Data Flow Diagrams (DFDs)

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Specific Instructional Objectives

At the end of this lesson the student will be able to: • • • Identify the activities carried out during the structured analysis phase. Explain what a DFD is. Explain why constructing DFDs are important in arriving at a good software design. • Explain what a data dictionary is. • Explain the importance of data dictionary. • Identify whether a DFD is balanced.

Structured Analysis

Structured analysis is used to carry out the top-down decomposition of a set of high-level functions depicted in the problem description and to represent them graphically. During structured analysis, functional decomposition of the system is achieved. That is, each function that the system performs is analyzed and hierarchically decomposed into more detailed functions. Structured analysis technique is based on the following essential underlying principles: • • • Top-down decomposition approach. Divide and conquer principle. Each function is decomposed independently. Graphical representation of the analysis results using Data Flow Diagrams (DFDs).

Data Flow Diagram (DFD)

The DFD (also known as a bubble chart) is a hierarchical graphical model of a system that shows the different processing activities or functions that the system performs and the data interchange among these functions. Each function is considered as a processing station (or process) that consumes some input data and produces some output data. The system is represented in terms of the input data to the system, various processing carried out on these data, and the output data generated by the system. A DFD model uses a very limited number of primitive symbols [as shown in fig. 5.1(a)] to represent the functions performed by a system and the data flow among these functions.

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External Entity

Process

Output

Data Flow

(a)

Data Store

number readnumber

validatenumber

data-item

validnumber

(b)

data-item

readnumber

validatenumber

validnumber

number

(c) Fig. 5.1 (a) Symbols used for designing DFDs (b), (c) Synchronous and asynchronous data flow Here, two examples of data flow that describe input and validation of data are considered. In Fig. 5.1(b), the two processes are directly connected by a data flow. This means that the ‘validate-number’ process can start only after the ‘readnumber’ process had supplied data to it. However in Fig 5.1(c), the two processes are connected through a data store. Hence, the operations of the two bubbles are independent. The first one is termed ‘synchronous’ and the second one ‘asynchronous’.

Importance of DFDs in a good software design

The main reason why the DFD technique is so popular is probably because of the fact that DFD is a very simple formalism – it is simple to understand and use. Starting with a set of high-level functions that a system performs, a DFD model

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hierarchically represents various sub-functions. In fact, any hierarchical model is simple to understand. Human mind is such that it can easily understand any hierarchical model of a system – because in a hierarchical model, starting with a very simple and abstract model of a system, different details of the system are slowly introduced through different hierarchies. The data flow diagramming technique also follows a very simple set of intuitive concepts and rules. DFD is an elegant modeling technique that turns out to be useful not only to represent the results of structured analysis of a software problem, but also for several other applications such as showing the flow of documents or items in an organization.

Data dictionary

A data dictionary lists all data items appearing in the DFD model of a system. The data items listed include all data flows and the contents of all data stores appearing on the DFDs in the DFD model of a system. A data dictionary lists the purpose of all data items and the...