Chapter 6 design

Design Engineering
• It covers the set of principles, concepts, and practices that lead to
the development of a high quality system or product.
• Goal of design engineering is to produce a model or representation
that depict:
• Firmness – program should not have any bug that inhibits its functions.
• Commodity – suitable to its intended use.
• Delight - pleasurable to use
• The design model provides detail about software data structures,
architecture, interfaces, and components that are necessary to
implement the system.

Software Design
• Software design model consists of 4 designs:
• Data/class Design
• Architectural Design
• Interface Design
• Component Design

Translating Analysis  Design

• Data/class design - Created by transforming the analysis model class-
based elements into classes and data structures required to implement the
software
• Architectural design - defines the relationships among the major
structural elements of the software, it is derived from the class-based
elements and flow-oriented elements of the analysis model
• Interface design - describes how the software elements, hardware
elements, and end-users communicate with one another, it is derived
from the analysis model scenario-based elements, flow-oriented
elements, and behavioral elements
• Component-level design - created by transforming the structural
elements defined by the software architecture into a procedural
description of the software components using information obtained from
the analysis model class-based elements, flow-oriented elements, and
behavioral elements

Why design is so important?
• It is place where quality is fostered.
• It provides us with representation of software that can be
assessed for quality.
• Only way that can accurately translate a customer’s
requirements into a finished software product.
• It serves as foundation for all software engineering
activities.
• Without design difficult to assess:
• Risk
• Test
• Quality

Design Process and Design Quality
• S/w design is an iterative process through which
requirements are translated into a “blueprint” for
constructing the s/w.
• As design iteration occur, subsequent refinement
leads to design representation at much lower levels
of abstraction.

Goal of design process
• The design must implement all of the explicit
requirements contained in the analysis model, and it
must accommodate all of the implicit requirements
desired by the customer.
• The design must be a readable, understandable guide for
those who generate code and for those who test and
subsequently support the software.
• The design should provide a complete picture of the
software, addressing the data, functional, and behavioral
domains from an implementation perspective.

Quality Guidelines
Characteristics of good design
• A design should exhibit an architecture that
• as been created using recognizable architectural styles or patterns,
• is composed of components that exhibit good design characteristics
and
• can be implemented in an evolutionary fashion
• A design should be modular; that is, the software should be logically
partitioned into elements or subsystems
• A design should contain distinct representations of data, architecture,
interfaces, and components.
• A design should lead to data structures that are appropriate for the
classes to be implemented and are drawn from recognizable data
patterns.
• A design should lead to components that exhibit independent functional
characteristics

Quality Guidelines (contd.)
• A design should lead to interfaces that reduce the
complexity of connections between components and with
the external environment.
• A design should be derived using a repeatable method that
is driven by information obtained during software
requirements analysis.
• A design should be represented using a notation that
effectively communicates its meaning.

Design Principles
• S/W design is both a process and a model.
• Design process - sequence of steps that enable the
designer to describe all aspects of the software to
be built.
• Design model - created for software provides a
variety of different views of the computer software

Design Principles
• The design process should not suffer from ‘tunnel vision.’ -
Designer should consider alternative approaches.
• The design should be traceable to the analysis model - a single
element of the design model often traces to multiple
requirements, it is necessary to have a means for tracking how
requirements have been satisfied by the design model.
• The design should not reinvent the wheel- use already exists
design pattern because time is short and resource are limited.
• The design should “minimize the intellectual distance” between
the software and the problem as it exists in the real world. –
design should be self-explanatory

Design Principles
• The design should exhibit uniformity and integration – before
design work begins rules of styles and format should be defined
for a design team.
• The design should be structured to accommodate change
• The design should be structured to degrade gently, even when
unusual 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
• Design concepts provide the necessary framework for “to get the thing on
right way”.
• Abstraction
• Refinement
• Modularity
• Architecture
• Control Hierarchy
• Structural Partitioning
• Data Structure
• Software Procedure
• Information Hiding

Abstraction
• At the highest level of abstraction – a solution is stated in broad terms
• At lower level of abstraction – a more detailed description of the solution is
provided.
• Two types of abstraction:
• Procedural abstraction: Sequence of instructions that have a specific and
limited function.
Ex. Open a door
open implies long sequence of activities (e.g. walk to the door, grasp knob,
turn knob and pull the door, etc).
• Data abstraction: collection of data that describes a data object.
Ex. Open a door. – door is data object.
• Data abstraction for door would encompass a set of attributes that describe
the door. (E.g. door type, swing direction, opening mechanism, etc.)

