Data Flow Graph (conversion)
Data Flow Diagram (DFD)
A Data Flow Diagram (DFD) is a tool used in the SA method to represent the system logic model. It graphically depicts the process of data flow and processing in the system, because it only reflects the logical functions required by the system, it is a function model.
It is a data flow chart of an airplane ticket booking system, which reflects the function of a travel agency to list the passenger information (name, age, unit, ID card number, travel time, destination, etc.) of the ticket booking system) enter the ticket reservation system. The system arranges flights for passengers and prints out a ticket for receiving tickets (with the payable accounts ). A passenger receives a ticket on the day before the plane departs. The system checks the ticket and sends the ticket to the passenger.
3.3.1 basic graphical symbols
A Data Flow chart has four basic graphical symbols:
: Arrow, indicating data flow;
0: circle or elliptic, indicating processing;
=: Parallel bars, indicating data storage;
□: Box, indicating the source or end point of the data.
(1) data stream. Data Stream is the path where data is transmitted in the system. Therefore, data streams are composed of fixed data. For example, a ticket consists of a passenger's name, age, unit, ID card number, date, destination, and other data items. Because a data stream is a flow of data, it must have a flow direction. In addition to the data stream between the data stream and the data storage, the data stream should be named by a noun or a noun phrase.
(2) Processing (also known as data processing ). Perform operations or transformations on data streams. Each processing should also have a name, usually a verb phrase, which briefly describes what processing is completed. In the layered data flow diagram, the processing should also be numbered.
(3) Data Storage (also called a file) refers to the temporary data stored. It can be a database file or any form of data organization.
(4) data source points or endpoints are entities (including personnel, organizations or other software systems) in the external environment of the software system, collectively referred to as external entities. Generally, only the top-level diagram of the data flow diagram is displayed.
3.3.2 steps for drawing a Data Flow Diagram
(1) first draw the input and output of the system, that is, draw the top-layer data flow diagram. A top-level flow chart contains only one processing method to indicate the developed system, and then considers the input data and output data streams of the system. A top-level chart is used to indicate the scope of the developed system and its data exchange relationship with the surrounding environment. The top-level diagram of the plane ticket booking system. (2) Draw a lower-layer data flow chart inside the system. Processing that is no longer decomposed is called basic processing. Generally, the layer number starts from 0 and adopts the principle of top-down and outward-internal. When a layer-0 data flow chart is drawn, the system that breaks down the top-layer data flow chart is a number of subsystems, and the data interface and Activity Relationship between each subsystem are determined. For example, the above ticket booking system can be divided into two parts by function. One part is the travel agency booking ticket, the other part is the passengers getting the ticket, and the two parts are linked through the data storage of the ticket file, layer-0 Data Flow Diagram 3-4.
(3) considerations.
① Name. Appropriate names make it easy for people to understand the meaning of data streams, data storage, and processing.
② Draw data streams rather than control flows. The data stream reflects the system's "what" and does not reflect "How to Do". Therefore, the data stream name on the arrow can only be a noun or a noun phrase. The whole figure does not reflect the processing execution sequence.
③ Do not draw material streams. Data Streams reflect data that can be processed by computers, and are not physical objects. Therefore, do not draw material streams for data flow diagrams of the target system.
④ Each processing has at least one input data stream and one output data stream, reflecting the source and processing result of the processed data.
⑤ Number. If a data flow graph is processed into another data flow graph, the upper layer graph is the parent graph, and the lower layer graph is the subgraph. The subgraph and all the processing operations should be numbered. ⑥ Balance between parent and subgraphs. The input and output data streams of subgraphs must be consistent with the input and output data streams processed by the parent graph. This means that the parent graph is balanced with the subgraphs.
7. Local Data Storage. When data storage in a data flow graph at a layer is not an external interface processed in the parent graph, but a data interface between some processes in the diagram, the data storage is called a local data storage.
Improve data flow diagrams. Note that a proper decomposition should be divided into several sub-processes with relatively independent functions, which can reduce the number of input and output data streams between processes and increase the comprehensibility of data flow graphs.
Figure 3-6 shows the equivalent DFD of Figure 3-2 drawn with this set of symbols.
3.3.3 flowchart example
