Data flow diagram symbols

About data flow diagram symbols

A data flow diagram, or DFD, shows how data moves through a system: where it comes from, what is done to it, where it is kept and where it goes. It has four symbols. A Process transforms data, an external entity sends data in or receives it from outside, a data store keeps it, and a data flow arrow carries it from one to another. Ed Yourdon, in Just Enough Structured Analysis, names the four as processes, flows, stores and terminators. The University of Notre Dame’s business analysis guide calls the DFD a visual representation of how data moves through a process or system. Each symbol has its own page.

Two notations

DFDs are drawn in one of two notations, which differ in shape, not meaning. The UK Health Security Agency, which asks for a data flow diagram with every application for its protected data, sets them out like this:

Yourdon calls the difference between process shapes purely cosmetic, as long as a diagram uses one shape throughout.

Where DFDs come from

DFDs are the main diagram of structured analysis. Yourdon names DeMarco’s book of 1978 and Gane and Sarson’s of 1977 among the early structured analysis textbooks, and says his own notation follows those classic books. He ties the circle to the Yourdon/DeMarco camp, since YOURDON Press published its textbooks, and the Gane and Sarson shape to their 1977 book and to consulting firms such as McDonnell Douglas Automation Company.

Levels

Classical structured analysis, Yourdon writes, began with a context diagram, a DFD with a single bubble for the entire system, and then divided the system into functions and data stores in a strictly top-down way. He adds that this did not work well, and that a newer approach, event partitioning, was added. Lower-level diagrams are known as leveled DFDs.

Numbers are not a sequence

A DFD shows what happens to data, not the order of the steps. Yourdon warns that readers may assume the numbers on bubbles mean a sequence, and says the numbering has nothing to do with it. A flowchart or a BPMN diagram shows the order of steps instead.