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Data Flow Diagrams

Updated on December 15, 2015
Figure 1. A physical data flow diagram, using the Yourdon/Coad notation
Figure 1. A physical data flow diagram, using the Yourdon/Coad notation
Figure 2. A logical DFD, with Gane/Sarson notation
Figure 2. A logical DFD, with Gane/Sarson notation
Figure 3. Standardized icon notation found in the Data Flow Diagram solution
Figure 3. Standardized icon notation found in the Data Flow Diagram solution | Source

When studying a business process or system that involves the transfer of data, it is common to use a data flow diagram (DFD) to visualize how that data is processed. While initially used exclusively in regards to the flow of data through a computer system, DFDs are now employed as a business modelling tool, describing business events and interactions, or physical systems involving data storage and transfer.

A DFD is a 2D diagram that appears something like a free-form flowchart. They can be divided into two broad categories — physical or logical data flow diagrams. They are not exclusive of each other; an interpretation of each can be placed over the same process, revealing different aspects of the data flow. The differences are, that a physical DFD shows how a system will be implemented, or how it currently operates — it includes the people involved, files, hardware, storage centres and other real-world elements. On the other hand, a logical DFD describes the necessity of certain operations and activities in order for data to be transferred from point A to point B.

DFDs are used by system analysts to create an overview of a business, to study and evaluate all its inputs and outputs, and to place each element within context along the data flow chain. Once an overall picture is achieved, each step can then be 'exploded' into a more detailed diagram of individual processes. The ideal scenario is first to create a visual representation of the current logical data flow — from here unnecessary processes can be dropped, and new features, inputs, outputs, activities and stored data can be added. This creates a new proposed logical data flow. With this new system, a new physical DFD can be devised, that takes into account all the proposed changes.

To ensure a measure of understanding when sharing diagrams with others, DFDs use a standardized notation system — it has been somewhat adapted for different needs over the years, but generally a DFD will use one of the two most prominent versions. Perhaps simplest of all is the Gane/Sarson notation, that uses three symbols and arrowed connectors to describe external entities, process and data stores, and the flow of data between them.

More in depth is the Yourdon/Coad notation (slightly adapted from DeMarco's version), with extra icons to describe multiple processes, process loops, conditions, and perhaps most importantly, different data states and object classes.

To produce professional and standardized data flow diagrams, most analysts will turn to a specialist drawing software, that can automate certain processes and cater for presentation and file sharing needs.


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