Process Models: Data Flow Diagrams

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Process ModelsData Flow Diagrams

• Special thanks goes to Dr. Jack T. Marchewka for
  many of the ideas and examples contained in these
  notes.

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Process Modeling

• Objective
• Understand the concept of business processes
• Understand and create Data Flow Diagrams.

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DFDs

• Visually represent
• data flows
• processes
• Emphasize the logic of system
• Provide documentation for Analysis Design

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Data Flow Diagrams
Data Flow
Process
File or Data Store
Source or Entity
Gane Sarson Methodology
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DFDs

• Do not show technical aspects
• Show interrelations of systems subsystems
• Facilitates
• developer understanding
• user-developer communication
• Completeness of data process definitions

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The Systems Concept
Output
Input
Process
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DFDs
1
Data Flow Input
Data Flow Output
Process
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Developing DFDs

• Context Diagram
• System Overview
• Little or no detail
• Zero Level Explosion
• More detailed
• First,Second, Third Levels
• More detailed levels

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Context DFD

• Basic overview of the system
• Contains only 1 process
• given the number zero
• Shows all external entities
• Major data flows among entities
• Generally no data stores
• Assign a name to the whole system

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Context DFD Examples
Apartment Rental
Receipts, Notices
Lease Cash Report
0
External Manager
Apartment Rental System
Tenant
Lease, Payments
Bank Deposit
Bank
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Recall on-line university registration (from
Use Case examples)
The system should enable the staff of each
academic department to examine the course offered
by their department, add and remove course, and
change the information about them (e.g., the
maximum number of students). It should permit
students to examine currently available courses,
add and drop courses to and from their schedules,
and examine the course for which they are
enrolled. Department staff should be able to
print a variety of reports about the courses and
the students enrolled in them. They system
should ensure that no student takes too many
course and that students who have any unpaid
fees are not permitted to register. (Assume that
a fees data store is maintained by the
university's financial office that the
registration system accesses but does not change.)
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Registration Context Diagram
0
Course information
Enrollment information
Department Staff
Students
Course offerings
Student schedules
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Recall real estate (from Use Case examples)
A Real Estate Inc. (AREI) sells houses. People
who want to sell their houses sign a contract
with AREI and provide information on their house.
This information is kept in a database by AREI
and a subset of this information is sent to the
citywide multiple-listing service used by all
real estate agents. AREI works with two types of
potential buyers. Some buyers have an interest
in one specific house. In this case, AREI prints
information from its database, which the real
estate agent uses to help show the house to the
buyer (a process beyond the scope of the system
to be modeled). Other buyers seek AREIs advice
in finding a house that meets their needs. In
this case, the buyer completes a buyer
information form that is entered into a buyer
database, and AREI real estate agents use its
information to search AREIs database and the
multiple-listing service for houses that meet
their needs. The results of these searches are
printed and used to help the real estate agent
show houses to the buyer.
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AREI Context Diagram
0
Buyer information
Sellers
Buyers
House information
House information
House Information
House Information
Multiple Listing Service
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Level 0 DFD

• Detail from exploding the context-level DFD
• Show less than 9 processes
• Number each process
• Show data stores (master files)
• Ignore the handling of exceptions
• Show all external entities

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Tenant Info
Level 0 DFD Apartment Rental
D1
Tenant File
Copy of lease
1
Lease
New Tenant Process
Bank Deposit
Receipt
Tenant
Bank
2
Payments
Collection Process
Notice
Cash Report
Unpaid Charges
Tenant Info
Ext. Mgr
3
D1
Tenant File
Delinquent Process
Delinquency Report
Delinquencies
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Level 0 DFD Registration
Course Offering Changes
Available course request
Available courses
Available courses
Course Offering List
Students
Available courses
Course enrollment
Course Offering Updates
Course Offerings
Student schedule
Course enrollment request
Dept Staff
Student schedule
Course information
Fee Payment History
Student Enrollment Report Request
Enrollment information
Student Enrollment Report
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Level 0 DFD AREI
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Create Child Diagrams

• Vertical Balancing
• A child diagram cannot produce output or receive
  input that the parent does not produce or receive
• External entities are not shown
• May contain additional data stores
• e.g.., transaction files
• Primitive when no longer exploded

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Level 1 DFD Apartment Rental
2.1
Deposit Checks
Deposit Receipts
Collect Security Deposit
Tenant Info
Bank Deposit
D1
Tenant File
Tenant Info
Bank Deposit
2.2
Rent Checks
Payment Receipts
Collect Rent
Cash Report
Unpaid Charges
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Level 2 DFD Apartment Rental
2.1.1
Deposit Check
Bank Deposit
Make Bank Deposit
Deposit Info
2.1.2
Update Info
Update Tenant File
D1
Tenant File
2.1.3
Tenant Info
Receipt
Create Receipt
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Level 1 DFD (1 of 3) Registration
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Level 1 DFD (2 of 3) Registration
Available Course Request
Available courses
2.1
Available Courses
2.2
Course enrollment add
Course to add to enrollment
Fee payment history
Students
Course enrollment add
2.3
Course to delete
Course to delete
2.4
Student enrollment information
Current schedule request
Student schedule
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Level 1 DFD (3 of 3) Registration
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Common Errors

• Forgetting data flows
• Arrow pointing wrong way
• Connecting data stores sources
• Incorrectly labeling processes or flows
• Too many processes (break it up!)
• Data flows entering a process must be different
  when leaving!

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Logical Physical DFDs

• Logical
• focuses on modeling the business
• system independent
• Physical
• focuses on modeling the system
• system dependent

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Logical DFDs - Current System

• Good for communication understanding
• Focus on business activities
• User view of the current system
• Model of the business

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Logical DFDs - New System

• Look for improvements
• effectiveness - i.e., doing the right thing
• efficiency - i.e., doing the thing right
• Transition from Analysis to Design

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Physical DFDs

• Use the new logical DFD to make transition to a
  CBIS
• Define Machine versus manual boundaries
• processes programs or manual procedures
• data stores data files or manual files
• Define controls, validations, security
• Define actual file names printouts
• Master transaction files

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Partitioning the Physical DFD

• Defining
• manual procedures
• automated procedures
• circled using a dotted line
• Batch versus On-Line Procedures

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2.1.1
Deposit Check
Bank Deposit
Make Bank Deposit
Deposit Info
2.1.2
Update Info
Update Tenant File
D1
Tenant File
2.1.3
Tenant Info
Receipt
Create Receipt