A data flow diagram (DFD) uses various symbols to show how the system transforms input data into useful information. It shows how data moves through an information system but does not show program logic or processing steps. A set of DFDs provides a logical model of what the system does, not how it is does it.
A set of DFDs can be likened to Google Maps. In Maps a user can zoom in and out of the map, changing the level of detail viewed. As one zooms in, more detail is exposed and the map is more focused on one spot to the exclusion of others. As one zooms out, the map contains less detail but has a view of a wider area.
A set of DFDs starts with a context diagram which provides a context for the system, how it interacts with External Entities. This level would be similar to Google Maps showing the North-East corner of Florida. Interstate and State Highways are illustrated and named, but little else. In the set of DFDs the next level of detail is a Diagram 0 DFD. In Google Maps this is akin to zooming into the map so that only the I295 perimeter is visible. Now, in addition to the detail provided at the previous zoom level, more detail of roads is provided. Major roads are drawn and named while lesser roads are merely drawn. Note that when zooming in from a context diagram to a Diagram 0 DFD, all the items on the context diagram are now contained in the Diagram 0 DFD and are named the same. The Diagram 0 DFD merely adds more level of detail and becomes more focused.
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