Ouline Of The Use And Functions Of Dfd (Data Flow Diagrams).

2341 words - 9 pages

Data Flow Diagrams (DFDs)1. IntroductionWhen system analysts attempt to understand the information requirements of users, they must be able to conceptualize how data moves through the organization, the processes or transformations that the data undergoes, and what the outputs are. Through a structured analysis technique called data flow diagrams (DFDs), the systems analyst can put together a graphical representation of data processes throughout the organization. The data flow approach emphasizes the logic underlying the system. By using combinations of only four symbols, the systems analyst can create a pictorial depiction of processes that will eventually provide solid system documentation.The data flow approach has four chief advantages over narrative explanations of the way data moves through the system (Kendall, 2002).·Freedom from committing to the technical implementation of the system too early.·Further understanding of the interrelatedness of systems and subsystems.·Communicating current system knowledge to user through data flow diagrams.·Analysis of a proposed system to determine if the necessary data and processes have been defined.2. Data Flow Diagram (DFD) NotionsProcesses:·Describe something actually happening, including any decisions that are made.·The process should be named clearly by using a verb-adjective-noun format for detailed processes.·Each process consists of a rectangle: the small left hand box contains the process number; to the right is entered the process location (which person or department causes the happening); the main box contains the process description (what is happening).·Processes can be decomposed into sub-processes, sub-sub-processes etc. - allowing more and more detail to be exposed. An asterisk indicates that the 'bottom level' of expansion has been reached.·The number of processes on a DFD should not usually exceed nine processes.Data Flows:·A data flow is shown by a line with an arrowhead, indicating the direction of flow.·Each data flow must have a label to explain what it is. Note that a data flow name must not include a verb. Data flows do not do anything- they simply exist.·The data may be carried in the form of documents or by any other means. No alternation of data can take place within a data flow - all changes are made by processes.Datastores:·A data store is a place where data comes to rest, it is anywhere that data is stored and retrieved.·An open-ended rectangle indicates a data store. In the small left-hand box is entered the reference 'D' (computer-held data) or 'M' (manually held data) followed by the data store number.·A data store is labelled with a description of the contents of the data store, using a noun.·If it is necessary (for the purpose of a neat layout) to show the same data store more than once on a diagram, all instances of it should have a duplicate line...

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