data flow

Data flow diagram (DFD/ data flow diagram) is the main tool to describe the system data flow. It uses a set of symbols to describe the whole picture of information in the whole system, and comprehensively reflects the flow, processing and storage of information in the system.

Data flow diagram has two characteristics: abstraction and generality. Abstraction means that the data flow chart removes the specific organization, workplace and material flow, leaving only the storage, flow, use and processing of information and data. Universality means that the data flow diagram links the processing processes of various services in the system to form a whole.

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In order to describe the information flow and processing process of complex software system, hierarchical DFD can be used to describe it. Hierarchical DFD can be divided into top layer, middle layer and bottom layer.

(1) Top floor. Determine the scope of the system, determine the input and output data flow, it explains the boundary of the system, abstracts the function of the whole system into a process, and there is only one top-level DFD.

(2) the middle layer. Below the top floor are several intermediate layers. An intermediate layer is not only the decomposition result of its upper layer processing, but also the abstraction of its lower layer processing, that is, it can be further decomposed.

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