Information is transformed as it flows through a computer-based system. The system accepts input in a variety of forms; applies hardware, s...
Information is transformed as it flows through a computer-based system. The system accepts input in a variety of forms; applies hardware, software, and human elements to transform it; and produces output in a variety of forms. Input may be a control signal transmitted by a transducer, a series of numbers typed by a human operator, a packet of information transmitted on a network link, or a voluminous data file retrieved from secondary storage. The transform(s) may comprise a single logical comparison, a complex numerical algorithm, or a rule-inference approach of an expert system. Output may light a single LED or produce a 200-page report. In effect, we can create a flow model for any computer-based system, regardless of size and complexity.
Structured analysis began as an information flow modeling technique. A computer- based system is represented as an information transform . A rectangle is used to represent an external entity; that is, a system element (e.g., hardware, a person, another program) or another system that produces information for transformation by the software or receives information produced by the software. A circle (sometimes called a bubble) represents a process or transform that is applied to data (or control) and changes it in some way. An arrow represents one or more data items (data objects). All arrows on a data flow diagram should be labeled. The double line represents a data store—stored information that is used by the software. The simplicity of DFD notation is one reason why structured analysis techniques are widely used.
It is important to note that no explicit indication of the sequence of processing or conditional logic is supplied by the diagram. Procedure or sequence may be implicit in the diagram, but explicit logical details are generally delayed until software design. It is important not to confuse a DFD with the flowchart.
Data Flow Diagrams
As information moves through software, it is modified by a series of transformations. A data flow diagram is a graphical representation that depicts information flow and the transforms that are applied as data move from input to output. The basic form of a data flow diagram, also known as a data flow graph or a bubble chart, is illustrated in the figure below.
The data flow diagram may be used to represent a system or software at any level of abstraction. In fact, DFDs may be partitioned into levels that represent increasing information flow and functional detail. Therefore, the DFD provides a mechanism for functional modeling as well as information flow modeling. In so doing, it satisfies the second operational analysis principle (i.e., creating a functional model)
A level 0 DFD, also called a fundamental system model or a context model, represents the entire software element as a single bubble with input and output data indicated by incoming and outgoing arrows, respectively. Additional processes (bubbles) and information flow paths are represented as the level 0 DFD is partitioned to revealmore detail. For example, a level 1 DFD might contain five or six bubbles with interconnecting arrows. Each of the processes represented at level 1 is a subfunction of the overall system depicted in the context model.
As we noted earlier, each of the bubbles may be refined or layered to depict more detail. A fundamental model for system F indicates the primary input is A and ultimate output is B. We refine the F model into transforms f1 to f7. Note that information flow continuity must be maintained; that is, input and output to each refinement must remain the same. This concept, sometimes called balancing, is essential for the development of consistent models. Further refinement of f4 depicts detail in the form of transforms f41 to f45. Again, the input (X, Y) and output (Z) remain unchanged.
The basic notation used to develop a DFD is not in itself sufficient to describe requirements for software. For example, an arrow shown in a DFD represents a data object that is input to or output from a process. A data store represents some organized collection of data. But what is the content of the data implied by the arrow or depicted by the store? If the arrow (or the store) represents a collection of objects, what are they? These questions are answered by applying another component of the basic notation for structured analysis—the data dictionary.
DFD graphical notation must be augmented with descriptive text. A process specification (PSPEC) can be used to specify the processing details implied by a bubble within a DFD. The process specification describes the input to a function, the algorithm that is applied to transform the input, and the output that is produced. In addition, the PSPEC indicates restrictions and limitations imposed on the process (function), performance characteristics that are relevant to the process, and design constraints that may influence the way in which the process will be implemented.
