Data flow model is a way of showing how data is processed by a system. At the analysis level, they should be used to model the way in which data is processed in the existing system. The notation used in this models represents functional processing, data stores and data movements between functions. Data flow models are used to show how data flows through a sequence of processing steps. The data is transformed at each step before moving on to the next stage. These processing steps or transformations are program functions when data-flow diagrams are used to document a software design.
The model shows how the order for the goods moves from process to process. It also shows the data stores that are involved in this process.There are various notation used for data-flow diagrams. In figure rounded rectangles represent processing steps, arrow annotated with the data name represent flows and rectangles represent data store (data sources). Data flow diagrams have the advantages that, unlike some other modelling notations, they are simple and intuitive.