Data Flow Diagrams

Advantages of Data Flow Approach

1. Freedom from committing to the technical implementation of the system too early. 2. Further understanding of the interrelatedness of systems and subsystems. 3. Communicating current system knowledge to users through data flow diagrams. 4. Analysis of a proposed system to determine if the necessary data and processes have been defined.

Some Mistakes

1. Spontaneous Generation 2. Black Hole 3. Gray Hole

Entities

A DFD shows only external entities that provide data to the system or receive output from the system. A DFD shows the boundaries of the system and how the system interfaces with the outside world. An entity name is the singular form of a department, outside organization, other information system, or person.

Data Flow Diagrams

A Data flow diagram is a graphical representation of the flow of data in an information system. It is capable of depicting incoming data flow, outgoing data flow and stored data.

Data Flow

A data flow is a path for data to move from one part of the information system to another. A data flow in a DFD represents one or more data items. For example, a data flow could consist of a single data item (such as a student ID number) or it could include a set of data (such as a class roster with student ID numbers, names, and registration dates for a specific class). The symbol for a data flow is a line with a single or double arrowhead.

Data Store

A data store is used in a DFD to represent data that the system stores because one or more processes need to use the data at a later time. For instance, instructors need to store student scores on tests and assignments during the semester so they can assign final grades at the end of the term.

Logical DFD vs Physical DFD

A logical DFD focuses on the business and business activities, while a physical DFD looks at how a system is implemented. So while any data flow diagram maps out the flow of information for a process or system, the logical diagram provides the "what" and the physical provides the "how."

Spontaneous Generation

A process produces output, but has no input data flow. Because it has no input, the process is called a spontaneous generation process.

Gray Hole

A process that has at least one input and one output, but the input obviously is insufficient to generate the output shown.

Black Hole

A process that has input, but produces no output.

Process

The name of the process appears inside the rectangle/circle. A process receives input data and produces output that has different content, form, or both. For instance, the process for calculating pay uses two inputs (pay rate and hours worked) to produce one output (total pay). Processes can be very simple or quite complex.