Design concepts in software engineering presentation

Prof.S.M.Sangve(ref. Soft. Engg. by Roger Pressman) 1
Design Concepts

Design
 Mitch Kapor, the creator of Lotus 1-2-3,
presented a “software design manifesto” in Dr.
Dobbs Journal. He said:
 Good software design should exhibit:
 Firmness: A program should not have any bugs that
inhibit its function.
 Commodity: A program should be suitable for the
purposes for which it was intended.
 Delight: The experience of using the program should
be pleasurable one.

Analysis Model -> Design Model
Analysis Model
use-cases - text
use-case diagrams
activity diagrams
swim lane diagrams
data flow diagrams
control-flow diagrams
processing narratives
f low- or ient ed
element s
behavior al
element s
class- based
element s
scenar io- based
element s
class diagrams
analysis packages
CRC models
collaboration diagrams
state diagrams
sequence diagrams
Da t a / Cla ss De sign
Arc hit e c t ura l De sign
Int e rfa c e De sign
Com pone nt -
Le v e l De sign
Design Model

Design and 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.

Quality Guidelines
 A design should exhibit an architecture that (1) has been created using
recognizable architectural styles or patterns, (2) is composed of components
that exhibit good design characteristics and (3) can be implemented in an
evolutionary fashion
 For smaller systems, design can sometimes be developed linearly.
 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.
 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
 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.
 The design should “minimize the intellectual distance” [DAV95] between
the software and the problem as it exists in the real world.
 The design should exhibit uniformity and integration.
 The design should be structured to accommodate change.
 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.
From Davis [DAV95]

Fundamental Concepts
 Abstraction—data, procedure, control
 Architecture—the overall structure of the software
 Patterns—”conveys the essence” of a proven design solution
 Separation of concerns—any complex problem can be more easily
handled if it is subdivided into pieces
 Modularity—compartmentalization of data and function
 Hiding—controlled interfaces
 Functional independence—single-minded function and low coupling
 Refinement—elaboration of detail for all abstractions
 Aspects—a mechanism for understanding how global requirements
affect design
 Refactoring—a reorganization technique that simplifies the design
 OO design concepts—Appendix II
 Design Classes—provide design detail that will enable analysis
classes to be implemented

Data Abstraction
door
implemented as a data structure
manufacturer
model number
type
swing direction
inserts
lights
type
number
weight
opening mechanism

Procedural Abstraction
open
implemented with a "knowledge" of the
object that is associated with enter
details of enter
algorithm

Architecture
“The overall structure of the software and the ways in
which that structure provides conceptual integrity for a
system.” [SHA95a]
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
Extra-functional 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.

Patterns
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
Also-known-as—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 trade-offs that must be considered when the pattern is
implemented
Related patterns—cross-references related design patterns

Separation of Concerns
 Any complex problem can be more easily
handled if it is subdivided into pieces that can
each be solved and/or optimized independently
 A concern is a feature or behavior that is
specified as part of the requirements model for
the software
 By separating concerns into smaller, and
therefore more manageable pieces, a problem
takes less effort and time to solve.

Modularity
 "modularity is the single attribute of software that allows
a program to be intellectually manageable" [Mye78].
 Monolithic software (i.e., a large program composed of a
single module) cannot be easily grasped by a software
engineer.
 The number of control paths, span of reference, number of
variables, and overall complexity would make
understanding close to impossible.
 In almost all instances, you should break the design into
many modules, hoping to make understanding easier
and as a consequence, reduce the cost required to build
the software.

