlier as the system is mode side by side.
5. Quicker user feedback is available leading to better solutions.
1. Leads to implementing and then repairing way of building systems.
2. Practically, this methodology may increase the complexity of the system as scope of the
system may expand beyond original plans.
4. Spiral Model: Boehm tried to incorporate the ‘project risk’ factor into a life cycle model. Each
phase is split roughly into four sections namely planning, risk analysis, development and
assessment. There are 4 phases namely feasibility, requirements analysis, design and coding and
The development spiral consists of four quadrants as shown in the figure above
The first step in each phase is to identify the objective, alternatives and constraints of that phase. The
next stage involves identifying the best alternative, by comparing all and performing risk analysis. One
the alternative is chose, prototypes are built using simulation and benchmarks. Each phase is completed
with a review by the people concerned with the project.
Specialized process models
1. Extreme programming includes creating a set of stories and assigning their priority. The
commitment defines the order for development. The objects are organized for development.
Designing occurs both after and before programming, in the form of design and refactoring.
Programming happens in the form of pain programming where two people work together at one
workstation. Writing test cases before starting the coding is a key feature of extreme
programming. Acceptance testing is followed by system testing.
2. Component-Based Development uses commercial-off-the-shelf (COTS) software components.
3. Unified Process is a use-case driven, architecture centric, iterative and incremental software
4. Dynamic Systems Development Method (DSDM) suggests an iterative software process. After
the feasibility study follows the business study that establishes the functional and information
requirements for the application. The iteration steps include Functional model iteration which
creates a prototype. New requirements might be generated through the prototype, by the user.
Subsequently or simultaneously designs are built or rebuilt. Implementation iteration places the
latest software increments.
5. Feature driven development emphasizes project management guidelines and techniques. The
processes defined for such development are to develop an overall model, build a feature list,
plan by feature, design by feature, build by feature..
Process improvement is about understanding existing processes and introducing process changes to
improve product quality reduce costs or accelerate schedules. Most process improvement work so
far has focused on defect reduction. This reflects the increasing attention paid by industry to quality.
However, other process attributes can also be the focus of improvement. Feedback is important for
process improvement to initiate.
Why is it difficult to improve a software process?
1. Not enough time: Developers, because of unrealistic schedules are left with no time to explore
problems of development and find solutions.
2. Lack of knowledge: many developers are not aware of best practices
3. Wrong motivation: The basic motivation should be to eradicate current difficulties and not just
to achieve a higher CMM level.
4. Insufficient commitment
Begins Improved future state
Do not quit here
Process improvement stages
Attributes of the current process are measured. These are a baseline for assessing
The current process is assessed and bottlenecks and weaknesses are identified.
Changes to the process that have been identified during the analysis are introduced.
Process used should depend on type of
product which is being developed
For large systems, management is usually the principal problem so you need a strictly
• For smaller systems, more informality is possible.
There is no uniformly applicable process which
should be standardised within an organisation
• High costs may be incurred if you force an inappropriate process on a development
• Inappropriate methods can also increase costs and lead to reduced quality.
Process analysis and modelling
1. Study an existing process to understand its activities.
2. Produce an abstract model of the process. You should normally represent this graphically.
Several different views (e.g. activities, deliverables, etc.) may be required.
3. Analyse the model to discover process problems. This involves discussing process activities with
stakeholders and discovering problems and possible process changes.
Process analysis techniques
• Published process models and process standards: It is always best to start process analysis
with an existing model. People then may extend and change this.
• Questionnaires and interviews: Must be carefully designed. Participants may tell you what
they think you want to hear.
• Ethnographic analysis: Involves assimilating process knowledge by observation. Best for in-
depth analysis of process fragments rather than for whole-process understanding.
Process change stages
• Improvement identification.
• Improvement prioritization.
• Process change introduction.
• Process change training.
• Change tuning.