Data Flow Diagram (DFD)

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.

Data flow diagrams are used by systems analysts to design information-processing systems but also as a way to model whole organizations. 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 conceptual, logical, and physical data models and object-oriented models.

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

In addition to PowerDesigner's standard Toolbox, a Data Flow Diagram toolbox is available to let you rapidly create objects specific to the diagram type:

Concept	Tool	Gane & Sarson	Yourdon	Description
Process				Location where data is transformed. See Processes (DFD).
Flow				Oriented link between objects, which conveys data. See Flows (DFD).
Data store				Repository of data. See Data Stores (DFD).
External entity				Source or destination of data. See External Entities (DFD).
Split/Merge				Splits a flow into several flows or merges flows from different sources into one flow. See Split/Merges (DFD).

This chapter outlines the specifics of PowerDesigner's support for data flow diagrams, and should be read in conjunction with Business Process Diagrams.

• Creating a Data Flow Diagram
  You create a data flow diagram from the business process diagram with the Data Flow Diagram process language attached.
• Processes (DFD)
  A process is an activity, which transforms and manipulates input data to produce output data. For example, in a model about the publication of books, selecting a manuscript is a process. Data is sent to the selection process in the form of a manuscript. During selection, the manuscript is transformed either into a manuscript that goes directly to the printer, or into a manuscript that must wait before it is printed.
• Flows (DFD)
  A flow conveys data between processes, external entities, and data stores and represents data in motion, such as computerized components, messages or bits, or non-computerized components, such as eggs or cake.
• Data Stores (DFD)
  A data store is the location where data resides permanently or temporarily. It responds to requests for storing and accessing data, but cannot initiate any actions. It represents data at rest, which can be computerized components, such as files or databases, or non-computerized components, such as names and addresses in an address book for example.
• External Entities (DFD)
  An external entity sends or receives data from the system. It can represent a person, a machine, an organization etc., that is external to the system being modeled. Flows outgoing from external entities go to processes.
• Split/Merges (DFD)
  A split/merge allows you to split a flow into several flows to send data to different destinations, or merge flows from different sources into one flow.
• DFD Process and Data Store Numbering
  Process and data store numbering is a convenient way of referencing processes and data stores in a DFD. For example, in a lively discussion about processes or data stores in a DFD it is easier to mention a process or a data store by its number instead of its name, which can sometimes be long or complex. Data store numbering is only available when using the Gane & Sarson methodology.
• DFD Balancing
  The concept of balancing states that all the incoming flows to a process and all the outgoing flows from a process in the parent diagram should be preserved at the next level of decomposition.