Data flow diagram symbols

Data flow diagrams use four main elements: external entities (sources or sinks of data), processes (activities that transform data), data stores (repositories that hold data), and data flows (arrows showing data movement between elements).

Yourdon-Coad uses circles for processes and parallel lines for data stores, while Gane-Sarson uses rectangles with rounded corners for processes and open-ended rectangles for data stores. Both notations represent the same concepts but with different visual styles.

External entities, also called actors, sources, sinks, or terminators, are systems or actors outside the main system that produce or consume data. They are typically placed at the boundaries of the diagram and represent the inputs and outputs of the system.

A process symbol represents an activity that changes or transforms data flows. All processes must have both inputs and outputs and are labeled with simple, descriptive names based on their function, such as "Ship Order."

A data store symbol represents data held for later access without generating operations or transforming data. A data store could consist of files held long-term or documents stored temporarily while waiting to be processed.

Arrows represent data flows, showing the movement and direction of data between external entities, processes, and data stores. They are labeled based on the type of data or its associated process.

Consistency in notation prevents confusion when reading the diagram. Since DFD symbols vary between notation types, maintaining one style throughout ensures clarity and effective communication.

A Level 0 DFD (context diagram) shows the system as a single process, Level 1 provides greater detail on main functions, and Levels 2 or higher illustrate increasingly detailed system functioning. DFDs rarely go beyond Level 2 due to complexity.