Thursday, 2 February 2017

Information about Data Flow Diagram

Data Flow Diagram

  • Definition

     A data flow diagram (DFD) is a graphical representation of the "flow" of data through an information system, modelling its process aspects. A DFD is often used as a preliminary step to create an overview of the system, which can later be elaborated.

    A DFD shows what kind of information will be input to and output from the system, where the data will come from and go to, and where the data will be stored. It does not show information about the timing of process or information about whether processes will operate in sequence or in parallel(which is shown on a flowchart).

    Data flow diagrams are also known as bubble charts.DFD is a designing tool used in the top-down approach to Systems Design.Data flow diagrams can be used in both Analysis and Design phase of the SDLC.There are different notations to draw data flow diagrams (Yourdon & Coad and Gane & Sarson[6]), defining different visual representations for processes, data stores, data flow, and external entities.

    External Entity 

    An external entity can represent a human, system or subsystem. It is where certain data comes from or goes to. It is external to the system we study, in terms of the business process. For this reason, people used to draw external entities on the edge of a diagram.


    A process is a business activity or function where the manipulation and transformation of data takes place. A process can be decomposed to finer level of details, for representing how data is being processed within the process. 


    Data Store

    A data store represents the storage of persistent data required and/or produced by the process. Here are some examples of data stores: membership forms, database table, etc.


    Data Flow

    A data flow represents the flow of information, with its direction represented by an arrow head that shows at the end(s) of flow connector. 

    The DFD may be used for any level of data abstraction. DFD can be partitioned into levels.              Each  level has more information flow and data functional details than the previous level.
   Highest level is Context Diagram. Some important points are:
  •  1 bubble (process) represents the entire system.
  •  Data arrows show input and output.
  •  Data Stores NOT shown. They are within the system.
Next Level is Level 0 DFD. Some important points are:
  • Level 0 DFD must balance with the context diagram it describes.
  • Input going into a process are different from outputs leaving the process.
  • Data stores are first shown at this level.
Next level is Level 1 DFD. Some important points are:
  • Level 1 DFD must balance with the Level 0 it describes.
  • Input going into a process are different from outputs leaving the process.
  • Continue to show data stores.
A DFD may look similar to a flow chart. However, there is a significant difference with the data flow diagram. The arrows in DFDs show that there is a flow of data between the two components and not that the component is sending the data that must be executed in the following component. A component in DFD may not continue execution when sending data and during execution of the component receiving the data. The component sending data can send multiple sets of data along several connections. In fact, a DFD node can be a component that never ends.