Data Flow Diagrams
12.4.1 Data Flow Diagrams
As information moves through software, it is modified by a series of transformations.
A data flow diagram is a graphical representation that depicts information flow and The DFD provides a
the transforms that are applied as data move from input to output. The basic form mechanism for
of a data flow diagram, also known as a data flow graph or a bubble chart, is illus- information flow
trated in Figure 12.10. modeling and
The data flow diagram may be used to represent a system or software at any level functional modeling.
of abstraction. In fact, DFDs may be partitioned into levels that represent increasing information flow and functional detail. Therefore, the DFD provides a mechanism for functional modeling as well as information flow modeling. In so doing, it satisfies the second operational analysis principle (i.e., creating a functional model) discussed in Chapter 11.
A level 0 DFD, also called a fundamental system model or a context model, repre- sents the entire software element as a single bubble with input and output data indi- cated by incoming and outgoing arrows, respectively. Additional processes (bubbles)
Refinement from one and information flow paths are represented as the level 0 DFD is partitioned to reveal DFD level to the next
more detail. For example, a level 1 DFD might contain five or six bubbles with inter- should follow an
connecting arrows. Each of the processes represented at level 1 is a subfunction of approximate 1:5 ratio,
reducing as the the overall system depicted in the context model. refinement proceeds.
As we noted earlier, each of the bubbles may be refined or layered to depict more detail. Figure 12.11 illustrates this concept. A fundamental model for system F indi- cates the primary input is A and ultimate output is B. We refine the F model into trans-
forms f 1 to f 7 . Note that information flow continuity must be maintained; that is, input
F I G U R E 12.11
Information flow
refinement
PA R T T H R E E C O N V E N T I O N A L M E T H O D S F O R S O F T WA R E E N G I N E E R I N G
and output to each refinement must remain the same. This concept, sometimes called balancing, is essential for the development of consistent models. Further refinement of f depicts detail in the form of transforms f to f . Again, the input (X, Y) and out-
Although information
flow continuity must put (Z) remain unchanged.
be maintained, The basic notation used to develop a DFD is not in itself sufficient to describe recognize that a data
requirements for software. For example, an arrow shown in a DFD represents a data item represented at
object that is input to or output from a process. A data store represents some orga- one level may be
refined into its nized collection of data. But what is the content of the data implied by the arrow or constituent parts at the depicted by the store? If the arrow (or the store) represents a collection of objects,
next level. what are they? These questions are answered by applying another component of the basic notation for structured analysis—the data dictionary. The use of the data dic- tionary is discussed later in this chapter.
DFD graphical notation must be augmented with descriptive text. A process spec- ification (PSPEC) can be used to specify the processing details implied by a bubble within a DFD. The process specification describes the input to a function, the algo- rithm that is applied to transform the input, and the output that is produced. In addi- tion, the PSPEC indicates restrictions and limitations imposed on the process (function), performance characteristics that are relevant to the process, and design constraints that may influence the way in which the process will be implemented.
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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