What is Data Flow Diagram ?

Data Flow Diagram :

The data flow diagram represents the flows of data between different process in a business. It is a graphical technique that depicts information flow and transforms that are applied as data from input to output. It provides a simple, intuitive method for describing business processes without focusing on the details of computer systems. DFDs are attractive techniques because they provide what users do rather than what computers do. In DFD, there are four symbols are used :

1. Process :

The circle represents the process. An activity that changes or transforms data flows. Since they transform incoming data to outgoing data, all processes must have inputs and outputs on a DFD.

2. Data Flow : 

The labeled arrows indicate incoming and outgoing data flow. Movement of data between external entities, processes and data stores is represented with an arrow symbol, which indicates the direction of flow.

3. Data Store :

The rectangle represents an external entity. A data store does not generate any operations but simply holds data for later access.

4. External Entity :

In Data Flow Diagrams external entities produce and consume data that flows between the entity and the system being diagrammed.

These data flows are the inputs and outputs of the DFD. Data objects are represented by labeled arrows, and transformations are represented by circles. The DFD is presented in a hierarchical fashion. That is, the first data flow model (sometimes called a level 0 DFD or context diagram) represents the system as a whole. Subsequent data flow diagrams refine the context diagram, providing increasing detail with each subsequent level.

In DFD there are various levels of DFD, which provide details about the input, processes, and output of a system. Note that the level of detail of process increases with increase in level(s). However, these levels do not describe the system's internal structure or behavior. These levels are listed below :

1. Level 0 DFD : This shows an overall view of the system. Level a DFD is also known as context diagram.

2. Level 1 DFD : This elaborates level a DFD and splits the process into a detailed form.2

3. Level 2 DFD : This elaborates level 1 DFD and displays the process(s) in a detailed form.

4. Level 3 DFD : This elaborates level 2 DFD and displays the process(s) in a detailed form.

Creating a Data Flow model :

The data flow diagram enables us to develop models of the information domain and functional domain. As the DFD is refined into greater levels of detail, you perform an implicit functional decomposition of the system. The Simple guidelines  the derivation of a data flow diagram :-

1. All icons must be labeled with meaningful names.

2. The DFD evolves through a number of levels of details.

3. Always begin with a context level diagram (also called level 0)

4. Always show external entities at level 0 and 1.

5. The level 0 data flow diagram should depict the software/system as a single bubble.

6. Primary input and output should be carefully noted.

7. Refinement should begin by isolating candidate processes, data objects, and data stores to be represented at the next level.

8. All arrows and bubbles should be labeled with meaningful names.

9. Information flow continuity must be maintained from level to level.

10. One bubble at a time should be refined.

Example of Data Flow Diagram :

The below figure shows the level 0 DFD diagram of food ordering system in restaurant.

Figure : Level 0 DFD for "food ordering system" in restaurant

Here customer, kitchen and restaurant managers are external entities.

  • The "food ordering system" accepts the food order from the customer and forwards the order to the kitchen.
  • When the service is provided to the customer, the system generates the bill.
  • A copy of customer bill can be submitted to the manager as a part of restaurant management report.
This level 0 DFD can be extended to level 1 DFD to show more details showing exact data flow and processes (i.e. transformers).

Figure : Level 1 DFD for "food ordering system" in restaurant

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