51 terms

Chapter 8 - Design Concepts

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Define a software design
a meaningful engineering representation of some software product that is to be built.
Discuss roughly the main reasons for a software design.
Designers must strive to acquire a repertoire of alternative design information and learn to choose the elements that best match the analysis model.
What is the connection between customer's requirements and and design
A design can be traced to the customer's requirements and can be assessed for quality against predefined criteria.
Expound on what occurs in the design process
During the design process the software requirements model is transformed into design models that describe the details of the data structures, system architecture, interfaces, and components. Each design product is reviewed for quality (i.e., identify and correct errors, inconsistencies, or omissions) before moving to the next software engineering action.
Describe Design Engineering
Encompasses the set of principles, concepts, and practices that lead to the development of a high quality system or product
Describe Design principles
establish an overriding philosophy that guides the designer as the work is performed
Describe Design concepts
must be understood before the mechanics of design practice are applied
What is the main goal of design engineering
is to produce a model or representation that is bug free (firmness), suitable for its intended uses (commodity), and pleasurable to use (delight)
List the Design Specification Models
Data/Class design
Architectural design
Interface design
Component-level design
Define Data/Class design
created by transforming the analysis model class-based elements (class diagrams, analysis packages, CRC models, collaboration diagrams) into classes and data structures required to implement the software
Define 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 (data flow diagrams, control flow diagrams, processing narratives) of the analysis model
Define 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 (use-case text, use-case diagrams, activity diagrams, swim lane diagrams), flow-oriented elements, and behavioral elements (state diagrams, sequence diagrams)
Define 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 form the analysis model class-based elements, flow-oriented elements, and behavioral elements
Software Quality Attributes
A good software must:
1. implement all explicit requirements from the analysis model and accommodate all implicit requirements desired by the user
2. be readable and understandable guide for those who generate code, test components, or support the system.
3. provide a complete picture (data, function, behavior) if the software from an implementation perspective
Design Quality Guidelines
A design should:
1.exhibit good architectural structure
2.be modular
3.contain distinct representations of data, architecture, interfaces, and components (modules)
4.lead to data structures that are appropriate for the objects to be implemented and be drawn from recognizable design patterns
5.lead to components that exhibit independent functional characteristics
6.lead to interfaces that reduce the complexity of connections between modules and with the external environment
7.be derived using a reputable method that is driven by information obtained during software requirements analysis
FURPS Quality Factors
Functionality
Usability
Reliability
Performance
Supportability
Generic Design Task Set
1. Select an architectural pattern appropriate to the software based on the analysis model

2. Partition the analysis model into design subsystems, design interfaces, and allocate analysis functions (classes) to each subsystem

3. Examine information domain model and design appropriate data structures for data objects and their attributes

4. Create a set of design classes

5. Translate analysis class into design class

6. Check each class against design criteria and consider inheritance issues

7. Define methods and messages for each design class
Select design patterns for each design class or subsystem after considering alternatives

8. Revise design classes and revise as needed

9. Design user interface

10. Review task analyses
Design Concepts
Abstraction
Software Architecture
Design Patterns
Modularity
Information Hiding
Functional Independence
Refinement
Refactoring
Objects
Class
Inheritance
Messages
Polymorphism
Design Classes
Modular Design Method Evaluation Criteria
Modular decomposability - provides systematic means for breaking problem into subproblems
Modular composability - supports reuse of existing modules in new systems
Modular understandability - module can be understood as a stand-alone unit
Modular continuity - side-effects due to module changes minimized
Modular protection - side-effects due to processing errors minimized
Effective Modular Design
Functional independence - modules have high cohesion and low coupling
Cohesion - qualitative indication of the degree to which a module focuses on just one thing
Coupling - qualitative indication of the degree to which a module is connected to other modules and to the outside world
Design Class Characteristics
Complete and sufficient
Primitiveness - each class method focuses on providing one service
High cohesion
Low coupling
Design Model
Process dimension (indicates design model evolution as design tasks are executed during software process) :
1. Architecture elements
2. Interface elements
3. Component-level elements
4. Deployment-level elements

Abstraction dimension (represents level of detail as each analysis model element is transformed into a design equivalent and refined):
1. High level (analysis model elements)
2. Low level (design model elements)

Many UML diagrams used in the design model are refinements of diagrams created in the analysis model
Data Design
High level model depicting user's view of the data or information
Design of data structures and operators is essential to creation of high-quality applications
Translation of data model into database is critical to achieving system business objectives
Reorganizing databases into data warehouse enables data mining or knowledge discovery that can impact success of business itself
Architectural Design
Derived from
1. Information about the application domain relevant to software
2. Relationships and collaborations among specific analysis model elements
3. Availability of architectural patterns and styles
Usually depicted as a set of interconnected systems that are often derived from the analysis packages
Interface Design
Interface is a set of operations that describes the externally observable behavior of a class and provides access to its operations
Component-Level Design
Describes the internal detail of each software component
Defines
- Data structures for all local data objects
- Algorithmic detail for all component processing functions
- Interface that allows access to all component operations
Deployment-Level Design
- Indicates how software functionality and subsystems will be allocated within the physical computing environment
- Modeled using UML deployment diagrams
Design Pattern Template
Name
Intent
Aliases
Motivation
Applicability
Structure
Participants
Collaborators
Consequences
Related patterns
Using Patterns in Design
Architectural patterns - define overall structure and relationships of software components (classes, packages, subsystems)

Design patterns - address specific design elements (component aggregations, component relationships, or communication)

Idioms (coding patterns) - language specific implementations for algorithms or communications mechanisms
Frameworks
Collection of "plug points" hooks or slots that enable it to be adapted to a specific problem domain

Plug points enable designers to integrate problem specific functionality within the skeleton

In an object-oriented context a skeleton is a collection of cooperating classes
The importance of software design can be summarized in a single word
Quality
Which of the following are areas of concern in the design model?
A) architecture
B) data
C) interfaces
Which of the following is not a characteristic common to all design methods?
configuration management
Which of these are characteristics of a good design?
implements all requirements in the analysis model

provides a complete picture of the software
Software design is an iterative generic process that may be applied without modification to any software project.
False
What types of abstraction are used in software design?
A) control
B) data
D) procedural
Which models can be used to represent the architectural design of a piece of software.
A) Dynamic models
B) Functional models
C) Structural models
Design patterns are not applicable to the design of object-oriented software?
False
Since modularity is an important design goal it is not possible to have too many modules in a proposed design.
False
Information hiding makes program maintenance easier by hiding data and procedure from unaffected parts of the program.
True
Cohesion is a qualitative indication of the degree to which a module
focuses on just one thing.
Coupling is a qualitative indication of the degree to which a module
is connected to other modules and the outside world.
When using structured design methodologies the process of stepwise refinement is unnecessary.
False
Software designs are refactored to allow the creation of software that is easier to integrate, easier to test, and easier to maintain.
True
Inheritance provides a mechanism by which changes to lower level classes can be propagated to all super classes quickly.
False
Polymorphism reduces the effort required to extend an object system by
enabling a number of different operations to share the same name.
3 of the five design class types
A) Business domain classes
C) Process classes
D) User interface classes
Which design model elements are used to depict a model of information represented from the user's view?
Data design elements
Which design is analogous to the floor plan of a house?
Architectural design
The deployment design elements specify the build order for the software components.
False
Frameworks and design patterns are the same thing as far as designers are concerned.
False
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