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DFD. Process modeling. To do tonight. 1. Learn to draw DFDs 2. Understand what does not belong in DFDs 3. Learn DFD Rules and Conventions. What DFD is?. A DFD is a network representation of a system. The system may be computerized, manual, or a hybrid of the two.

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DFD

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DFD

Process modeling


To do tonight..

1. Learn to draw DFDs

2. Understand what does not belong in DFDs

3. Learn DFD Rules and Conventions


What DFD is?

  • A DFD is a network representation of a system. The system may be computerized, manual, or a hybrid of the two.

  • The DFD portrays the system in terms of its component pieces, with all interfaces among the components indicated

  • DFDs should not show flow of control or control

    information

  • DFDs are not really flow diagrams in the traditional sense--they are a tool for functional decomposition


DFDs-- Four notational symbols


Reading Data Flow Diagrams


Identifying Processes

  • Direct Identification

  • Top-Down

  • Bottom-Up

  • Outward-In

  • Functional Sequence


Identifying Data Stores

  • Examine Nouns

  • Identify significant ‘things’ about which

    you need to store information

  • Avoid description in terms of physical devices

  • Data Store =>e.g. “Applications”

    ¹ Bill’s File


DFD: Conventions - Processes

  • Process names should be descriptive.

  • Processes should have a single Action Verb

    and a Singular Object.

Tip: Name data flows before naming Processes.


DFD: Conventions - Data Flows

  • Names reflect not only data which moves,

    but what we know about the data.


DFD: Common Mistakes


DFD: Common Mistakes


DFD: Common Mistakes


DFD: Common Mistakes

  • Black Holes and Miracles


DFD: Balancing and Leveling


Working toward a leveled set of DFDs


Working toward a leveled set of DFDs


Stopping

  • When do we stop creating more levels?

    – Each Process performs only ONE function

  • Signs indicating need to partition further:

    – process is difficult to name

    – has too many inputs or too many outputs

    – would be easier to understand if broken down


Signs of Errors

  • Leveling/Balancing Errors

  • Deceptive Process Labels

  • Flow of Control Information

  • Too many data flows

  • Never-used Inputs

  • Widely varied depths of explosion


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