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Data Flow Diagramming Rules

Understand the basic rules of data flow diagramming, including the flow of data between symbols and the decomposition of DFDs. Learn how to draw correct data flow diagrams and analyze different levels of decomposition.

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Data Flow Diagramming Rules

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  1. Data Flow Diagramming Rules • Basic rules that apply to all DFDs • Inputs to a process are always different than outputs • Objects always have a unique name • In order to keep the diagram uncluttered, you can repeat data stores and sources/sinks on a diagram 8.1

  2. Process No process can have only outputs (a miracle) No process can have only inputs (black hole) A process has a verb phrase label Data Store Data cannot be moved directly from one store to another Data cannot move directly from an outside source to a data store Data cannot move directly from a data store to a data sink Data store has a noun phrase label Data Flow Diagramming Rules 8.2

  3. Source/Sink Data cannot move directly from a source to a sink A source/sink has a noun phrase label Data Flow A data flow has only one direction of flow between symbols A fork means that exactly the same data goes from a common location to two or more processes, data stores or sources/sinks Data Flow Diagramming Rules 8.3

  4. Data Flow Diagramming Rules • Data Flow (Continued) • A join means that exactly the same data comes from any two or more different processes, data stores or sources/sinks to a common location • A data flow cannot go directly back to the same process it leaves • A data flow to a data store means update • A data flow from a data store means retrieve or use • A data flow has a noun phrase label 8.4

  5. 8.5

  6. Incorrect and Correct ways to draw data flow diagram 8.6

  7. Decomposition of DFDs • Functional decomposition • Act of going from one single system to many component processes • Repetitive procedure • Lowest level is called a primitive DFD • Level-N Diagrams • A DFD that is the result of n nested decompositions of a series of subprocesses from a process on a level-0 diagram 8.7

  8. Figure 8-4Context diagram of Hoosier Burger’s food ordering system 8.8

  9. Figure 8-5Level-0 DFD of Hoosier Burger’s food ordering system 8.9

  10. Level-1 diagram showing the decomposition of process 1.0 from the level-0 diagram for Hoosier Burger’s food ordering system 8.10

  11. Level-1 diagram showing the decomposition of process 4.0 from the level-0 diagram for Hoosier Burger’s food ordering system 8.11

  12. Level-2 diagram showing Process 4.3 from the level-1 diagram for process 4.0 for Hoosier Burger’s food ordering system 8.12

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