Software design and Software engineering.pptx

SOFTWARE DESIGN
Prepared by Dr.T.Thendral 15.07.23
Prepared by
Dr.T.Thendral
Assistant Professor
Department of Computer Science
Sri Ramaskrishna College of Arts & Science for Women
Coimbatore

• Software design and Software engineering
• The Design process
• Design principles
• Design concepts
• Effective modular design
• Software Architecture

SOFTWARE DESIGN AND SOFTWARE
ENGINEERING
• Software design is the first of
three technical activities—
design, code generation, and
test—that are required to
build and verify the software
• The design task produces a
data design, an architectural
design, an interface design,
and a component design
• The data design transforms
the information domain model
created during analysis into
the data structures that will be
required to implement the
software

• The architectural design defines the relationship
between major structural elements of the software,
the “design patterns”
• The interface design describes how the software
communicates within itself, with systems that
interoperate with it and with humans who use it
• An interface implies a flow of information (e.g., data
and/or control) and a specific type of behavior
• The component-level design transforms structural
elements of the software architecture into a
procedural description of software components

• The importance of software design can be
stated with a single word—quality
• Design is the place where quality is fostered in
software engineering

Translating the analysis model into
a software design

THE DESIGN PROCESS
• Software design is an iterative process
through which requirements are translated
into a “blueprint” for constructing the
software

Design and Software Quality
• Throughout the design process, the quality of
the evolving design is assessed with a series of
formal technical reviews or design
walkthroughs

Three characteristics of a good design:

• A design should exhibit an architectural
structure that
• (1) has been created using recognizable design
patterns
• (2) is composed of components that exhibit
good design characteristics, and
• (3) can be implemented in an evolutionary
fashion, thereby facilitating implementation
and testing

• 2. A design should be modular
• 3. A design should contain distinct representations of data,
architecture, interfaces, and components (modules).
• 4. A design should lead to data structures that are
appropriate for the objects
• 5. A design should lead to components that exhibit
independent functional characteristics
• 6. A design should lead to interfaces that reduce the
complexity of connections between modules and with the
external environment
• 7. A design should be derived using a repeatable method
that is driven by information obtained during software
requirements analysis

The Evolution of Software Design
• Early design work concentrated on criteria for the
development of modular programs and methods
for refining software structures in a top-down
manner
• Procedural aspects of design definition evolved
into a philosophy called structured programming
• Later work proposed methods for the translation
of data flow or data structure into a design
definition

• Methods have a number of common
characteristics:
• (1) a mechanism for the translation of analysis
model into a design representation
• (2) a notation for representing functional
components and their interfaces
• (3) heuristics for refinement and partitioning, and
• (4) guidelines for quality assessment

Example
DESIGN PRINCIPLES

DESIGN PRINCIPLES
• Software design is both a process and a model
• Design should be Creative skill, past
experience, a sense of what makes “good”
software, and an overall commitment to
quality are critical success factors for a
competent design
• The design model is the equivalent of an
architect’s plans for a house

• e.g., a three-dimensional rendering of the
house
• slowly refines the thing to provide guidance
for constructing each detail (e.g., the
plumbing layout)

• The design process should not suffer from
“tunnel vision.”- onsider alternative
approaches
• The design should be traceable to the
analysis model- Because a single element of
the design model often traces to multiple
requirements
• Requirements have been satisfied by the
design model!

