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Chapter 9 - Structured Analysis Wiki

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Chapter 9
From Structured Analysis Wiki

Dataflow Diagrams
“Form ever follows function.”
-- Louis Henri Sullivan (http://en.wikipedia.org/wiki/Louis_Sullivan)
“The Tall Office Building Artistically Considered,”(http://www.njit.edu/v2/Library/archlib/pub-domain/sullivan-1896-tallbldg.html) Lippincott’s Magazine, March 1896
UGG Bailey Button (http://www.baileybuttonugg.net)

Contents
1 INTRODUCTION
2 THE COMPONENTS OF A DFD
2.1 The Process
2.2 The Flow
2.3 The Store
2.4 The Terminator
3 GUIDELINES FOR CONSTRUCTING DFDs
3.1 Choosing Meaningful Names
3.2 Number the Processes
3.3 Avoid Overly Complex DFDs
3.4 Redraw the DFD As Many Times As Necessary3.5 Make Sure That Your DFD Is Logically Consistent
4 LEVELED DFDs
5 Extensions to the DFD for real-time systems
6 SUMMARY
7 REFERENCES
8 QUESTIONS AND EXERCISES
9 ENDNOTES

INTRODUCTION
IN THIS CHAPTER, YOU WILL LEARN:

1.
2.
3.
4.

The components of a dataflow diagram (http://en.wikipedia.org/wiki/Dfd) ;
How to draw a simple dataflow diagram;
Guidelines for drawingsuccessful dataflow diagrams; and
How to draw leveled dataflow diagrams.

In this chapter, we will explore one of the three major graphical modeling tools of structured analysis: the dataflow

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Chapter 9 - Structured Analysis Wiki

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diagram. The dataflow diagram is a modeling tool that allows usto picture a system as a network of functional processes,
connected to one another by “pipelines” and “holding tanks” of data. In the computer literature, and in your conversations
with other systems analysts and business users, you may use any of the following terms as synonyms for dataflow
diagram:

Bubble chart
DFD (the abbreviation we will use throughout this book)
Bubble diagramProcess model (or business process model)
Business flow model
Work flow diagram
Function model
“A picture of what’s going on around here”
The dataflow diagram is one of the most commonly used systems-modeling tools, particularly for operational systems in
which the f unctions of the system are of paramount importance and more complex than the data that the system
manipulates. DFDs werefirst used in the software engineering field as a notation for studying systems design issues (e.g.,
in early structured design books and articles such as (Stevens, Myers, and Constantine (http://www.research.ibm.com
/journal/sj/132/ibmsj1302C.pdf) . 1974), (Yourdon and Constantine (http://www.amazon.com/exec/obidos
/ASIN/0138544719/edyourdonswebsit) , 1975), (Myers, 1975), et al.). In turn,the notation had been borrowed from earlier
papers on graph theory, and it continues to be used as a convenient notation by software engineers concerned with direct
implementation of models of user requirements.
This is interesting background, but is likely to be irrelevant to the users to whom you show DFD system models; indeed,
probably the worst thing you can do is say, “Mr. User, I’dlike to show you a top-down, partitioned, graph-theoretic model
of your system.” Actually, many users will be familiar with the underlying concept of DFDs, because the same kind of
notation has been used by operations research scientists for nearly a century to build work-flow models of organizations.
This is important to keep in mind: DFDs can be used not only to model information-processingsystems, but also as a way
of modeling whole organizations, that is, as a tool for business planning and strategic planning.
We will begin our study of dataflow diagrams by examining the components of a typical dataflow diagram: the process, the
flow, the store, and the terminator. We will use a fairly standard notation for DFDs, following the notation of such classic
books as (DeMarco...