Chapter 21 Working with the Data Flow Diagram

Data Flow Diagram Basics

What is a Data Flow Diagram?

The Data Flow Diagram (DFD) is a graphical representation of the flow of data through an information system. It enables you to represent the processes in your information system from the viewpoint of data. The DFD lets you visualize how the system operates, what the system accomplishes and how it will be implemented, when it is refined with further specification.

There are two types of DFDs:

Logical data flow diagrams are implementation-independent. They describe the system, rather than how activities are accomplished.
Physical data flow diagrams are implementation-dependent. They describe the actual entities (devices, department, people, etc.) involved in the current system.

Both types of data flow diagrams support a top-down approach to systems analysis, whereby analysts begin by developing a general understanding of the system and gradually explode components in greater details.

DFDs can also be grouped together to represent a sub-system of the system being analyzed.

A data flow diagram can look as follows:

Data flow diagrams are used by systems analysts to design information-processing systems but also as a way to model whole organizations.

PowerDesigner support

The DFD is part of the Business Process Model. You create a data flow diagram using the Data Flow Diagram process language, which is part of the Analysis family. For more information, see "What is a BPM?" in the "Getting Started with the Business Process Model" chapter.

PowerDesigner DFD support:

Both Gane & Sarson and Yourdon graphical notations, which are model options that you select from the Model Options dialog box. For more information, see Introducing Data Flow Diagram Concepts.
Automatic processes and data stores numbering, see Process and data store numbering
Data flow diagram balancing, see Data flow diagram balancing
V6 PAM import as data flow diagrams, see Importing a ProcessAnalyst Model into a DFD

You build a DFD at the very beginning of your business process modeling in order to model the functions your system has to carry out and the interaction between those functions together with focusing on data exchanges between processes. You can associate data with data models, such as Conceptual Data Models, Physical Data Models, or Object-Oriented Data Models.

This chapter outlines the specifics of PowerDesigner's support for data flow diagrams, and should be read in conjunction with the "Building Analysis Business Process Model" chapter in Part One of this book.