14. Data Flow Diagrams (cont)

Data Store

Many systems need to keep a persistent copy of data in order to support various processes. The data store can be an electronic one in the form of a database or it could be a physical one in the form of a filing system. It may be useful in the DFD to show the difference between the two by using perhaps 'D' to denote an electronic store and 'M' for a paper based data store. D and M are arbitrary letters but you should remain consistent in the DFD you draw.

So an electronic database data store in the example looks like

Should the data store need to be repeated, a good way to indicate this is to use a double line in the graphic, like so

Where Invoice copy in the example is accessed by two separate processes.