Data Flow Diagram
Data Flow Diagram (DFD) is the part of the SSADM method (Structured Systems Analysis and Design Methodology), intended for analysis and information systems projection. Data Flow Diagrams are intended for graphical representation of data flows in the information system and for analysis of data processing during the structural projection. By means of data flow diagrams it is possible to show visually the work of the information system and results of this work. Data Flow Diagram visualizes processes, data depositories and external entities in information systems and data flows connecting these elements.
Data Flow Diagram consists of following components:
This variant of presentation of Data Flow Diagram objects got the name of Yourdon – de Marco notation.
Objects of Data Flow Diagrams are interpreted in the following way:
Every diagram object should have a name. Each data flow is denoted with indication of transferred data and with the possibility of indication of the format of these data. Data Flow Diagram should not involve more than 10 objects, excluding arrows, representing data flows. In case of more complicated system the totality of several objects (as a rule, processes) is united and represented on the diagram in the form of one object. This complicated compound object is presented in the form of a separate flows diagram. Each component has a number, at this, diagrams illustrating compound objects are numerated starting from the number of an object which they describe. For example components of the diagram of the description of an object with number 5 will be numerated 5.1, 5.2 etc. Generally Process Flow Diagram depicts just the major equipment without details.
For clearness there is a possibility to duplicate notations of used data depositories and external entities. Processes can’t be duplicated. For example if one the same data depository is used in several different processes it is better to duplicate it on the diagram but not to create several intricate data flow arrows from one object to several processes. This concerns external entities as well. Duplicates of data depositories are marked with the double line from the left side, external entities duplicates - with the asterisk.
Data flows on the diagram can ramify and merge that implies branching or confluence of data in the information system.
Example 1. Data Flow Diagram
Thus the information system is represented by a planner in the form of the high level DFD in which objects are worked out in details by diagrams of the lower level with the preset level of detailed elaboration. There is also another way of looking at which all events in the system are described at once and each event is represented in the form of process transforming data flows, further these subruns are grouped for getting diagrams of the higher level.
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