The label is the name of the terminating entity (for example, “Web application book”), person, or group of people. Terminators, external entities or people with which the system communicates, are shown as rectangles. The label is generally the plural of the name of the items carried by the flow into and out of the store-for example, “Web links.” (This implies that the same items go in and out, so the flows into and out of the store will have the same labels.) Stores (places where data are stored) are shown as two parallel lines or an open-ended rectangle. The label should say what kind of information or item moves along the flow-for example, “Web link.” The label should be a word, phrase, or short sentence that says what the process does-for example, “Find information on DFDs.”įlows are shown as arrowed lines (either straight or curved). A process shows a part of the system that transforms inputs into outputs. Processes or functions are shown as circles or sometimes as rounded rectangles. It also identifies internal data stores that must be present in order for the system to do its job and shows the flow of data between the various parts of the system. The Level 1 DFD shows how the system is divided into subsystems (processes), each of which deals with one or more of the data flows to or from an external agent, and which together provide all of the functionality of the system as a whole. This context-level DFD is next “exploded” to produce a Level 1 DFD that shows some of the detail of the system being modeled. The context diagram shows the entire system as a single process and gives no clues as to its internal organization.
The system’s interactions with the outside world are modelled purely in terms of data flows across the system boundary. This helps to create an accurate drawing in the context diagram. It is common practice to draw the context-level data flow diagram first, which shows the interaction between the system and external agents that act as data sources and data sinks.
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