Lecture 8: Chapter 8
◼ Design Concepts
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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.
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Analysis Model -> Design Model
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Design and Quality Goals
◼ 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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How to achieve the Quality
◼ 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.
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Fundamental Concepts in Design
◼ 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—manifestation of separation of concerns
◼ Information Hiding—controlled interfaces, no details of algorithms/data
◼ 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
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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
Describes the door object 7
Procedural Abstraction
open
details of enter
algorithm
implemented with a "knowledge" of the
object that is associated with enter
Sequence of instructions for a function
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Software 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.
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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
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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.
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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.
◼ BUT: Pay attention to integration costs too.
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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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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.
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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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Aspects
◼ From the requirements analysis
◼ Use case, feature, data structure, etc.
◼ 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.
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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.
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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.
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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
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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.
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The Design Model
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Design Model Elements
◼ Data elements
◼ Data model --> data structures
◼ Data model --> database architecture
◼ Architectural elements
◼ Like floor plan of a house
◼ 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
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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).
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Interface Elements
Like windows, doors, etc. of a house
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Component Elements
◼ Specifies the details of components
◼ Similar to the plumbing, electrical, details of every
room in a floor plan
◼ SensorManagement performs all functions
regarding sensors
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Deployment Elements
◼ How subsystems will
be allocated in the
physical environment
◼ Computing
environment but no
details about hardware
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