25-04-2012, 04:42 PM
Dataflow Diagrams
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In this chapter, we will explore one of the three major graphical modeling tools of structured analysis: the
dataflow diagram. The dataflow diagram is a modeling tool that allows us to 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 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 diagram
* Process model (or business process model)
* Business flow model
* Work flow diagram
* Function model
* “A picture of what’s going on around here”
The components of a DFD
a typical DFD for a small system. Before we examine its components in detail, notice
several things:
* It hardly needs to be explained at all; one can simply look at the diagram and understand it. The
notation is simple and unobtrusive and, in a sense, intuitively obvious. This is particularly important
when we remember who is supposed to be looking at Figure 9.1 — not the systems analyst, but the
user! If the user needs an encyclopedia in order to read and understand the model of his system, he or
she probably won’t bother to do either.
* The diagram fits easily onto one page. This means two things: (1) someone can look at the diagram
without being overwhelmed, and (2) the system that is being modeled by the diagram is not very complex.
What do we do if the system is intrinsically complex, for example, so complex that there would be
literally hundreds of circles and lines in the diagram? We will discuss this in Section
Summary
As we have seen in this chapter, the dataflow diagram is a simple but powerful tool for modeling the functions
in a system. The material in Sections 9.1, 9.2, and 9.3 should be sufficient for modeling most classical
business-oriented information systems. If you are working on a real-time system (e.g., process control, missile
guidance, or telephone switching), the real-time extensions discussed in Section 9.4 will be important;
for more detail on real-time issues, consult (Ward and Mellor, 1985).