BITS - SEWP ZC461

Software Engineering

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 Analysis Model -> Design
Model

Component -
sc enario- based f low- oriented L evel Design
elements elements
use-cases - text data flow diagrams
use-case diagrams control-flow diagrams
activity diagrams processing narratives
swim lane diagrams
Int erfac e Design
Analysis Model

A rc hit ec t ural Design

c lass- based behavioral
elements elements
class diagrams state diagrams
analysis packages sequence diagrams
CRC models Dat a/ Class Design
collaboration diagrams

Design Model
What makes a quality design? 2
The Design Process
Design is an iterative process to
translate requirements into a
“blueprint”

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Design and Software
Quality
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
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Design Quality Criteria
A design should exhibit an architecture
 (1) has been created using recognizable architectural styles or
patterns,
 (2) is composed of components that exhibit good design
characteristics
 (3) 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.
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Design Quality Criteria
contd.
A design should lead to components that
exhibit independent functional characteristics
A design should lead to interfaces that reduce
the complexity of connections between
components and with the external
environment
A design should be represented using a
notation that effectively communicates its
meaning
A design should be derived using a repeatable
method driven by information obtained during
requirement analysis 6
Design
Principles
*Software design is both a process and a model*

The design process should not suffer from ‘tunnel

vision’
The design should be traceable to the analysis
model
The design should not reinvent the wheel – Design
patterns
The design should “minimize the intellectual
distance” between the software and the problem
as it exists in the real world
The design should exhibit uniformity and
integration 7
Design Principles
The design should be structured to degrade
gently, even when aberrant 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
Quality Factors
 External
 Observed by users – speed, reliability, correctness etc.
 Internal
 Important for software engineers
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Design
Concepts
Abstraction—data, procedure,
control
Architecture—the overall structure of
the software – conceptual integrity
of system
Patterns—”conveys the essence” of
a proven design solution
Modularity—compartmentalization of
data and function 9
Design Concepts contd.
Hiding—controlled interfaces
Functional independence—single-minded
function and low coupling
Refinement—elaboration of detail for all
abstractions
Structural Partitioning
 Horizontal
 Vertical – also called Factoring
 a reorganization technique that simplifies the
design
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Data
Abstraction door

manufacturer
model number
type
swing direction
inserts
lights
   type
   number
weight
opening mechanism

implemented as a data structure

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Procedural
Abstraction
open

details of enter 
algorithm

implemented with a "knowledge" of the  
object that is associated with enter

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Architecture
“The overall structure of the software and the ways in 
which that structure provides conceptual integrity for a 
system.” 
Structural properties.  This aspect of the architectural design representation 
defines the components of a system (e.g., modules, objects, filters) and the manner 
in which those components are packaged and interact with one another. For 
example, objects are packaged to encapsulate both data and the processing that 
manipulates the data and interact via the invocation of methods 
Extra­functional properties.  The architectural design description should address 
how the design architecture achieves requirements for performance, capacity, 
reliability, security, adaptability, and other system characteristics.
Families of related systems.  The architectural design should draw upon 
repeatable patterns that are commonly encountered in the design of families of 
similar systems. In essence, the design should have the ability to reuse 
architectural building blocks. 

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Pattern
s
Design Pattern Template
Pattern name—describes the essence of the pattern in a short but expressive name 
Intent—describes the pattern and what it does
Also­known­as—lists any synonyms for the pattern
Motivation—provides an example of the problem 
Applicability—notes specific design situations in which the pattern is applicable
Structure—describes the classes that are required to implement the pattern
Participants—describes the responsibilities of the classes that are required to implement 
the pattern
Collaborations—describes how the participants collaborate to carry out their 
responsibilities
Consequences—describes the “design forces” that affect the pattern and the potential 
trade­offs that must be considered when the pattern is implemented
Related patterns—cross­references related design patterns

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Modular
Design
easier to build, easier to change, easier to fix ...

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Modularity: Trade-
offs
What is the "right" number of modules 
for a specific software design?

module development cost 

      cost of
    software
module
integration
cost

optimal number number of modules
   of modules
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Information Hiding
module •  algorithm
controlled
interface •  data structure
•  details of external interface
•  resource allocation policy

clients "secret"

a specific design decision

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Why Information
Hiding?
reduces the likelihood of “side effects”
limits the global impact of local design
decisions
emphasizes communication through
controlled interfaces
discourages the use of global data
leads to encapsulation—an attribute of
high quality design
results in higher quality software
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Stepwise
Refinement
open

walk to door;
reach for knob;
open door; repeat until door opens
turn knob clockwise;
walk through; if knob doesn't turn, then
close door.     take key out;
    find correct key;
    insert in lock;
endif
pull/push door
move out of way;
end repeat
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Effective Modular Design
Functional Independence
 Is achieved by single-minded
development
 Is measured by two qualitative
criteria
 Cohesion
 Coupling

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Functional
Independence
COHESION  ­  the degree to which a 
module performs one and only one 
function. 
 
