Transcript DFD - Virginia Commonwealth University

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
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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”
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
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Leveling/Balancing Errors
Deceptive Process Labels
Flow of Control Information
Too many data flows
Never-used Inputs
Widely varied depths of explosion