Software engineering introduction

Software Engineering

by
Vishal Singh
(vishal11380@yahoo.com)

Meaning:
Software engineering is the field of computer
science that deals with the building of
software systems which are so large or so
complex that they are built by a team or
teams of engineers.

Software Engineering is the use of
techniques, methods and
methodologies to develop high quality
software which is
 Reliable
 Easy to understand
 Useful
 Modular
 Efficient
 Modifiable
 Reusable
 Good user interface
 Well documented
 Delivered in cost effective and timely manner

Software engineering goals
 a. Maintainability
It should be possible to easily introduce
changes to the software without increasing
the complexity of the original system
design.
 b. Reliability

The software should prevent failure in
design and construction as well as recover
from failure in operation.

 c. Efficiency
The software system should use the
resources that are available in an optimal
manner.
 d. Understandability
The software should accurately model the
view the reader has of the real world. Since
code in a large, long-lived software system
is usually read more times than it is written,
it should be easy to read at the expense of
being easy to write, and not the other way
around.

Software engineering principles

 a. Abstraction
"The essence of abstraction is to extract
essential properties while omitting
inessential detail."
 b. Information Hiding
The code should contain no unnecessary
detail. Elements that do not affect other
portions of the system are inaccessible to
the user

 c. Modularity
The code is purposefully structured. Components
of a given module are logically or functionally
dependent.
 d. Localization
The breakdown and decomposition of the code is
rational. Logically related computational units are
collected together in modules.
 e. Uniformity
The notation and use of comments, specific
keywords and formatting is consistent and free
from unnecessary differences in other parts of
the code.

 f.Completeness
Nothing is blatantly missing from any module. All
important or relevant components are present
both in the modules and in the overall system as
appropriate.
 g. Conformability

The modules of the program can be tested
individually with adequate rigor. This gives rise
to a more readily alterable system and enables
the reusability of tested components.

The Software Life Cycle

A sample waterfall life cycle model
comprises the following phases:
 Requirements analysis and
specification
 Design and specification
 Coding and module testing
 Integration and system testing
 Delivery and maintenance

Software engineering introduction

Classification of Software Qualities

External versus Internal Qualities
We can divide software qualities into
external and internal qualities. The
external Qualities are visible to the user of
the system and the internal qualities are
those that concerns the developer of the
system.

Representative Qualities
1. Correctness, Reliability, and Robustness
 Correctness: A program is functionally correct if
it behaves according to the specification of the
functions it should provide (called functional
requirements specifications).
 Reliability: Informally, software is reliable if the
user can depend on it.
 Robustness: A program is robust if it behaves
"reasonably," even in circumstances that were
not anticipated in the requirements specification-
for example, when it encounters incorrect input
data or some hardware malfunction (say, a disk
crash).

2. Performance
 Any engineering product is expected to meet a
certain level of performance. Unlike other
disciplines, in software engineering we often
equate performance with efficiency. We will
follow this practice here. A software system is
efficient if it uses computing resources
economically.
 Performance is important because it affects the
usability of the system. If a software system is
too slow, it reduces the productivity of the users,
possibly to the point of not meeting their needs.

3. User Friendliness
 A software system is user friendly if its
human users find it easy to use.
 For example, a novice user may
appreciate verbose messages, while an
experienced user grows to detest and
ignore them. Similarly, a nonprogrammer
may appreciate the use of menus, while a
programmer may be more comfortable
with typing a command.

4.Verifiability
 A software is verifiable if its properties can
be verified easily. For example, the
correctness or the performances of a
software system are properties we would
be interested in verifying.
 Modular design, disciplined coding
practices and the use of an appropriate
programming language all contribute to
verifiability.

 Maintainability
 The term “software maintenance” is
commonly used to refer to the
modifications that are made to a software
system after its initial release.
 There is evidence that maintenance costs
exceed 60% of the total costs of the
software.

 Reusability
 Reusability is an important factor. In
product evolution, we modify a product to
build a new version of that same product;
in. product reuse, we use it-perhaps with
minor changes-to build another product.
Reusability appears to be more applicable
to software components than to whole
products but it certainly seems possible to
build products that are reusable.

 Portability
 Software is portable if it can run in
different environments. The term
"environment" can refer to a hardware
platform or a software environment such
as a particular operating system. With the
proliferation of different processors and
operating systems, portability has become
an important issue for software engineers.

 Understandability
 Some software systems are easier to
understand than others. Of course, some tasks
are inherently more complex than others. For
example, a system that does weather
forecasting, no matter how well it is written, will
be harder to understand than one that
prints a mailing list. Given tasks of inherently
similar difficulty, we can follow certain
guidelines to produce more understandable
designs and to write more understandable
programs.

 Interoperability
 Interoperabilityrefers to the ability of a
system to coexist and cooperate with
other systems-for example, a word-
processor's ability to incorporate a chart
produced by a graphing package, or the
graphics package's ability to graph the
data produced by a spreadsheet, or the
spreadsheet's ability to process an image
scanned by a scanner.

