Process Modeling

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Process Modeling

• By
• Bob Larson

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Objectives

• Describe why the data flow diagram is called a
  process model
• Recognize and describe an abstraction
• Identify the four elements of a data flow diagram
• Construct a data flow diagram
• Decompose a data flow diagram

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Modeling Systems

• Process model
• Diagrams how data flows through the system
• Data model
• Diagrams the relationships between data files
• Object model
• Diagrams the relationships between enterprise
  objects
• System models
• Diagram system functions, hardware and software

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The Data Flow Diagram (DFD)

• View of what people and procedures do to
  transform data into information
• Composed of four elements
• External entity
• Originator of data (source)
• Receiver of information (sink)
• Process a series of steps that manipulate data
• Data store place to keep data for later
  reference
• Data flow describes data and information
  elements passing between external entities,
  processes, and data stores

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DFD and the Six CIS Components
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Illustrated DFD Symbols

• Standard rules for construction
• Data flows must originate or terminate at a
  process
• Data stores must have at least one entry and one
  exit data flow
• Processes must have at least one entry and one
  exit data flow

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Note difference between Source and Sink
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Bottom-Up DFD creation

• Develop a narrative of the system
• Underline the action words
• Develop a sequential list of the action words
• Eliminate tasks that do not transform data
• Identify cohesive tasks
• Fit all remaining tasks to a cohesive task
• Develop an IPO chart for each cohesive task

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Data Flow Diagrams - Example

• Portion of Silhouette Sea Charters narrative
• Sometimes the office manager cannot determine
  whether the request is reasonable. When this
  happens, the request is forwarded to the owner,
  who makes this determination, contacts the
  customer if necessary, and either adjusts the
  request or rejects it outright. All requests are
  returned to the office manager for scheduling and
  filing.

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Bottom-Up DFD Worksheet
Action words identified in the narrative
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Evaluate Special Requests
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Partial Data Flow Diagram Sketch
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Top-Down DFD Creation

• Solicit oral answers to the question What is the
  first task of this system?
• Continue with What happens next?
• Repeat Step 2 until the response is We do it all
  over again, or We are finished.

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The First Context Diagram

• Shows a single process
• Connected only to the external entities
• Establishes the system boundaries

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Revised Context Diagram
Add two new external entities and several new
data flows appear, reflecting our revised
understanding of the system.
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Internal and External Entities

• External Entities
• Something or someone who either generates data or
  receives information
• Operations of such entities are beyond analysts
  control
• Students, data processing, lab managers, and
  teachers in our example
• All entities are by definition external
• Dont get tempted to identify a person or an
  object that performs a task as an entity

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Level Decomposition

• Any single process in the context diagram is
  composed of a collection of well-defined
  activities called cohesive tasks
• Task definition begins by locating the events
  within a process
• Events identify the end of one task and the
  beginning of another
• Once a task is identified, the analyst can
  identify the inputs and outputs

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Task Issue Access Cards

• Issue Access Cards INPUTS
• class rosters (Source Data Processing)
• access cards for registered students (Source
  Data Processing)
• student registration (Source Data Processing)
• signed non-piracy agreement (Source Student)
• name on class rosters (Data Store Roster)
• Issue Access Cards OUTPUTS
• access cards (Sink Student)
• lab handbook (Sink Student)
• card numbers already issued (Sink Data
  Processing)
• checkoff on class rosters (Data Store Roster)
• new lab account (Data Store Time)
• nonpiracy agreement (Data Store Agreements)

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Issue Access Cards Task DFD
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Logical and Physical DFDs

• Logical DFDs remove all reference to the
  implementation specifics of the system
• Physical DFDs specify the real world objects that
  are used to make the system work
• During the analysis phase
• Develop the physical DFD
• Example Add the numbers with a calculator
• Abstract the logical DFD
• Example Compute the total
• During the design phase
• Develop the logical DFD
• Example Accumulate the total
• Develop the physical DFD
• Example Input the numbers with a bar-code scanner

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First-Level DFD
Shadow indicates the entity is depicted twice
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Data Flow Walk-Through
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TKSystem DFD Levels
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Fin