Data Flow Diagrams (DFD) - Part I

This unit deals with one of the major techniques for recording the requirements of a user for a new computer application. An initial diagram is constructed to show the processes which are being implemented in an existing system. The diagram helps to show how information is used to produce the functions that are required by the current system. It also shows what information is provided to the system and what information is provided form the system. Other benefits include the documentation of who is using the system and what data will be stored. By careful construction of the DFDs (data flow diagrams) the boundaries of the system to be built may be clearly identified. This helps to clarify what will and what will not be constructed. It will also show the interaction that may be required with other systems.

The data flow diagrams should also have some associated documentation. This is necessary as the diagrams are meant as a visual representation of the way in which information is processed. There is limited space on the diagrams so that documentation to explain, refine and describe further details of what is shown need to be kept somewhere in the proposed system documentation. The data flow diagrams and the associated documentation together combine to form a data flow model. This is also commonly called a process model.

The user requirements when complete are used as a basis for the development of the system. Later when the system has been developed it can be tested against the initial requirements to see whether the user’s needs have been met.

Development and purpose of DFDs

Before attempting to construct an initial DFD it is necessary to gather and digest information that helps us to understand how data is processed in the current system. Fact-finding techniques are used for this purpose and are discussed in another Unit. As the DFD is constructed a systems analyst will often come across areas of doubt where the precise way to model the system is unclear. This is a natural part of the development and should not be regarded with alarm. In fact, it is expected and it is a consequence of attempting to model the current situation that questions will be asked to clarify the exact processes which are taking place. Sometimes the analyst will make an assumption and then check this with the user at a subsequent meeting.

Results of interviews, documents, reports, questionnaires etc. will all play a part in helping the analyst to gain an insight into the current processes. Where a system is being developed form scratch the analyst will work with the user to develop the proposed DFDs.

When all the information about the current system is gather it should be possible to construct the DFDs to show:

• the information that enters and leaves the system
• the people/roles/other systems who generate and/or receive that information
• the processes that occur in the system to manipulate the information
• the information that is stored in the system
• the boundary of the system indicating what is (and what is not) included

As a staring point, it is sometimes useful to construct a document flow diagram. This diagram shows how ‘documents’ are passed around an organization to fulfill the current requirements of the system under investigation. The term’ documents’ is interpreted very loosely and usually translates to information.
Sometimes before beginning to produce the set of data flow diagrams a document flow diagram is generated. This helps to establish the system boundary so we can decide which parts of the system we are modelling and which parts we are not. The document flow diagram shows the different documents (or information sources in the system and how they flow from a source to a recipient.

Here is an example of a document flow diagram shown below.

Here ellipses represent either the source or recipient of the ‘documents’ and named arrows show the direction of transfer and the nature of the information being exchanged. This kind of diagram is a first step in understanding what information is in the system and how it is used to perform the required functions.

Another useful feature of the construction of this type of diagram is that it enables a sensible discussion of where the system boundary should lie. In other words it is important to establish what is to be included in the proposed information system and what is not. To indicate this, a system boundary line is constructed on the document flow diagram.

*** Please remember that drawing the system boundary is very very important.