Data Flow Diagram (DFD) is a graphical representation of data flow in any system. It is capable of illustrating incoming data flow, outgoing data flow and store data. Data flow diagram describes anything about how data flows through the system.
Sometimes people get confused between data flow diagram and flowchart. There is a major difference between data flow diagram and flowchart. The flowchart illustrates flow of control in program modules. Data flow diagrams illustrate flow of data in the system at various levels. Data flow diagram does not have any control or branch elements.
Types of DFD :
DFD is of two types:
- Logical DFD:
Logical data flow diagram mainly focuses on the system process. It illustrates how data flows in the system. Logical DFD is used in various organizations for the smooth running of system. Like in a Banking software system, it is used to describe how data is moved from one entity to another. - Physical DFD:
Physical data flow diagram shows how the data flow is actually implemented in the system. Physical DFD is more specific and close to implementation.
| Design Feature | Logical | Physical |
|---|---|---|
| What the model depicts | How the business operates. | How the system will be implemented (or how the current system operates). |
| What the processes represent | Business activities. | Programs, program modules, and manual procedures. |
| What the data stores represent | Collections of data regardless of how the data are stored. | Physical files and databases, manual files. |
| Type of data stores | Show data stores representing permanent data collections. | Master files, transition files. Any processes that operate at two different times must be connected by a data store. |
| System controls | Show business controls. | Show controls for validating input data, for obtaining a record (record found status), for ensuring successful completion of a process, and for system security (example: journal records). |
Figure illustrated below shows a logical data flow diagram and a physical data flow diagram for a grocery store cashier. The CUSTOMER brings the ITEMS to the register; PRICES for all ITEMS are LOOKED UP and then totaled; next, PAYMENT is given to the cashier; finally, the CUSTOMER is given a RECEIPT. The logical data flow diagram illustrates the processes involved without going into detail about the physical implementation of activities. The physical data flow diagram shows that a bar code—the universal product code (UPC) BAR CODE found on most grocery store items—is used. In addition, the physical data flow diagram mentions manual processes such as scanning, explains that a temporary file is used to keep a subtotal of items, and indicates that the PAYMENT could be made by CASH, CHECK, or DEBIT CARD. Finally, it refers to the receipt by its name, CASH REGISTER RECEIPT.
Developing Logical Data Flow Diagrams
To develop such a diagram, first construct a logical data flow diagram for the current system. There are a number of advantages to using a logical model, including:
- Better communication with users.
- More stable systems.
- Better understanding of the business by analysts.
- Flexibility and maintenance.
- Elimination of redundancies and easier creation of the physical model.
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