Data Flow Diagram (DFD) Explained: Systems Analysis Essentials

What Is a Data Flow Diagram (DFD)?

A Data Flow Diagram (DFD) is a graphical representation of the flow of data through an information system. A DFD can also be used to display data processing within structured design. It is common practice for a designer to draw a connection to the first-level DFD, which shows the interaction between the system and external entities. This context-level DFD is "exploded" to reveal more details of the system being modeled.

History and Purpose of DFDs

Data flow diagrams were invented by Larry Constantine, the original developer of structured design, based on the computer model of Martin and Estrin, known as the "Data flow chart." The DFD is one of three essential perspectives of the Structured Systems Analysis and Design Method (SSADM).

Project sponsors and end-users should be informed and consulted at all stages of a system evolution. With a DFD, users can visualize:

• How the system works.
• What the system will achieve.
• How the system will be implemented.

By comparing the old system's data flow diagrams with the new system's flow charts, developers can establish differences and implement improvements to create a more efficient system. DFDs provide the end-user with a physical idea of how data will ultimately flow and how it affects the structure of the entire system. The development of a DFD helps in identifying transaction data in the data model.

Classification of DFD Levels

Diagrams are classified into the following levels:

• Level 0: Context Diagram.
• Level 1: High-level diagram.
• Level 2: Detail or expansion diagram.
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Detailed DFD Level Definitions

Level 0: Context Diagram

The context diagram only draws the process flows between main processes; data stores do not appear at this level.

Level 1: Top-Level Diagram

This level reflects all processes that describe the main functions. Processes at this level can be interrelated, provided they exist within a storage or external entity that unites the groups.

Level 2: Detail or Expansion Diagram

At this level, processes can be interrelated directly or through data storage. Processes at levels 1 and 2 must always maintain the inputs and outputs defined in the context diagram.