Modularity: Trade-offs
What is the "right" number of modules
for a specific software design?
optimal number
of modules
cost of
software
number of modules
module
integration
cost
module development cost

Information Hiding
module
controlled
interface
"secret"
• algorithm
• data structure
• details of external interface
• resource allocation policy
clients
a specific design decision

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

Stepwise Refinement
open
walk to door;
reach for knob;
open door;
walk through;
close door.
repeat until door opens
turn knob clockwise;
if knob doesn't turn, then
take key out;
find correct key;
insert in lock;
endif
pull/push door
move out of way;
end repeat

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

Functional Independence
 Functional independence is achieved by developing
modules with "single-minded" function and an "aversion"
to excessive interaction with other modules.
 Cohesion is an indication of the relative functional
strength of a module.
 A cohesive module performs a single task, requiring little
interaction with other components in other parts of a
program. Stated simply, a cohesive module should (ideally)
do just one thing.
 Coupling is an indication of the relative interdependence
among modules.
 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.

Aspects
 Consider two requirements, A and B.
Requirement A crosscuts requirement B “if a
software decomposition [refinement] has been
chosen in which B cannot be satisfied without
taking A into account. [Ros04]
 An aspect is a representation of a cross-cutting
concern.

Aspects—An Example
 Consider two requirements for the SafeHomeAssured.com WebApp.
Requirement A is described via the use-case Access camera
surveillance via the Internet. A design refinement would focus on
those modules that would enable a registered user to access video
from cameras placed throughout a space. Requirement B is a generic
security requirement that states that a registered user must be
validated prior to using SafeHomeAssured.com. This requirement is
applicable for all functions that are available to registered SafeHome
users. As design refinement occurs, A* is a design representation for
requirement A and B* is a design representation for requirement B.
Therefore, A* and B* are representations of concerns, and B* cross-
cuts A*.
 An aspect is a representation of a cross-cutting concern. Therefore,
the design representation, B*, of the requirement, a registered user
must be validated prior to using SafeHomeAssured.com, is an aspect
of the SafeHome WebApp.

Refactoring
 Fowler [FOW99] 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 corrected to yield a better
design.

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 extend the design

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 or between the
user and the application.

The Design Model
process dimension
archit ect ure
element s
int erface
element s
component -level
element s
deployment -level
element s
low
high
class diagrams
analysis packages
CRCmodels
use-cases - text
use-case diagrams
activity diagrams
swim lane diagrams
collaboration diagrams data flow diagrams
data flow diagrams
state diagrams
sequence diagrams
state diagrams
sequence diagrams
design class realizations
subsystems
subsystems
refinements to:
deployment diagrams
class diagrams
analysis packages
CRC models
component diagrams
design classes
activity diagrams
sequence diagrams
refinements to:
component diagrams
design classes
activity diagrams
sequence diagrams
subsystems
component diagrams
design classes
activity diagrams
sequence diagrams
analysis model
design model
Requirements:
constraints
interoperability
targets and
configuration
technical interface
design
Navigation design
GUIdesign

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” (Chapters 9 and 12)
 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
 Deployment elements

Architectural Elements
 The architectural model [Sha96] is derived from
three sources:
 information about the application domain for the
software to be built;
 specific requirements model elements such as data
flow diagrams or analysis classes, their relationships
and collaborations for the problem at hand, and
 the availability of architectural patterns (Chapter 12)
and styles (Chapter 9).

Interface Elements
Cont rolPanel
LCDdisplay
LEDindicators
keyPadCharacteristics
speaker
wirelessInterface
readKeyStroke()
decodeKey ()
displayStatus()
lightLEDs()
sendControlMsg()
Figure 9 .6 UML int erface represent at ion forCont rolPa ne l
KeyPad
readKeystroke()
decodeKey()
<<int erface>>
WirelessPDA
KeyPad
MobilePhone

Component Elements
SensorManagement
Sensor

Deployment Elements
Figure 9 .8 UML deployment diagram forSafeHome
Personal computer
Security
homeManagement
Surveillance
communication
Cont rol Panel CPI server
Security homeownerAccess
externalAccess

Design concepts in software engineering presentation

More Related Content

Similar to Design concepts in software engineering presentation

Recently uploaded

Design concepts in software engineering presentation