Refinement
• Refinement is actually a process of elaboration.
• begin with a statement of function (or description of information)
that is defined at a high level of abstraction.
• That is, the statement describes function or information
conceptually but provides no information about the internal
workings of the function or the internal structure of the information.
• Refinement causes the designer to elaborate on the original
statement, providing more and more detail as each successive
refinement (elaboration) occurs.
• Abstraction and refinement are complementary concepts.
Abstraction enables a designer to specify procedure and data and
yet suppress low-level details.
• Refinement helps the designer to expose low-level details as design
progresses.

Modularity
• Architecture and design pattern embody modularity.
• Software is divided into separately named and addressable
components, sometimes called modules, which are
integrated to satisfy problem requirement.
• modularity is the single attribute of software that allows a
program to be intellectually manageable
• It leads to a “divide and conquer” strategy. – it is easier to
solve a complex problem when you break into a
manageable pieces.
• Refer fig. that state that effort (cost) to develop an
individual software module does decrease if total number of
modules increase.
• However as the no. of modules grows, the effort (cost)
associated with integrating the modules also grows.

Modularity and software cost
• Undermodularity and overmodularity should be
avoided. But how do we know the vicinity of M?
• We modularize a design so that development can be
more easily planned.
• Software increments can be defined and delivered.
• Changes can be more easily accommodated.
• Testing and debugging can be conducted more
efficiently and long-term maintained can be conducted
without serious side effects.

Architecture
• Software architecture suggest “ the overall structure of the
software and the ways in which that structure provides
conceptual integrity for a system.
• No. of different models can use to represent architecture.
• Structural Model- represent architecture as an organized
collection of components
• Framework model – Increase level of design abstraction by
identifying repeatable architectural design framework.
• Dynamic model – address behavior of the program
architecture and indicating how system or structure
configuration may change as a function.
• Process Model – focus on design of the business or technical
process that the system must accommodate.
• Functional models – used to represent the functional
hierarchy of a system.

Control Hierarchy
• Control hierarchy, represents the organization of
program components (modules) and implies a
hierarchy of control.
• Different notations are used to represent control
hierarchy for those architectural styles that are
amenable to this representation.
• The most common is the treelike diagram that
represents hierarchical control for call and return
architectures.

Control Hierarchy
• In fig. depth and width provide an indication of the number
of levels of control and overall span of control.
• Fan-out is a measure of the number of modules that are
directly controlled by another module. Fan-in indicates how
many modules directly control a given module.
• A module that controls another module is said to be
superordinate to it, and conversely, a module controlled by
another is said to be subordinate to the controller.
Ex. Module M is superordinate to modules a, b, and c.
Module h is subordinate to module e

Structure partitioning
• Two types of Structure partitioning:
• Horizontal Partitioning
• Vertical Partitioning
• Horizontal partitioning defines separate branches of the
modular hierarchy for each major program function.
• Control modules, represented in a darker shade are used to
coordinate communication & execution between its functions.
• Horizontal partitioning defines three partitions—input, data
transformation (often called processing) and output.
• Benefits of Horizontal partitioning:
• software that is easier to test
• software that is easier to maintain
• propagation of fewer side effects
• software that is easier to extend

• On the negative side (Drawback), horizontal partitioning
often causes more data to be passed across module
interfaces and can complicate the overall control of
program flow.
• Vertical partitioning, often called factoring, suggests
that control (decision making) and work should be
distributed top-down in the program structure.
• Top-level modules should perform control functions and
do little actual processing work.
• Modules that reside low in the structure should be the
workers, performing all input, computation, and output
tasks.

• A change in a control module (high in the structure)
will have a higher probability of propagating side
effects to modules that are subordinate to it.
• A change to a worker module, given its low level in
the structure, is less likely to cause the propagation of
side effects.
• For this reason vertically partitioned structures are
less likely to be susceptible to side effects when
changes are made and will therefore be more
maintainable.

Data Structure
• Data structure is a representation of the logical relationship among
individual elements of data
• A scalar item is the simplest of all data structures. It represents a single
element of information that may be addressed by an identifier.
• When scalar items are organized as a list or contiguous group, a
sequential vector is formed.
• When the sequential vector is extended to two, three, and ultimately,
an arbitrary number of dimensions, an n-dimensional space is created.
Most common n-dimensional space is the two-dimensional matrix
• A linked list is a data structure that organizes contiguous scalar items,
vectors, or spaces in a manner (called nodes) that enables them to be
processed as a list.
• A hierarchical data structure is implemented using multilinked lists that
contain scalar items, vectors, and possibly, n-dimensional spaces.