 Productivity
 Productivity is a quality of the software
production process: it measures the
efficiency of the process and, as we said
before, is the performance quality applied
to the process. An efficient process results
in faster delivery of the product.

 Timeliness
 Timeliness is a process-related quality that
refers to the ability to deliver a product on
time. Historically, timeliness has been
lacking in software production processes
leading to the "software crisis," which in
turn led to the need for-and birth of
software engineering itself. Even now,
many current processes fail to result in a
timely product.

 Visibility
A software development process is visible
if all of its steps and its current status are
documented clearly. The idea is that the
steps and the status of the project are
available and easily accessible for
external examination.

 SDLC Models
There are various software development
approaches defined and designed which are
used/employed during development process of
software, these approaches are also referred as
"Software Development Process Models".
 Some of the most commonly used Software
developments models are the following:
 Waterfall model
 Spiral model
 Prototyping model
 Iterative model

Waterfall Model

 Waterfallapproach was first Process
Model to be introduced and followed
widely in Software Engineering to ensure
success of the project. In "The Waterfall"
approach, the whole process of software
development is divided into separate
process phases.

The stages of "The Waterfall Model" are:

 Requirement Analysis & Definition
 System & Software Design
 Implementation & Unit Testing
 Integration & System Testing
 Operations & Maintenance

Waterfall Model Common Error
Common Errors in Requirements Analysis

 Problem 1: Customers don't (really) know what
they want
 Problem 2: Requirements change during the
course of the project
 Problem 3: Customers have unreasonable
timelines
 Problem 4: Communication gaps exist between
customers, engineers and project managers
 Problem 5: The development team doesn't
understand the politics of the customer's
organization

Prototype Model
A prototype is a working model that is
functionally equivalent to a component of
the product. In many instances the client
only has a general view of what is
expected from the software product. In
such a scenario where there is an
absence of detailed information regarding
the input to the system, the processing
needs and the output requirements, the
prototyping model may be employed.

The process of prototyping involves the following steps:
 Identify basic requirements
Determine basic requirements including the input and
output information desired. Details, such as security, can
typically be ignored.
 Develop Initial Prototype
The initial prototype is developed that includes only user
interfaces.
 Review
The customers, including end-users, examine the
prototype and provide feedback on additions or changes.
 Revise and Enhancing the Prototype
Using the feedback both the specifications and the
prototype can be improved. Negotiation about what is
within the scope of the contract/product may be
necessary. If changes are introduced then a repeat of
steps #3 ands #4 may be needed.

Advantages of Prototyping

 Reduced time and costs
 Improved and increased user involvement
Disadvantages of Prototyping
 Insufficient analysis
 User confusion of prototype and finished
system
 Developer attachment to prototype
 Excessive development time of the prototype
 Expense of implementing prototyping

Spiral Model
 The spiral model was defined by Barry Boehm in
his 1988 article
 The Spiral Model

The spiral model, also known as the spiral
lifecycle model, is a systems development method
(SDM) used in information technology (IT). This
model of development combines the features
of the prototyping model and the waterfall
model. The spiral model is intended for large,
expensive and complicated projects.

The steps in the spiral model can be
generalized as follows:
1. The new system requirements are defined in
as much detail as possible. This usually
involves interviewing a number of users
representing all the external or internal users
and other aspects of the existing system.
2. A preliminary design is created for the new
system.
3. A first prototype of the new system is
constructed from the preliminary design. This
is usually a scaled-down system, and
represents an approximation of the
characteristics of the final product.

4. A second prototype is evolved by a fourfold procedure:
(1) evaluating the first prototype in terms of its
strengths, weaknesses, and risks; (2) defining the
requirements of the second prototype; (3) planning and
designing the second prototype; (4) constructing and
testing the second prototype.
5. At the customer's option, the entire project can be
aborted if the risk is deemed too great. Risk factors
might involve development cost overruns, operating-
cost miscalculation, or any other factor that could, in
the customer's judgment, result in a less-than-
satisfactory final product.
6. The existing prototype is evaluated in the same
manner as was the previous prototype, and, if
necessary, another prototype is developed from it
according to the fourfold procedure outlined above.

7. The preceding steps are iterated until the
customer is satisfied that the refined
prototype represents the final product
desired.
8. The final system is constructed, based
on the refined prototype.
9. The final system is thoroughly evaluated
and tested. Routine maintenance is
carried out on a continuing basis to
prevent large-scale failures and to
minimize downtime.

Advantages of Spiral model
 Estimates (i.e. budget, schedule, etc.)
become more realistic as work
progresses, because important issues are
discovered earlier.
 It is more able to cope with the (nearly
inevitable) changes that software
development generally entails.

Disadvantages of Spiral model
 Highly customized limiting re-usability
 Applied differently for each application
 Risk of not meeting budget or schedule

Software engineering introduction

Recommended

More Related Content

What's hot (20)

Viewers also liked (10)

Similar to Software engineering introduction (20)

Recently uploaded (20)

Software engineering introduction