Data Flow Diagram (DFD/Data Flow Chart), is a description of the data flow of the system is a major tool, which uses a set of symbols to describe the entire system of information in the whole picture, a comprehensive reflection of the flow of information in the system, processing and storage.

The data flow diagram has two characteristics: abstraction and generalization. Abstract refers to the data flow diagram to the specific organization, workplace, material flow are removed, leaving only information and data storage, flow, use and processing. Generalization, on the other hand, refers to the data flow diagram to link the system's processing of various business processes are considered to form an overall.

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In order to describe the information flow and processing of a complex software system, a hierarchical DFD can be used to describe it, and a hierarchical DFD has a top layer, an intermediate layer, and a bottom layer.

(1) Top layer. Determine the scope of the system, determine the input and output data flow, it describes the boundaries of the system, the function of the whole system is abstracted into a processing, the top layer DFD only one sheet.

(2) Intermediate layer. Below the top layer are a number of intermediate layers, and a given intermediate layer is both the result of the decomposition of the processing of the layer above it and an abstraction of a number of processing of the layer below it, i.e., it is further decomposable.

(3) Bottom layer. If the processing of a DFD cannot be further decomposed, this DFD is the bottom layer. The processing of the bottom DFD is composed of basic processing, and the so-called basic processing refers to the processing that cannot be further decomposed.