DESIGN CONCEPT AND
PRINCIPLES
 What is a design?
A meaningful representation of something that is
to be built
 In the software engineering context, design
addresses four major areas
Data
Interfaces
Architecture
Components
Software Engineering
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DESIGN CONCEPT AND
PRINCIPLES
Who does it?
Software engineers design computer
based system, but the skills required at
each level of design work is different.
Why it is important? What is the output
of this phase?
How do I ensure that I have done it
right?
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DESIGN CONCEPT AND
PRINCIPLES
The design activity is often divided into
two sub phases
System design ( top-level-design )
Detailed design (low-level-design)
Responsible Person: System designer
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DESIGN CONCEPT AND
PRINCIPLES
Software design sits in the technical
kernel of software engineering
Each of the elements of the analysis
model provides information that is
necessary to create the four design
models required for complete design
specification
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DESIGN CONCEPT AND
PRINCIPLES
The data design transforms the
information domain model created
during analysis into data structure that
will required to implement the software
Data objects and the relationship – E-R
model
Detailed data content depicted in – data
dictionary
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DESIGN CONCEPT AND
PRINCIPLES
Part of the data design may occur in
conjunction with design of software
architecture
More detailed data design occurs as the
software component is designed
The architectural design- defines the
relationship between major structural
elements of the software
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DESIGN CONCEPT AND
PRINCIPLES
The interface design- Describes how
the software communicates within itself,
with systems that interoperate with it,
and with humans who use it
The component level design- transforms
structural elements of software
architecture into a procedural
description of software components
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DESIGN CONCEPT AND
PRINCIPLES
Why design is so important?
Can be stated with a single word- Quality
Without design we build an unstable
systemOne that will fall when small changes are
made
One that may be difficult to test
One whose quality cannot be assessed until
late in the software process
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DESIGN CONCEPT AND
PRINCIPLES
Design and software qualityThroughout the design process the quality
of the evolving design is assessed with a
series of formal technical reviews
What is a good design?
McGlaughlin suggests some characters
that serves as a guide for the evaluation of
a good design
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DESIGN CONCEPT AND
PRINCIPLES
 They are as follows:
The design must implement all of the explicit
requirements contained in the analysis model.
The design must be readable, understandable,
guide for those who generate code and for those
who test and subsequently support the software
Design should provide a complete picture of
software, addressing the data , functional and
behavioral domain from an implementation
perspective
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DESIGN CONCEPT AND
PRINCIPLES
Technical criteria for a good design:
1.Should exhibit an architectural structure
that
Has been created using recognizable design
pattern
Composed of components that exhibit good
design characteristics
Can be implemented in an evolutionary
fashion, which will help the implementation and
testing
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DESIGN CONCEPT AND
PRINCIPLES
2.A design should be modular
3.A design should contain distinct
representation of data, architecture ,
interfaces, and components
4. A design should lead to data
structures that are appropriate for the
objects to be implemented.
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DESIGN CONCEPT AND
PRINCIPLES
 5. A design should lead to components that
exhibits independent functional characters
 6. A design should lead to interfaces that
reduce the complexity of connections
between modules and with external
environment
 7. A design should be derived using a
repeatable method that is driven by
information obtained during software
requirement analysis.
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DESIGN CONCEPT AND
PRINCIPLES
 Evolution of a software design:
 It is a continuous process spread over last
four decades
Early design work concentrated on development
of modular programs and methods, for refining
software structure in a top-down manner.
Procedural aspect of design definition evolved into
philosophy of structured programming
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DESIGN CONCEPT AND
PRINCIPLES
Evolution of software continued:
Later work proposed methods for the
translation of data flow / data structure into
the design definition
Newer design approach, proposed an
object oriented approach for the solution of
the problem
Today the emphasis in software design is
on the software architecture.
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DESIGN CONCEPT AND
PRINCIPLES
Design principles:
Design is both process and the model.
Design process is a sequence of steps,
enables the designers to describe all the
aspects of software to be built.
Design model begins by representing the
totality of the thing to be built and slowly
refines the thing to provide guidance for
constructing each detail
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DESIGN CONCEPT AND
PRINCIPLES
 A set of principles of software design are as
follows:
1. The design process should not suffer from
“tunnel vision”.
2.The design should be traceable to the analysis
model.
3.The design should not reinvent the wheel
4.The design should minimize the ‘intellectual
distance’ between the software and the problem
as it exits in the real world
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DESIGN CONCEPT AND
PRINCIPLES
5.The design should exhibit uniformity and
integration
 6.The design should be structured to
accommodate change
7.Design is not coding, coding is not design
8.The design should be assessed for quality as it
is created, not after fact
The design should be reviewed to minimize
conceptual (semantic ) errors
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DESIGN CONCEPT AND
PRINCIPLES
Terms
Abstraction: When we consider modular
solution of any problem, many level of
abstractions can be posed
Highest level of abstraction: solution is
stated in broad terms using languages of
problem environment.
At the lowest level more procedural
orientation is taken
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DESIGN CONCEPT AND
PRINCIPLES
 Refinement:
Is a top down design strategy, originally proposed
by ‘WIRTH’
A program is developed by successfully refined
level of procedural detail
 Modularity:
Software architecture describes Modularity as
software is divided into separately named and
addressable components, called modules , which
are integrated to satisfy the problem requirement
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DESIGN CONCEPT AND
PRINCIPLES
 How to define an effective module?
Answer lies in the methods used to define
modules within a system
Mayer defines some criteria, that enables us to
evaluate an effective module
Modular decomposability
Modular composability
Modular understandability
Modular protection
A system may be designed modularly even if its
implementation is monolithic
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DESIGN CONCEPT AND
PRINCIPLES
Control hierarchy:
Also called the program structure,
represents the organization of the
program components and implies a
hierarchy of control
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DESIGN CONCEPT AND
PRINCIPLES
 Structural partitioning:
If the architectural system is hierarchical, program
can be partitioned both horizontally as well as
vertically
 Data structure:
Is a representation of the logical relationship
among individual elements of data. Data structure
is as important as program structure to the
representation of software architecture
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DESIGN CONCEPT AND
PRICIPLES
 Software procedure:
Program structure defines control hierarchy
without regard to the sequence of processing and
decisions
 Information hiding:
Modules must be characterized by design
decisions that hides from all others.
Modules should be specified in such a manner
such that the information within a module is
inaccessible to other modules those have no need
to those information
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