Partitioning the Analysis Model
22.2.1 Partitioning the Analysis Model
One of the fundamental analysis principles (Chapter 11) is partitioning. In OO system design, we partition the analysis model to define cohesive collections of classes, rela-
The concepts of tionships, and behavior. These design elements are packaged as a subsystem. coupling and cohesion
In general, all of the elements of a subsystem share some property in common. (Chapter 13) can be
They all may be involved in accomplishing the same function; they may reside within applied at the
the same product hardware, or they may manage the same class of resources. Sub- subsystem level. Strive
to achieve good systems are characterized by their responsibilities; that is, a subsystem can be iden- functional
tified by the services that it provides [RUM91]. When used in the OO system design independence as you
context, a service is a collection of operations that perform a specific function (e.g., design subsystems.
managing word-processor files, producing a three-dimensional rendering, translat- ing an analog video signal into a compressed digital image).
As subsystems are defined (and designed), they should conform to the following design criteria:
? What
• The subsystem should have a well-defined interface through which all com-
criteria
munication with the rest of the system occurs.
guide us in the
• With the exception of a small number of “communication classes,” the
design of
classes within a subsystem should collaborate only with other classes within
subsystems?
the subsystem. • The number of subsystems should be kept low. •
A subsystem can be partitioned internally to help reduce complexity. When two subsystems communicate with one another, they can establish a
client/server link or a peer-to-peer link [RUM91]. In a client/server link, each of the subsystems takes on one of the roles implied by client and server. Service flows from server to client in only one direction. In a peer-to-peer link, services may flow in either direction.
When a system is partitioned into subsystems, another design activity, called lay- ering, also occurs. Each layer [BUS96] of an OO system contains one or more sub- systems and represents a different level of abstraction of the functionality required to accomplish system functions. In most cases, the levels of abstraction are deter- mined by the degree to which the processing associated with a subsystem is visible to an end-user.
CHAPTER 22
OBJECT-ORIENTED DESIGN
For example, a four-layer architecture might might include (1) a presentation layer (the subsystems associated with the user interface), (2) an application layer (the sub- systems that perform the processing associated with the application), (3) a data for- matting layer (the subsystems that prepare the data for processing), and (4) a database layer (the subsystems associated with data management). Each layer moves deeper into the system, representing increasingly more environment-specific processing.
Buschmann and his colleagues [BUS96] suggest the following design approach for layering:
? 1. Establish layering criteria. That is, decide how subsystems will be grouped in
How do I
create a
a layered architecture.
layered design?
2. Determine the number of layers. Too many introduce unnecessary complex- ity; too few may harm functional independence.
3. Name the layers and allocate subsystems (with their encapsulated classes) to
a layer. Be certain that communication between subsystems (classes) on one layer and other subsystems (classes) at another layer follow the design phi- losophy for the architecture. 8
4. Design interfaces for each layer.
5. Refine the subsystems to establish the class structure for each layer.
6. Define the messaging model for communication between layers.
7. Review the layer design to ensure that coupling between layers is minimized (a client/server protocol can help accomplish this).
8. Iterate to refine the layered design.
Parts
» The Concurrent Development Model
» SUMMARY Software engineering is a discipline that integrates process, methods, and tools for
» PEOPLE In a study published by the IEEE [CUR88], the engineering vice presidents of three
» THE PROCESS The generic phases that characterize the software process—definition, development,
» THE PROJECT In order to manage a successful software project, we must understand what can go
» METRICS IN THE PROCESS AND PROJECT DOMAINS
» Extended Function Point Metrics
» METRICS FOR SOFTWARE QUALITY
» INTEGRATING METRICS WITHIN THE SOFTWARE PROCESS
» METRICS FOR SMALL ORGANIZATIONS
» ESTABLISHING A SOFTWARE METRICS PROGRAM
» Obtaining Information Necessary for Scope
» An Example of LOC-Based Estimation
» QUALITY CONCEPTS 1 It has been said that no two snowflakes are alike. Certainly when we watch snow
» SUMMARY Software quality assurance is an umbrella activity that is applied at each step in the
» R diagram 1.4 <part-of> data model; data model <part-of> design specification;
» SYSTEM MODELING Every computer-based system can be modeled as an information transform using an
» Facilitated Application Specification Techniques
» Data Objects, Attributes, and Relationships
» Entity/Relationship Diagrams
» Hatley and Pirbhai Extensions
» Creating an Entity/Relationship Diagram
» SUMMARY Design is the technical kernel of software engineering. During design, progressive
» Data Modeling, Data Structures, Databases, and the Data Warehouse
» Data Design at the Component Level
» A Brief Taxonomy of Styles and Patterns
» Quantitative Guidance for Architectural Design
» Isolate the transform center by specifying incoming and outgoing
» SUMMARY Software architecture provides a holistic view of the system to be built. It depicts the
» The User Interface Design Process
» Defining Interface Objects and Actions
» D E S I G N E VA L U AT I O N
» Testing for Real-Time Systems
» Organizing for Software Testing
» Criteria for Completion of Testing
» The Transition to a Quantitative View
» The Attributes of Effective Software Metrics
» Architectural Design Metrics
» Component-Level Design Metrics
» SUMMARY Software metrics provide a quantitative way to assess the quality of internal product
» Encapsulation, Inheritance, and Polymorphism
» Identifying Classes and Objects
» The Common Process Framework for OO
» OO Project Metrics and Estimation
» Event Identification with Use-Cases
» SUMMARY Object-oriented analysis methods enable a software engineer to model a problem by
» Partitioning the Analysis Model
» Designing Algorithms and Data Structures
» Program Components and Interfaces
» SUMMARY Object-oriented design translates the OOA model of the real world into an
» Testing Surface Structure and Deep Structure
» Deficiencies of Less Formal Approaches 1
» What Makes Cleanroom Different?
» Design Refinement and Verification
» SUMMARY Cleanroom software engineering is a formal approach to software development that
» Structural Modeling and Structure Points
» Describing Reusable Components
» SUMMARY Component-based software engineering offers inherent benefits in software quality,
» Guidelines for Distributing Application Subsystems
» Middleware and Object Request Broker Architectures
» An Overview of a Design Approach
» Consider expert Web developer will create a complete design, but time and cost can be appropriate
» A Software Reengineering Process Model
» Reverse Engineering to Understand Data
» Forward Engineering for Client/Server Architectures
» SUMMARY Reengineering occurs at two different levels of abstraction. At the business level,
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