Se ii unit2-software_design_principles

1
Software Design Principles
“Producing the software blueprint”

2
Lecture Objectives
To understand the importance of design in
developing quality software
To describe the translation from the
requirements analysis model to the design
model
To understand the principles that guide
proper design of software

3
What Is Design?
Explaining the idea/concept of something
Usually with graphical diagrams
With the intention to build
The design is a representation of a
product or a system with sufficient detail
for implementation

4
The Second Task
Problem
Models
Solution
Analysis
Design
Development
Testing

5
Design Principles
Data design is the first design activity, which results in fewer complexes,
modular and efficient program structure. The data objects, attributes, and
relationships depicted in entity relationship diagrams and the information
stored in data dictionary provide a base for data design activity.
Architectural Design major seeks to integrate engineering and architecture in
ways that blend innovative architectural design with cutting-edge engineering
technologies.
User interface design (UI) or user interface engineering is the design of
user interfaces for machines and software, such as computers, home
appliances, mobile devices, and other electronic devices, with the focus on
maximizing usability and the user experience.

6
Designing Software
From our understanding of the problem,
we start building the software
Translate the analysis model into the
design model
Map the information from the analysis
model to the design representations - data
design, architectural design, interface
design, procedural design

7
Design Principles
Design process should be improvise via new
technology
The design should be traceable to the analysis
model
The design should “minimize intellectual
distance” between the software and the problem
in the real world

8
Design Principles (Continued)
The design should exhibit uniformity and
integration
The design should be structured to
accommodate change
The design should be structured to
degrade gently.

9
Design Principles (Continued)
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 errors

10
Component of System
System software is the software used to manage and
control the hardware components and which allow
interaction between the hardware and the other types
of software. The most obvious type of system software
is the computer's operating system but device
drivers are also included within this category. There
are three components of system software:
1.System Software
2.Standard Software
3. Application Software

11
Design Concepts
Abstraction
Refinement
Modularity
Software
Architecture
Control Hierarchy
Structural Partitioning
Data Structure
Software Procedure
Information Hiding
Fundamental concepts which provide
foundation to design correctly:

12
Abstraction
In software engineering and computer
science, Abstraction is a technique for arranging
complexity of computer systems. It works by establishing
a level of complexity on which a person interacts with the
system, suppressing the more complex details below the
current level. Conceptual abstractions may be formed by
filtering the information content of a concept or an
observable phenomenon, selecting only the aspects which
are relevant for a particular purpose.

13
Abstraction
Identifying important features for representation
There are many levels of abstraction depending
on how detailed the representation is required
Data abstraction - representation of data objects
Procedural abstraction - representation of
instructions

14
Refinement is a generic term of computer science that
encompasses various approaches for
producing correct computer programs and simplifying
existing programs to enable their formal verification.
In formal methods, program refinement is
the verifiable transformation of an abstract (high-
level) formal specification into a concrete (low-
level) executable program.
Stepwise refinement allows this process to be done in stages.
Logically, refinement normally involves implication, but
there can be additional complications.
Refinement

15
Refinement
Stepwise refinement - top-down design Starting
at the highest level of abstraction, every step of
refinement ‘decompose’ instructions into more
detailed instructions
Complementary to abstraction

16
In software engineering, modularity refers to the extent to which
a software/Web application may be divided into smaller modules.
Software modularity indicates that the numbers of applications
modules are capable of serving a specified business domain.
Modularity is an important Software Engineering principle. It is a
practical application of the principle of “Separation of Concerns" by
dividing a complex system into simpler and more manageable modules.
The ubiquitous television set is an example of a system made up of a
number of modules – speakers, projection tube, channel buttons, volume
buttons, etc. Each module has its own defined functionality but when
they are put together synergistically, the complete functionalities of a
television are realized.
Modularity

17
Modularity
Software is divided into separately named
and addressable modules
“Divide and conquer” approach - problem
is broken into manageable pieces
Solutions for the separate pieces then
integrated into the whole system

19
The alternative approach is to take a complete system
and decompose it into its modules. This approach is
known as the decomposition or top-down approach.
A car, for example, can be decomposed into a number of
sub-systems: body, fuel system, electrical system,
transmission, etc.
The electrical system can be further decomposed into
modules such as battery, alternator, lights, etc.

20
Software architecture refers to the high level structures of a software
system, the discipline of creating such structures, and the
documentation of these structures. These structures are needed to
make the software system.
Each structure comprises software elements, relations among them,
and properties of both elements and relations.
“The software architecture of a program or computing system is the
structure or structures of the system, which
comprise software elements, the externally visible properties of those
elements, and the relationships among them.
Software Architecture

21
Software Architecture
Modules can be integrated in many ways to produce the
system
Software architecture is the overall structure of the
software
The hierarchy of components and how they interact, and
the structure of data used by the components
Use of framework models, and possible reuse of
architectural patterns

22
Software Architecture Patterns
Recurring pattern help designers reuse
successful designs by base of new designs on
prior experience.
A designer who is familiar with such patterns
can apply them immediately to design problems
without having to rediscover them.

23
Why use Design Patterns?
Reuse successful practices
Improve communication
Step towards a software engineer’s
handbook

24
Examples of Software
Architecture
S1
S2
S3
S4
S5
S3
S2 S4 S5
S1
Batch
sequential
pattern
Program structure

Control Hierarchy
25
A program structure that represents the organization of a
program component and implies a hierarchy of control.
A hierarchical control system is a form of control system
in which a set of devices and software is arranged in a
hierarchical tree. When the links in the tree are
implemented by a computer network, then that
hierarchical control system is also a form of networked
control system.

26
Control Hierarchy
Hierarchy of modules representing the
control relationships
A super-ordinate module controls another
module
A subordinate module is controlled by
another module
Measures relevant to control hierarchy:
depth, width, fan-in, fan-out

27
Structure Terminology
g hed
i
f
a b c
M
Fan-out
Fan-in
Width
Depth

28
Structural Partitioning
Program structure partitioned horizontally and
vertically
Horizontal partitioning defines separate
branches for each major program function -
input, process, output
Vertical partitioning defines control (decision-
making) at the top and work at the bottom

29
Software Procedure
Processing details of individual modules
Precise specification of processing,
including sequence of events, exact
decision points, repetitive operations, and
data organization/structure
Procedure is layered - subordinate
modules must be referenced in
processing details

30
Information Hiding
Information (procedure and data) contained
within a module is inaccessible to other modules
that have no need for such information
Effective modularity is achieved by independent
modules, that communicate only necessary
information
Ease of maintenance - testing, modification
localized and less likely to propagate

Se ii unit2-software_design_principles

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