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More On Data Flow Modeling

More On Data Flow Modeling. CMSC 445 Pressman Chapters 11/12. Basic idea of Dataflow Modeling. Information flow through the system is modified by a series of transformations. Dataflow diagrams (DFD) DFDs capture these transformations in graphical form. Overview of Lecture.

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More On Data Flow Modeling

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  1. More On Data Flow Modeling CMSC 445 Pressman Chapters 11/12

  2. Basic idea of Dataflow Modeling • Information flow through the system is modified by a series of transformations. Dataflow diagrams (DFD) • DFDs capture these transformations in graphical form.

  3. Overview of Lecture • Components and notation of DFD's • Three important properties of DFD • Constructing a DFD • Some observations and hints

  4. Components and notation • Alternative notations possible (see Pressman Page 313)

  5. Another DFD Example • See also Pressman page 313 for another example

  6. Important DFD properties • Information flow in any system can be represented • ”Bubbles" can be refined to represent more detail • They describe flow of data rather than flow of control • Makes DFD implementation independent • Bottom level is a PSPEC • see Pressman page page 318

  7. Constructing a DFD • Start with Context diagram (Level 0 DFD)which shows only a single process (represents the entire system), and external entities • Identify the functions to be performed • Show the information flow between functions and identify data stores and external entities • Repeatedly elaborate on the DFD ( bottom up and top down ) • After each elaboration is complete validate the DFD: • identify missing functions • identify functions that need more detail • rearrange in higher order bubbles • identify features that do not meet with the user's approval

  8. Constructing a DFD • We always start with a physical representation of the problem • That is translated into a requirements spec • From the spec a context diagram is derived • Process detailed in Pressman pgs. 323 -325 • We’ll look at another example

  9. Context Diagram Example • Suppose that the requirements said the following Letters of complaint received from citizens are entered into a complaints master file by the City Clerk’s office. The date, department code, and a complaint description are stored for each letter. Weekly reports are produced from the complaints master file and given to the City Manager. Two weekly reports are produced: a department summary and a detail list.

  10. Identifying Flows, Processes and Sources, and Sinks

  11. Resulting Context Diagram

  12. Considerations for the Level One Diagram

  13. Level - 1 Diagram City Complaint Handling System

  14. Some observations and hints • All Bubbles should be connected • At each Level there should be no more than 5 - 7 sub bubbles • Refine only one Process Bubble at a time • When refining the information flow: information continuity must be maintained • Deciding when to stop may be difficult

  15. When to stop decomposing • Reduced each process to a single decision or calculation or single database operation • Data store represents data about a single entity • System user doesn’t want to see any more detail • Every data flow doesn’t need to be split further to show the differences in data handling • Each business form or transaction, on-line display, report is a single data flow

  16. How to get started building logical DFDs • Start with a physical DFD, then get rid of all physical references • Organize all the information in the problem by type • data (data flows, data stores, data elements) • people (sources/sink or physical) • activities (processes) • hardware/software (physical) • places (physical) • Build the decomposition diagram first to identify activities (processes), then work on lower level DFDs

  17. DFD Rule Review Checklist • All data flows should be labeled. (These are the most common labeling omissions.) • If you can’t name a data flow, re-evaluate it’s existence -- is it really a data flow? • External entities (or agent) are outside the system: sources or sinks of data. • Individuals performing functions within the system are NOT entities -- their functions are captured as processes. • If you find yourself naming a data flow a verb -- take another look -- it may be a process! Data flows should be noun clauses. • All external entities shown on sub-diagrams (Level 0+) should be on the context level diagram. • Make certain your labels are meaningful

  18. References for Lecture • Pressman Chapters 11/12 • Online Sources: • nyquist.ee.ualberta.ca/~wjoerg/SE/SE_Notes-F/SE_What-F/SE_Req-F/SE_DFD.htm • www.wright.edu/coba/msis/mis321

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