Objectives: Introduction Over View of System Analysis and Design
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| 6.4.1.2 Constructing a DFD Several rules of thumb are used in drawing DFDs. 1. Process should be named and numbered for easy reference. Each name should be representative of the process. 2. The direction of flow is from top to bottom and from left to right. Data traditionally flow from the source (upper left corner) to the destination (lower right corner, although they may flow back to a source. One way to indicate this is to draw along flow line back to the source. An alternative way is to repeat the source symbol as a destination. Since it is used more than one in the DFD, it is marked with a short diagonal in the lower right corner. 3. When a process is exploded into lower- level details, they are numbered. 4. The names of data stores, sources and destinations are written in capital letters. Process and data flow names have the first letter of each word capitalized. How detailed should a DFD be As mentioned earlier, the DFD is designed to aid communication. If it contains dozens of processes and data stores, it gets too unwieldy. The rule of thumb is to explode the DFD to a functional level, so that the next sublevel does not exceeded 10 processes. Beyond that, it is best to take each function separately and expand it to show the explosion of the single process. If a user wants to know what happens within a given process, then the detailed explosion of that process maybe shown. A DFD typically shows the minimum contents of data stores. Each data store should contain all the data elements that flow in and out. Questionnaires can be used to provide information fora first cut. All discrepancies, missing interfaces, redundancies, and the like are then accounted for- often through interviews. The DFD methodology is quite effective, especially when the required design is unclear and the user and the analyst need a notational language for communication. The DFD is easy to understand after a brief orientation. The main problem however is the large number of iterations that often are required to arrive at the most accurate and complete solution. Download 0.94 Mb. Share with your friends:
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