• The design should not reinvent the wheel-
Systems are constructed using a set of design
patterns
• These patterns should always be chosen as an
alternative to reinvention
• Time is short and resources are limited
• The design should “minimize the intellectual
distance” between the software and the
problem as it exists in the real world.
• structure of the software design should mimic
the structure of the problem domain

• The design should exhibit uniformity and
integration-
• A design is uniform if it appears that one
person developed the entire thing.
• Rules of style and format should be defined
for a design team before design work begins
• design is integrated if care is taken in defining
interfaces between design components

• The design should be structured to accommodate
change
• The design should be structured to degrade
gently, even when a’berrant data, events, or
operating conditions are encountered
• Design is not coding, coding is not design
• The design should be assessed for quality as it is
being created, not after the fact
• The design should be reviewed to minimize
conceptual (semantic) errors

DESIGN CONCEPTS
• Each helps the software engineer to answer
the following questions:
• What criteria can be used to partition
software into individual components?
• How is function or data structure detail
separated from a conceptual representation of
the software?
• What uniform criteria define the technical
quality of a software design?

• The beginning of wisdom for a [software
engineer] is to recognize the difference
between getting a program to work, and
getting it right“
• Fundamental software design concepts
provide the necessary framework for "getting
it right"

Abstraction
• Abstraction:
• When we consider a modular solution to any problem,
many levels of abstraction can be posed
• Procedural Abstraction
• An example of a procedural abstraction would be the word
open for a door
• Data abstraction
• A data abstraction is a named collection of data that
describes a data object
• In the context of the procedural abstraction open, we can
define a data abstraction called door
• data object, the data abstraction for door (e.g., door type,
swing direction, opening mechanism, weight, dimensions)

• Control abstraction is the third form of
abstraction used in software design
• control abstraction implies a program control
mechanism without specifying internal details

Refinement
• Stepwise refinement is a top-down design
strategy originally proposed by Niklaus Wirth
• In each step (of the refinement), one or several
instructions of the given program are
decomposed into more detailed instructions
• Refinement is actually a process of elaboration
• Abstraction enables a designer to specify
procedure and data and yet suppress low-level
details
• Refinement helps the designer to reveal low-level
details as design progresses

• Every refinement step implies some design
decisions.
• It is important that . . .
• the programmer be aware of the underlying
criteria (for design decisions) and of the
existence of alternative solutions . . .

Modularity
• software is divided into separately named and
addressable components, often called
modules, that are integrated to satisfy
problem requirements.

• Modular decomposability - reduce the complexity of the
overall problem, thereby achieving an effective modular
solution
• Modular composability-If a design method enables existing
(reusable) design components to be assembled into a new
system
• Modular understandability-If a module can be understood
it will be easier to build and easier to change
• Modular continuity-If small changes to the system
requirements result in changes to individual modules
• Modular protection-If an aberrant condition occurs within
a module and its effects are constrained within that module
side effects will be minimized.

Objective
• What Is Architecture?
• Why Is Architecture Important?

Software Architecture
• Software architecture the overall structure of
the software
• The ways in which that structure provides
conceptual integrity for a system
• First program was divided into modules,
software systems have had architectures,
and programmer
• Effective software architecture and its explicit
representation and design have become
dominant themes in software engineering

What Is Architecture?
• The architecture of a building, many different
attributes come to mind
• EXAMPLE:
• It is the manner in which the various
components of the building are integrated to
form a cohesive whole
• It is the beautiful feel of the structure—the
visual impact of the building—and the way
textures, colors, and materials are combined
to create the external facade and the internal
“living environment

• The architecture is not the operational
software.
• It is a representation that enables a software
engineer to
• (1) analyze the effectiveness of the design in
meeting its stated requirements
• (2) consider architectural alternatives at a
stage when making design changes is still
relatively easy,
• (3) reducing the risks associated with the
construction of the software

Why Is Architecture Important?
• Three key reasons that software architecture is
important:
• Representations of software architecture are an
enabler for communication between all parties
(stakeholders) interested in the development of a
computer-based system
• The architecture highlights early design decisions -
impact on all software engineering work that follows -
the ultimate success of the system as an operational
entity
• Architecture “constitutes a relatively small,
intellectually graspable model of how the system is
structured and how its components work together”

Summary of Unit II
• Software design and Software engineering
• The Design process
• Design principles
• Design concepts
• Effective modular design

Software design and Software engineering.pptx

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