Data Flow Diagramming Rules

1
Data Flow Diagramming Rules

• Data stores and sources/sinks
• no data flows between two data stores must be a
  process in between
• no data flows between a data store and a source
  or sink must be a process in between
• no data flows between two sources/sinks
• such a data flow is not of interest, or
• there is a process that moves that data

2
Customer Information
Customer Information
Customer Data
Customer Data
Customer Preferences
Customer Data
Customer Data
D1
D1
Customer Preferences
Customer Preferences
D2
Customer Preferences
D2
3
Customer Information
Customer Information
Customer Data
Customer Data
Customer Data
D1
Customer Data
Customer Data
D1
2.2
Extract Customer Preferences
Customer Preferences
Customer Preferences
Customer Preferences
D2
Customer Preferences
D2
4
Customer
Customer Information
Customer
Customer Data
Customer Data
D1
Customer Data
Customer Data
D1
5
Service Information
Doctor
Diagnosis
Bill
Patient
6
Data Flow Diagramming Rules

• Processes
• a process must have at least one input
• a process must have at least one output
• a process name (except for the context level
  process) should be a verb phrase
• usually three words verb, modifier, noun
• on a physical DFD, could be a complete sentence

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1.0
Demographic Data
Gather Data
Survey Responses
Final Report
8
2.0
Visa Authorization
BETTER
2.0
Total Sales Records
BETTER
2.0
Inspect Finished Products
BETTER
9
Data Flow Diagramming Rules

• Data flows
• data flows are unidirectional
• a data flow may fork, delivering exactly the same
  data to two different destinations
• two data flows may join to form one only if the
  original two are exactly the same
• no recursive data flows
• data flows (and data stores and sources/sinks)
  are labelled with noun phrases

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Customer Order
Order Information
Order Total
Order Information
Order Total
3.0
3.0
Print Delivery Instructions
Print Delivery Instructions
11
Customer Order
Customer Address
Customer Order
Customer Address
Customer Information
3.0
3.0
Print Delivery Instructions
Print Delivery Instructions
12
Daily Sales
Cumulative To-Date Sales
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Data Flow Diagramming Guidelines

• The inputs to a process are different from the
  outputs
• Every object in a DFD has a unique name

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Customer Data
Customer Data
Customer Data
Valid Customer Data
15
Order
Customer Data
Customer Data
Customer Data
Order
16
Customer Data
Customer Data
Only if these are exactly the same
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Employee Master
D1
Employee Record
Hours Worked
1.0
2.0
Gross Pay
Calculate Gross Pay
Calculate Withholding Amount
Employee Time Record
Employee Time File
D2
Withholding
3.0
4.0
Employee Record
Employee Master
D1
Net Pay
Calculate Net Pay
Print Employee Paycheck
Check Reconciliation Record
Employee Paycheck
Check Reconciliation
D3
18
Withholding Tables
D4
Number of Dependents
Employee Master
D1
Withholding Rates
Hours Worked
Employee Record
5.0
1.0
2.0
Create Time Record
Gross Pay
Calculate Gross Pay
Calculate Withholding Amount
Employee Time Record
Employee Time Record
Withholding Amount
Employee Time File
Gross Pay
D2
Employee Master
D1
3.0
4.0
Employee Record
Net Pay
6.0
Calculate Net Pay
Print Employee Paycheck
Paycheck Information
Reconcile Pay Check
Employee Paycheck
Check Reconciliation Record
Check Reconciliation
D3
19
Data Elements

• Indivisible pieces of data
• Data flows and data stores are made up of data
  elements
• Like attributes on an ER diagram
• The data elements of a data flow flowing in or
  out of a data store must be a subset of the data
  elements in that data store