Data Flow Diagrams
This page was sourced from the Systems Analysis and Design toolbox developed by the Canberra Institute of Technology.
Why do we create them?
A set of data flow diagrams (DFDs) is a modelling tool used by system analysts to help them analyse a business and its business needs.
The creation of these diagrams is part of a process for the analyst to document what and how the data flows around an organisation. The employees of the organisation need to provide the details. Often the details are contradictory or incomplete, so DFDs become a good tool for clarifying business processes.
System analysts use DFDs to discuss with their clients and people working in the organisation what data is used, how the old system works and how it will link to the new system. DFDs allow the systems analyst to create a series of models which can be signed off or agreed to by the client representative as an accurate reflection of business processes. Once this analysis has been completed and agreed to, the design phase can proceed. A specification of requirements is then prepared.
DFDs consist of a number of symbols and words which, when combined, describe either an existing system for processing data or a new system being designed to manage the data for an organisation. They can be either
There are many levels of dataflow diagrams. The number of levels depends upon the complexity of the data processing occurring. Each model of a business operation must have a context diagram and a level 0 diagram. Many models have another three levels: levels 1, 2 and 3. When no further complexity needs explaining and when the processing of data within a levelled diagram can be explained on one A4 sheet, then it is time to stop the process of 'decomposition' of creating another lower level diagram to add clarity.
A set of levelled diagrams enables an analyst to check that the details needed to move onto the design phase are accurately recorded. Click here to read more about levelling.
A physical DFD of a current system is often created first because this is what users are most comfortable with. They usually know how data moves around the system by linking it to people or places.
Logical (or functional)
A logical data flow diagram is created to move users and management from what they do now and to demonstrate the independence of processing data from people and places. It is a functional view of the business. These functions may, however, be performed in a variety of ways, not just one.