In the late 1970s data-flow diagrams (DFDs) were introduced and popularized for structured analysis and design (Gane and Sarson 1979). DFDs show the flow of data from external entities into the system, showed how the data moved from one process to another, as well as its logical storage. Figure 1 presents an example of a DFD using the Gane and Sarson notation. There are only four symbols:
Squares representing external entities, which are sources or destinations of data.
Rounded rectangles representing processes, which take data as input, do something to it, and output it.
Arrows representing the data flows, which can either be electronic data or physical items.
Open-ended rectangles representing data stores, including electronic stores such as databases or XML files and physical stores such as or filing cabinets or stacks of paper.
Figure 1. Enrolling in the university.
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