Data Flow Graph

Data Flow Graph

A data flow chart is also called a data flow chart date flow digoal and DFD. It is a graphical tool that helps you understand and analyze system data flows, it accurately describes the functions, inputs, outputs, and data storage of the system logically, and is an important part of the system logic model.

Basic components of a data flow chart

Data Stream: It is composed of a set of fixed data, indicating the data flow. It is worth noting that the data stream rather than the control flow is described in the data flow diagram. In addition to data flowing to or from data storage, you do not need to name the data flowing out. Each Data Stream must have a proper name to reflect the meaning of the data stream.

Processing: The processing description describes the transformation between the input data stream and the output data, that is, the processing after which the input data stream becomes the output data. Each processing task has a name and a number. The number indicates the hierarchy and graph of the data flow chart in which the processing is located. You can see which processing is used to separate the data flow chart.

Data storage: Data Storage refers to data temporarily stored. Each data storage has a name.

External entity: an external entity is a person or organization that exists outside the software system. It points out the source of the data or the place where the data generated by the system is required.

External entity processing data stream

Hierarchical Data Flow Diagram Design Method

Step 1: Draw the input and output of the subsystem

Consider the entire system as a large processing, and then draw an input and output diagram based on the external entities from which the data system receives data streams and the external entities from which the system sends data streams. This figure is called a top-level chart.

Step 2: Draw the interior of the subsystem

The processing of top-level graphs is divided into several processes, and these processes are connected with data streams, so that the input data of the top-level graphs is converted into the output data stream of the top-level graphs after several processing processes. This graph is called a layer-0 graph. The process of drawing a data flow chart from a processing table is the decomposition of the processing.

The following methods can be used to determine processing:

When the composition or value of the data stream changes, we should plot a processing item. The processing function is to achieve this change, which can also be determined based on the system functions.

Method for Determining data stream

When a user processes several pieces of data as a unit (the data arrives together and is processed together), the user can regard the data as a data stream.

About Data Storage

Data to be used at a certain time in the future can be organized into a data storage.

Step 3: Inside the image processing

We regard each processing as a small system and the input and output data streams processed as input and output streams of a small system. As a result, we can draw a DFD chart for each small system like a 0-layer graph.

Step 4: Describe the decomposition graph of sub-processing

For each processing in the DFD diagram decomposed in step 3, the decomposition process in step 3 is repeated until the undecomposed processing in the diagram is simple enough (that is, it cannot be decomposed again ). Now, we have a set of hierarchical data flow diagrams.

Step 5: map data streams and process numbers

For a software system, the data flow diagram may have many layers, and each layer has many layers. To differentiate different processing and different DFD subgraphs, numbers should be assigned to each graph for ease of management.

● There is only one top-level graph, and there is only one processed graph, so you do not need to enter it.

● There is only one layer-0 image, and the processing numbers in the image are respectively 0.1, 0.2 ,..., Or 1 or 2.

● A subgraph is the processed number in the parent graph.

● The processing number in a subgraph is composed of the graph number, dot, and serial number, such as 1.12 and 1.3.

Notes:

1. Data Stream, processing, data storage, and external entity naming should be appropriate. The name should reflect the actual meaning of the component to avoid empty names.

2. Draw a data flow chart instead of a control flow chart.

3. A processed output data stream should not have the same name as the input data stream.

4. agree that one processing has multiple data streams and one processing, and one processing has two identical output data streams with different processing flows.

5. maintain a balance between the parent and subgraphs. That is to say, the input and output streams processed by a parent graph must be in the same quantity and name as the input and output data streams of its subgraphs. It is worth noting that assume that an input (output) data stream in the parent graph corresponds to several input (output) data streams in the subgraph, the data items in the subgraphs that constitute these data streams are exactly the data streams in the parent graph, so they are still balanced.

6. In the top-down decomposition process, if a data storage occurs for the first time and is only related to a processing, the data storage should be used as the internal file of the processing instead of being drawn.

7. Maintain data conservation, that is, the data in all output data streams processed by a processing operation must be directly obtained from the output stream or generated through the processing.

8. Each processing operation must have both input and output data streams.

9. In the entire data flow diagram, each data storage must have both read and write data streams. However, in a sub-graph, there may be only read, no write, or only write, no read.

Pay special attention to the above points, which are all the goals of the examination. In the past two years, the exam content has been concentrated on the points 2, 5, 6, and 8.

For example:

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 functional 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. In addition to the data stream between the data stream and the data storage, the data stream should be named using nouns or nouns.

(2) Processing (also known as data processing ). Perform operations or transformations on data streams. Each processing task must also have a name. Generally, it is a verb phrase that describes in a concise manner what processing is completed. In the layered data flow diagram, the processing should also be numbered.

(3) Data Storage (also known as a file) refers to the temporary storage of data, which 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 current 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 represent 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 the sequence uses the principle of "top-down" and "outgoing. 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". Therefore, the data stream name on the arrow can only be a noun or a noun phrase. The entire figure does not reflect the processing sequence.

③ Do not draw material streams. Data Streams reflect data that can be processed by computers, rather than 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. Assume that a data flow graph is processed and decomposed into another data flow graph, the upper layer graph is the parent graph, and the lower layer graph is the subgraph directly. The subgraph and all its processing operations must 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 is the balance between the parent graph and the subgraphs.

7. Local Data Storage. When the data storage in a data flow graph at a layer is not the external interface corresponding to the processing in the parent graph, but the data interface between some processes in this graph, the data storage is called 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 add the comprehensibility of data flow graphs.

Figure 3-6 shows the DFD equivalent to Figure 3-2 drawn by the attention symbol.

3.3.3 flowchart example

Data Flow Graph