Understanding Data Flow Diagrams (DFD)

A Data Flow Diagram (DFD) is a visual representation of the flow of information within a system. It shows how data enters and leaves the system, what transforms the data, and where data is stored. DFDs are a fundamental tool in system analysis and design, helping analysts understand and document processes and data handling.

DFDs use specific symbols to represent different components of the system. Understanding these symbols is key to interpreting and creating DFDs accurately.

Components of a Data Flow Diagram and Their Symbols

A typical Data Flow Diagram consists of four main components:

• Process: Represents a transformation of data. It takes input data flows and produces output data flows.
• Data Store: Represents a place where data is held, such as a database file or a filing cabinet. Data can be stored or retrieved from a data store.
• Data Flow: Represents the movement of data between processes, data stores, and external entities. It is shown as an arrow.
• External Entity: Represents a source or destination of data outside the boundary of the system being modeled. External entities interact with the system by sending or receiving data.

Representing External Entities in a DFD

External entities are crucial in a DFD as they define the system's boundaries and its interactions with the outside world. Examples of external entities include customers, suppliers, other systems, or specific departments that interact with the system being diagrammed.

In the standard notation for Data Flow Diagrams (like Yourdon-DeMarco or Gane-Sarson), an external entity is consistently represented by a Rectangle.

Let's look at how each option represents components in DFD:

• Circle or Oval: Represents a Process.
• Ellipse: Sometimes used for Processes, similar to a Circle, depending on the notation standard.
• Diamond: Often used in flowcharts to represent a decision point, but not a standard symbol for external entities in typical DFD notations.
• Rectangle: Represents an External Entity.

Therefore, based on standard DFD notation, external entities are represented by a rectangle.

Revision Table: DFD Symbols Summary

Additional Information on DFDs

DFDs can be drawn at different levels of detail. A Context Diagram is the highest-level DFD, showing the system as a single process and its interactions with external entities. A Level 0 DFD breaks down the single process into major functions or processes within the system. Subsequent levels (Level 1, Level 2, etc.) further decompose processes into more detailed sub-processes. This hierarchical approach helps manage complexity when modeling large systems.

DFDs can also be categorized as Logical DFDs or Physical DFDs. A Logical DFD focuses on the business activities and data flows, showing what happens. A Physical DFD shows how the system is actually implemented, including specific technology, hardware, and people involved.