A data flow diagram (DFD) shows how the functions, or processes, of a system relate to each other via the data that flows between them. They are most valuable in showing ‘what’, (the logical view) but can also be used to show the ‘how’ (the physical view).
Data Flow Diagram – a process modelling tool
- Data flow diagrams (DFD) show how the functions, or processes, of a system relate to each other via the data that flows between them. They are most valuable in showing ‘what’, (the logical view) but can also be used to show the ‘how’ (the physical view).
- Data flow diagrams can be used to model the processes in a current system and, separately, a proposed new system. In addition they can show either physical or logical views.
- Data flow diagrams are not concerned with control or sequence. As a tool they are of limited use to designers, but are of greater use to analysts in communicating to the client and to the designers.
- DFDs are levelled so as to reduce the complexity at any one level.
Rules for Drawing a Data Flow Diagram
The four elements used in data flow diagrams are:
Data Flow
- Show the movement of data between processes, into and out of data stores and to and from terminators
- An arrow head indicates the direction of the data flow
- The data must have a meaningful name
- They do not show control
Process
- Indicates a function, this should be a transformation of data
- A functional primitive is the lowest process level
Data Store
- Shows data at rest
- May be manual or automatic accumulation of data
Terminator
- Shows the originator and/or receiver of the overall system data
- Represents people, organisations and systems just beyond the scope of the system under study
- Marks the boundary of the model
- Only appear in the highest level DFD – usually the context diagram
Download the PDF version of this guide to DFDs: How To Draw Data Flow Diagrams
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