Overview of Data Flow Diagram


Data Flow Symbols
  • Unique Names for Data Flows.  No two data flows should ever have the same name.  (Remember, data is transformed by a process, so the input and output data flows from a process will have different names.)
  • Data Flows should be consistent and cascade among levels.
    • For example, if a “Student” EE provides an application to a process on a level 1 DFD (Figure 1 or Figure 2 or Figure 3), then it must appear on the Figure 0 DFD and the context diagram
    • As a quick double-check before you turn in home work and projects: Count all the data flows on your context diagram from each EE.  On the level 0 diagram, the counts should be the same.  Cascade this rule for each subordinate level.
      • NOTE: You should perform this same check for each data store.

Process Symbols
  • DFD Process symbols have 3-parts: a numeric id on top, a verb-object process label in the middle, and an actor-place-system identifier on the bottom.  (Process symbols on a Figure 0 diagram do not have to have an actor-place identifier if the process is exploded and there is more than one actor-place involved in the lower level process.)
  • Context Diagrams should have a single process circle that is labeled with a noun phrase for the system or overall process,
  • Processes transform data; printing, sending, retrieving, etc are not DFD  processes.
  • Process numbering should be consistent across levels of diagrams
  • No more than 7 process bubbles should be on any diagram.  Processes that are more complex should be “exploded” on a lower level diagram.

Environmental Entity (EE) Symbols
  • Environmental entities should be consistent and cascade among levels.
    • For example, if a “Supplier” EE appears on a level 1 DFD (Figure 1 or Figure 2 or Figure 3), then it must appear on the Figure 0 DFD and the context diagram
    • The number of EEs on the Context Diagram and Figure 0 diagram should be the same.   The numbers will not be the same on the lower level diagrams, since each lower level diagram is a subset of the process, but they should be consistent. 
  • No data flows between environmental entities.  Environmental entities can not “talk” directly to each other.
    • The rationale – If environmental entities could communicate directly, they wouldn’t be using the diagrammed system.  (They’d be operating outside the system.)
  • If an environmental entity is repeated on the same diagram, then each instance should be numbered separately; e.g. EE1/1 and EE1/2 identify the 1st and 2nd instance of EE1

Data Store Symbols
  • Data Stores should be named and numbered using D1, D2, etc or S1, S2 etc.
  • Data Stores should be consistent across levels of diagrams
  • Data Stores do not communicate with each other or EEs directly.  A process is required for sending data to a data store and retrieving data from a data store

Consistency across levels
  • Number and label all diagrams.  To maintain consistency, each environmental entity, process, and data store should be labeled using the same number and text.  These numbers and text descriptions should be the same across all levels.

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