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Data Flow Diagram

A data flow diagram (DFD) is a graphical representation of the "flow" of data through an information system. A data flow diagram can also be used for the visualization of data processing (structured design). It is common practice for a designer to draw a context-level DFD first which shows the interaction between the system and outside entities. This context-level DFD is then "exploded" to show more detail of the system being modeled.

Data flow diagram notation

External agents/Terminators

are outside of the system being modeled. Terminators (also referred to as sources or sinks, depending on whether data flows from or into them) represent where information comes from or where it goes. In designing a system, we have no idea about what these terminators do or how they do it.

Processes

modify the inputs in the process of generating the outputs

Data Stores

represent a place in the process where data comes to rest. A DFD does not say anything about the relative timing of the processes, so a data store might be a place to accumulate data over a year for the annual accounting process.

Data Flows

are how data moves between terminators, processes, and data stores (those that cross the system boundary are known as IO or Input Output Descriptions).

Real Time link

communication back and forth between external agent and a process as the process is executing.

Developing a DFD

Using Top-Down Approach

1. The system designer makes a context level DFD, which shows the interaction (data flows) between the system (represented by one process) and the system environment (represented by terminators).

2. The system is decomposed in lower level DFD (Zero) into a set of processes, data stores, and the data flows between these processes and data stores.

3. Each process is then decomposed into an even lower level diagram containing its subprocesses.

4. This approach then continues on the subsequent...