Data Flow Diagram-2 (hierarchical data flow diagram)

For complex practical problems, there are usually dozens or even dozens or hundreds of processes on the data flow diagram. Such a data flow diagram looks unclear. The hierarchical data flow diagram can solve this problem well.

System S in

Figure 1.5 system processing data flow diagram

Its Input and Output reflect the relationship or interface between the system and the external environment. The data processing requirement cannot be indicated only in this figure, and further details need to be made. If s can be divided into three subsystems S1, S2, and S3, You can plot their processing and related data streams. Subsystem S1 can be further subdivided into s1.1, s1.2, s1.3, and so on, as shown in Figure 1.6.

Figure 1.6 Hierarchical Data Flow Diagram

According to this method, the system is gradually refined and decomposed from top to bottom, and the hierarchical data flow diagram is used to reflect the structural relationship, so that the whole system can be clearly displayed and easily understood. The following issues should be taken into account when drawing a hierarchical data flow diagram:

1) No.

To facilitate management and reading, numbers of graphs and their processing at each level are required. The hierarchy numbers are from top to bottom (system diagram), 0-layer diagram, 1-layer diagram, and so on. The relationship between the graph layers is parent-child, the lower graph is a subgraph, and the upper graph is a parent graph. The ID of the subgraph is the number processed by its parent graph. The number processed in the subgraph consists of the subgraph number, decimal point, and local number. In this number, the number of decimal places in the graph number is the level Number of the graph, and the number before the last decimal point is the number of its parent graph. For example, a graph numbered 3.2.1 is a subgraph in a 2-layer graph and its parent graph numbered 3.2.

2) Data balance between parent and subgraphs

Subgraphs are refined by parent graphs. Therefore, the input and output data streams of subgraphs must be consistent with the input and output data streams processed in the parent graph.

3) Degree of Decomposition

A specific process is divided into two or three processes at a time, which may require too many layers, but too much decomposition is difficult to understand. According to the research results of psychology, there are no more than seven questions that can be effectively handled at the same time. Therefore, the number of subprocesses refined by each processing operation generally cannot exceed 7. This decomposition process can be stopped when the subprocessing obtained is 10 minutes simple.

4) Chart format

For a large system, there may be dozens or dozens of data flow diagrams, which are generally bound into a book. To facilitate collection, sorting, and reading, you can design a chart similar to Figure 1.7 for formatting. Assume that the university teaching administration system is 2nd sub-processes in the 0-layer data flow diagram of the university teaching management system. It is a subgraph in the 1-layer data flow diagram.

Figure 1.7 data flow chart format