What is a data flow diagram (DFD)

What is a data flow diagram (DFD)

 

The Data Flow Diagram (DFD) is a graphical representation of the flow of data through an information system. It enables you to represent the processes in your information system from the viewpoint of data. The DFD lets you visualize how the system operates, what the system accomplishes and how it will be implemented, when it is refined with further specification.

At its simplest, a data flow diagram looks at how data flows through a system. It concerns things like where the data will come from and go to as well as where it will be stored. But you won't find information about the processing timing (e.g. whether the processes happen in sequence or in parallel).

We usually begin with drawing a context diagram, a simple representation of the whole system. To elaborate further from that, we drill down to a level 1 diagram with additional information about the major functions of the system. This could continue to evolve to become a level 2 diagram when further analysis is required. Progression to level 3, 4 and so on is possible but anything beyond level 3 is not very common. Please bear in mind that the level of detail asked for depends on your process change plan.

DFDs represent the following:

1. External devices sending and receiving data

2. Processes that change that data

3. Data flows themselves

4. Data storage locations

 The hierarchical DFD typically consists of a top-level diagram (Level 0) underlain by cascading lower level diagrams (Level 1, Level 2…) that represent different parts of the system.

Concept	Tool	Gane & Sarson	Yourdon	Description
Process				Location where data is transformed.
Flow				Oriented link between objects, which conveys data.
Data store				Repository of data.
External entity				Source or destination of data. See External entity.
Split/Merge				Splits a flow into several flows or merges flows from different sources into one flow.

DFDs only involve four symbols. They are:

• Process
• Data Object
• Data Store
• External entity

	Process Transform of incoming data flow(s) to outgoing flow(s).
	Data Flow Movement of data in the system.
	Data Store Data repositories for data that are not moving. It may be as simple as a buffer or a queue or a s sophisticated as a relational database.
	External Entity Sources of destinations outside the specified system boundary.

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