Data flow diagrams (DFDs) are a means for creating system blueprints and communicating with clients and users. The creation of DFDs is an early step in the structured analysis approach to system analysis. The DFD Yourdon/DeMarco diagram is derived from a freehand “paper and pencil” technique that uses bubbles as process symbols and arced arrows as data flows.
A DFD is a diagram that shows where the data come from, where they go, where they are stored, and what happens to them on the way. These four things are the only things that can happen to data. DFDs are a way of showing the overall picture of a system, and some of the detail.
In general, most process models of systems start with a context diagram, on which a process symbol, some external entities, one or more data stores, and some data flows are pictured. The process symbol is traditionally labeled “The System,” or words to that effect. That symbol expands to a “Child” diagram which can have 5 to 9 process symbols, each of which can expand to a child diagram, and so on, in an analysis process known as creating a set of leveled diagrams: see Creating a set of leveled diagrams.
