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Data Flow Diagrams

Data Flow Diagrams. Outline Components of DFD The structured analysis methodology DFD creation Guidelines Common errors. Components of DFD. External entity Process Data flow Data store *Data dictionary. Structured Analysis. Create a logical model of the system (draw a DFD)

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Data Flow Diagrams

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  1. Data Flow Diagrams • Outline • Components of DFD • The structured analysis methodology • DFD creation • Guidelines • Common errors

  2. Components of DFD • External entity • Process • Data flow • Data store • *Data dictionary

  3. Structured Analysis • Create a logical model of the system (draw a DFD) • Put the detail in a data dictionary • Define the logic of the processes • Define contents and immediate access for data stores • Create a functional specification

  4. DFD Creation • Collect entities and their inputs/outputs with the system • Draw the context level DFD (Level 0) • Update data dictionary • Expand DFDs into next level DFDs • Repeat 3 and 4 until the overall DFDs are sufficient enough to satisfy all the requirements in detailed expression

  5. Data Dictionary • Organize a dictionary to maintain the information of all components completely and consistently. • Double check titles, numbers, labels, and names with DFDs after creating DD.

  6. Logic of Processes • Use decision tree to fetch possible branches for handling the input data flows • Use structured English (or PDL) to express the processes which perform these branches

  7. Data Stores • Are these logical data stores the simplest possible? Can they be combined? Should they be combined? • What immediate accesses to the data stores will be needed, and how valuable is each type of access? • *Any existing data stores will do the job?

  8. Some Guidelines • Choose meaningful names for processes, data stores, data flows, and external entities • Hierarchically number the processes • Avoid overly complex DFDs (no more than half a dozen bubbles per diagrams) • Redraw the DFDs as many times as necessary

  9. Some Guidelines • Make sure your DFDs are logically consistent • Avoid processes w/o input flows and/or output flows • Beware of unlabeled components • Beware of read-only/write-only data stores

  10. Common Errors • Only verb or noun in the process names • Labels are inconsistent in 2 levels • Labels are inconsistent in DFD and DD • Decomposition brings data stores into upper processes • Decomposition generates new data stores outside upper process

  11. Common Errors • Data flows out but no data flows in, or vice versa • Two flows are connected together • The titles/numbers are different between two levels • Flows missing after decomposition • Redundant names for different components

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