Data Flow Diagrams (DFD) and Difference between Logical and Physical DFD

• A data flow diagram represents a software system as a labeled, directed graph.
• Data flow diagram illustrates the flow of data among a set of components.
• The components may be tasks, software components, or even abstractions of the functionality that will be included in the software system.

The data flow diagram (DFD) serves two purposes:

1. Provides an indication of how data are transformed as they move through the system.
2. Depicts the functions (and sub functions) that transform the data flow.

Symbols Used In DFD

Types of data flow diagrams (DFDs)

Data flow diagrams (DFDs) are categorized as either logical or physical. A logical DFD focuses on the business and how the business operates. It describes the business events that take place and the data required and produced by each event. On the other hand, a physical DFD shows how the system will be implemented.

Design Feature	Logical	Physical
What the model depicts	How the business operates	How hue system will be implemented (or how the current system operates)
What the process represent	Business activities	Programs, program modules and manual procedures
What the data stores represent	Collection of data, regardless of how the data is stored	Physical files and databases, manual files
Type of data source	Show data stores representing permanent data collections	Master files, transaction files. Any processes that operate at two different times must be connected by a data source
System controls	Show business controls	Show controls for validating input data, for obtaining a record (record found status), for ensuring successful completion of a process and for system security (example: Journal records)

A context diagram depicts the entire process being modeled as a single task, or box, in a process diagram. The diagram shows important interactions between the system and external agents.

A subsystem DFD contains one process per major subsystem. The DFD shows important interfaces with external agents. Each process is further represented by another DFD—an event-partitioned DFD (or diagram zero) for that subsystem.

An event-partitioned system model contains one process per event. The model shows important interfaces with external agents. If no subsystem DFD is created, the event-partitioned system model is also called diagram 0.

A DFD fragment is a portion of an event-partitioned system model that shows the process, external agents, data stores, and data flows needed to respond to a single event.

A process decomposition is a DFD that shows the internal implementation details of a single process on another DFD.

What are the advantages of DFDs?

• DFDs depict flow and boundary.
• DFD represents a graphical display of the process and is therefore a usable document that can be shown to both users and programmers. It serves the users as process verification.It serves the programmers as the schematic (blueprint) of the system from a technical perspective.
• It can be used in JAD sessions and as part of the business specification documentation.
• Most important, it can be used for maintaining and enhancing a process.