COUPLING  ­  the degree to which a 
module is "connected" to other 
modules in the system.

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Sizing Modules: Two
Views
What's  How big 
inside?? is it??

MODULE

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Refactorin
g
Fowler defines refactoring in the following
manner:
 "Refactoring is the process of changing a
software system in such a way that it does not
alter the external behavior of the code [design]
yet improves its internal structure.”
When software is refactored, the existing
design is examined for
 redundancy
 unused design elements
 inefficient or unnecessary algorithms
 poorly constructed or inappropriate data
structures
 or any other design failure that can be 23
OO Design
Concepts
Design classes
 Entity classes
 Boundary classes
 Controller classes
Inheritance—all responsibilities of a
superclass is immediately inherited by all
subclasses
Messages—stimulate some behavior to
occur in the receiving object
Polymorphism—a characteristic that
greatly reduces the effort required to 24
Design
Classes
Analysis classes are refined during design to become entity
classes
Boundary classes are developed during design to create
the interface (e.g., interactive screen or printed reports)
that the user sees and interacts with as the software is
used.
 Boundary classes are designed with the responsibility of

managing the way entity objects are represented to

users.
Controller classes are designed to manage
 the creation or update of entity objects;

 the instantiation of boundary objects as they obtain

information from entity objects;
 complex communication between sets of objects;
 validation of data communicated between objects or25
Inheritanc
e
Design options:
 The class can be designed and built from scratch. That
is, inheritance is not used
 The class hierarchy can be searched to determine if a
class higher in the hierarchy (a superclass)contains most
of the required attributes and operations. The new class
inherits from the superclass and additions may then be
added, as required
 Characteristics of an existing class can be overridden
and different versions of attributes or operations are
implemented for the new class.

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Messages

:SenderObject

message (<parameters>)
:ReceiverObject

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Polymorphis
m
Conventional approach …

case of graphtype:
if graphtype = linegraph then DrawLineGraph (data);
if graphtype = piechart then DrawPieChart (data);
if graphtype = histogram then DrawHisto (data);
if graphtype = kiviat then DrawKiviat (data);
end case;

All of the graphs become subclasses of a general class called graph. 
Using a concept called overloading, each subclass defines an 
operation called draw. An object can send a draw message to any 
one of the objects instantiated from any one of the subclasses. The 
object receiving the message will invoke its own draw operation to 
create the appropriate graph. 

graphtype draw
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The Design
Model
high

analysis model

class diagrams
analysis packages
use-cases - text class diagrams
Requirements:
CRC models use-case diagrams constraints
analysis packages
collaboration diagrams
activity diagrams CRC models interoperability
data flow diagrams swim lane diagrams collaboration diagrams targets and
control-flow diagrams collaboration diagrams data flow diagrams
processing narratives state diagrams control-flow diagrams
conf iguration
sequence diagrams processing narratives
state diagrams
sequence diagrams

design class realizations

subsystems
collaboration diagrams technical interface component diagrams
design class realizations
design design classes
subsystems
Navigation design activity diagrams
collaboration diagrams
GUI design sequence diagrams
component diagrams
design model design classes
refinements to: activity diagrams
refinements to: sequence diagrams
component diagrams
design class realizations design classes
subsystems activity diagrams
low collaboration diagrams sequence diagrams deployment diagrams

architecture interface component-level deployment-level

elements elements elements elements
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process dimension
Design Model
Elements
Data elements
 Data model --> data structures
 Data model --> database architecture
Architectural elements
 Application domain
 Analysis classes, their relationships, collaborations and
behaviors are transformed into design realizations
 Patterns and “styles”
Interface elements
 the user interface (UI)
 external interfaces to other systems, devices, networks
or other producers or consumers of information
 internal interfaces between various design components.
Component elements
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Deployment elements
Interface
Elements
MobilePhone

WirelessPDA

Cont rolPanel

LCDdisplay
LEDindicators
keyPadCharacteristics K eyPad
speaker
wirelessInterface

readKeyStroke()
decodeKey ()
displayStatus()
lightLEDs()
sendControlMsg()

<<int erface>>
K eyPad

readKeystroke()
decodeKey()

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Figure 9 .6 UML int erface represent at ion for Cont rolPanel
Component
Elements

SensorManagement
Sensor

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Deployment
Elements
Cont rol Panel CPI server

Security homeownerAccess

Personal computer

externalAccess

Security Surveillance

homeManagement communication

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Figure 9 .8 UML deployment diagram for SafeHome
Framework
s
A framework is not an architectural
pattern, but rather a skeleton with a
collection of “plug points” (also called
hooks and slots) that enable it to be
adapted to a specific problem
domain.

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