How to Construct a Data Flow Diagram
Data Flow Diagrams are used to illustrate how information will move between interrelated systems in the accomplishment of a given function, task or process. The visual description of this information flow is typically rendered at several levels of granularity, with each given level showing how data is fed into a given process step and subsequently passed along at the completion of that step. This article shows how to approach Data Flow Diagrams in general.
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Instructions
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Context Diagram
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At the highest level, any given process may be seen as a single step, which is fed information, perhaps stores information, and ultimately passes information along. The Level 0 Data Flow Diagram, or Context Diagram, is designed to represent only the inputs and outputs of the overall process.
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Draw a circle in the middle of your work space. This will represent your overall process.
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Arrange rectangles around this process figure. Draw one rectangle for each external system that either contributes to or draws from your central process.
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Remain at a high level with these labels. If you were illustrating telephone order placement, you might show a circle labeled "Order" surrounded by rectangles like Customer, Manufacturer, and Logistics---if logistics were managed by an outside company.
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Draw arrows (flows) between these figures to show how information will move to and from these different systems. Label each arrow with the name of the information exchanged.
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Add a new page and label it Figure 0. This diagram is also meant to represent the entire process on a page, but this level of detail will show all the major functions and interfaces that power the process in the Context Diagram.
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Keep Figure 0 detail low as well. Your Order Placement process might show the Call Center, Inventory Management, Shipping and Invoicing as major process components at this level.
Sublevels
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Pick a single flow (an arrow) from the Figure 0 diagram. That will become the focus for your next, more detailed diagram.
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In a new drawing space, using the line chosen from the Figure 0 diagram as your new central process, illustrate with circles, squares and lines the processes and data sources that feed and are fed from this process. In this example, if you chose a flow between the Manufacturer and the Inventory Management process, you might now see a goods-ordering and separate receiving process at this more detailed view.
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Avoid driving too quickly into too much detail. Too many details can make the diagrams cluttered and obscure the relationships between the data and the processes working on the data.
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In these more detailed views, you may show multiple processes, but allow yourself to create as many levels of detail as necessary to ensure any given diagram remains clear and concise for people unfamiliar with the system.
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Continue drawing and labeling the flows that illustrate which data moves to and from each given process. Select the next flow to expand with detail until you have reached the point at which no further details are useful.
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Return to the Figure 0 diagram and begin detailing the next data flow, and continue until all the Figure 0 lines have been fully decomposed into subprocesses and the information flowing between them.
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Tips & Warnings
Though different nomenclatures have arisen for the elements of a Data Flow Diagram, if you use the following, your diagram will be quickly understood:
Circle---represents a process
Square---represents a system external to the process
Two parallel, horizontal lines---represents a data storage system
Arrow---called a "flow," represents directional movement of data
Aesthetics are important. A diagram that is not pleasing to look at will not be as readily understood by its viewers. Take time to right-size the level of details and to arrange the elements of the picture for clarity and balance.