Software Procedure
• Software procedure focuses on the processing
details of each module individually.
• Procedure must provide a precise specification of
processing, including sequence of events, exact
decision points, repetitive operations, and even
data organization and structure.

Information Hiding
• The principle of information hiding suggests that modules
be "characterized by design decisions that (each) hides from
all others modules.“
• In other words, modules should be specified and designed
so that information (algorithm and data) contained within a
module is inaccessible to other modules that have no need
for such information.
• The intent of information hiding is to hide the details of data
structure and procedural processing behind a module
interface.
• It gives benefits when modifications are required during
testing and maintenance because data and procedure are
hiding from other parts of software, unintentional errors
introduced during modification are less.

EFFECTIVE MODULAR DESIGN
• Effective modular design consist of three things:
• Functional Independence
• Cohesion
• Coupling

Functional Independence
• Functional independence is achieved by developing
modules with "single-minded“ function and an "aversion" to
excessive interaction with other modules.
• In other words - each module addresses a specific sub-
function of requirements and has a simple interface when
viewed from other parts of the program structure.
• Independence is important –
• Easier to develop
• Easier to Test and maintain
• Error propagation is reduced
• Reusable module.

Functional Independence
• To summarize, functional independence is a key to
good design, and design is the key to software
quality.
• To measure independence, have two qualitative
criteria: cohesion and coupling
• Cohesion is a measure of the relative functional
strength of a module.
• Coupling is a measure of the relative
interdependence among modules.

Cohesion
• Cohesion is a natural extension of the information hiding concept
• A cohesive module performs a single task within a software
procedure, requiring little interaction with procedures being
performed in other parts of a program
• Simply state, a cohesive module should (ideally) do just one thing.
• We always strive for high cohesion, although the mid-range of the
spectrum is often acceptable.
• Low-end cohesiveness is much "worse" than middle range, which is
nearly as "good" as high-end cohesion.
• So. designer should avoid low levels of cohesion when modules are
designed.

Cohesion
• When processing elements of a module are related
and must be executed in a specific order,
procedural cohesion exists.
• When all processing elements concentrate on one
area of a data structure, communicational cohesion
is present.
• High cohesion is characterized by a module that
performs one distinct procedural task.

Types of cohesion
• A module that performs tasks that are related
logically is logically cohesive.
• When a module contains tasks that are related by
the fact that all must be executed with the same
span of time, the module exhibits temporal
cohesion.
• At the low-end of the spectrum, a module that
performs a set of tasks that relate to each other
loosely, called coincidentally cohesive.

Coupling
• Coupling depends on the interface complexity between
modules, the point at which entry or reference is made to a
module, and what data pass across the interface
• In software design, we strive for lowest possible coupling.
Simple connectivity among modules results in software that
is easier to understand and less prone to a "ripple effect"
caused when errors occur at one location and propagate
through a system.
• It occur because of design decisions made when structure
was developed.

Coupling
• Coupling is characterized by passage of control between
modules.
• “Control flag” (a variable that controls decisions in a
subordinate or superordinate module) is passed between
modules d and e (called control coupling).
• Relatively high levels of coupling occur when modules are
communicate with external to software.
• External coupling is essential, but should be limited to a
small number of modules with a structure.

• High coupling also occurs when a number of modules
reference a global data area.
• Common coupling, no. of modules access a data item in a
global data area
• So it does not mean “use of global data is bad”. It does
mean that a software designer must be take care of this
thing.

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.
• Later work proposed methods for the translation of data
flow or data structure into a design definition.
• Today, the emphasis in software design has been on
software architecture and the design patterns that can be
used to implement software architectures.

Characteristics are common to all design
methods
• A mechanism for the translation of analysis model
into a design representation,
• A notation for representing functional components
and their interfaces.
• Heuristics for refinement and partitioning
• Guidelines for quality assessment.

Design quality attributes
• Acronym FURPS –
• Functionality
• Usability
• Reliability
• Performance
• Supportability

• Functionality – is assessed by evaluating the feature set and
capabilities of the program.
• Functions that are delivered and security of the overall
system.
• Usability – assessed by considering human factors, consistency
& documentation.
• Reliability – evaluated by
• measuring the frequency and severity of failure.
• Accuracy of output results.
• Ability to recover from failure and predictability of the
program.
• Performance - measured by processing speed, response time,
resource consumption, efficiency.
• Supportability – combines the ability to extend the program
(extensibility), adaptability and serviceability.

Chapter 6 design

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