Data Flow Diagram (DFD)
The Data Flow Diagram (DFD) is a graphical representation of the flow of data through an information system. It enables you to represent the processes in your information system from the viewpoint of data. The DFD lets you visualize how the system operates, what the system accomplishes and how it will be implemented, when it is refined with further specification. Data flow diagrams are used by systems analysts to design information-processing systems but also as a way to model whole organizations. You build a DFD at the very beginning of your business process modeling in order to model the functions your system has to carry out and the interaction between those functions together with focusing on data exchanges between processes. You can associate data with conceptual, logical, and physical data models and object-oriented models. There are two types of DFDs, both of which support a top-down approach to systems analysis, whereby analysts begin by developing a general understanding of the system and gradually break components out into greater detail: •
Logical data flow diagrams - are implementation-independent and describe the system, rather than how activities are accomplished. •
Physical data flow diagrams - are implementation-dependent and describe the actual entities (devices, department, people, etc.) involved in the current system. DFDs can also be grouped together to represent a sub-system of the system being analyzed. A data flow diagram can look as follows:
PowerDesigner support for DFD includes:
Support for the Gane & Sarson and Yourdon notations, which you choose between by selecting Tools > Model Options. •
Automatic processes and data stores numbering (see Process and Data Store Numbering). •
Data flow diagram balancing.
Data Flow Diagram specific validation rules. Power Designer may perform automatic corrections to your model or output errors and warnings that you will have to correct manually. In addition to PowerDesigner's standard Toolbox, a Data Flow Diagram toolbox is available to let you rapidly create objects specific to the diagram type: Concept
Gane & Sarson
Location where data is transformed. See Process.
Oriented link between objects, which conveys data. See Flow.
Repository of data. See Data store.
Source or destination of data. See External entity.
Splits a flow into several flows or merges flows from different sources into one flow. See Split/merge.
Structured English or "pseudocode" consists of the following elements: 1.
Operation statements written as English phrases executed from the top down 2.
Conditional blocks indicated by keywords such as IF, THEN, and ELSE 3.
Repetition blocks indicated by keywords such as DO, WHILE, and UNTIL Use the following guidelines when writing Structured English: 1.
Statements should be clear and unambiguous
Use one line per logical element
All logic should be expressed in operational, conditional, and repetition blocks 4.
Logical blocks should be indented to show relationship
Keywords should be capitalized
Examples of common keywords
START, BEGIN, END, STOP, DO, WHILE, DO WHILE, FOR, UNTIL, DO UNTIL, REPEAT, END WHILE, END UNTIL, END REPEAT, IF THEN, IF, ELSE, IF ELSE, END IF, THEN, ELSE THEN, ELSE IF, SO, CASE, EQUAL, LT, LE, GT, GE, NOT, TRUE, FALSE, AND, OR, XOR, GET, WRITE, PUT, UPDATE, CLOSE, OPEN, CREATE, DELETE, EXIT, FILE, READ, EOF, EOT, WITH,RETURN,SORT
Example of Structured English
A bank will grant loan under the following conditions
If a customer has an account with the bank and had no loan outstanding, loan will be granted. 2.
If a customer has an account with the bank but some amount is outstanding from previous loans then loan will be granted if special approval is given. 3.
Reject all loan applications in...
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