Process Models:Data Flow Diagrams Special thanks goes to Dr. Jack T. Marchewka for many of the ideas and examples contained in these notes.
Process Modeling • Objective: • Understand the concept of business processes • Understand and create Data Flow Diagrams.
DFDs • Visually represent • data flows • processes • Emphasize the logic of system • Provide documentation for Analysis & Design
Data Flow Diagrams Data Flow Process File or Data Store Source or Entity Gane & Sarson Methodology
DFDs • Do not show technical aspects • Show interrelations of systems & subsystems • Facilitates • developer understanding • user-developer communication • Completeness of data & process definitions
The Systems Concept Output Input Process
DFDs 1 Data Flow Input Data Flow Output Process
Developing DFDs • Context Diagram • System Overview • Little or no detail • Zero Level Explosion • More detailed • First,Second, Third Levels • More detailed levels
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
Context DFD Examples Apartment Rental Receipts, Notices Lease Cash Report 0 External Manager Apartment Rental System Tenant Lease, Payments Bank Deposit Bank
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.)
Course Registration System Registration Context Diagram 0 Course information Enrollment information Department Staff Students Course offerings Student schedules
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 AREI’s 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 AREI’s 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.
AREI System AREI Context Diagram 0 Buyer information Sellers Buyers House information House information House Information House Information Multiple Listing Service
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
Level 0 DFD: Apartment Rental Tenant Info D1 Tenant File Copy of lease 1 Lease New Tenant Process Receipt Bank Deposit Tenant Bank 2 Payments Collection Process Notice Cash Report Unpaid Charges Tenant Info Ext. Mgr 3 D1 Tenant File Delinquent Process Delinquency Report Delinquencies
1 Maintain department course offerings D3 D2 D1 Fees 2 Course Offerings Maintain student enrollments Enrollments 3 Course Enrollment Reports 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
Maintain house seller information D4 D2 D3 D1 Sales Contract 1 Sales Contract Sellers Multiple Listing Services File House information House information House information House information Offered Houses Buyers Sales Contracts House information Generate requested report 2 House information request Buyers Buyer information form Buyer information House information Level 0 DFD: AREI
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
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
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 Receipt Tenant Info Create Receipt
Produce course offering list D2 Department ID 1.1 Course offering list Course Offering List Request Department ID 1.2 New Course New Course information Dept. Staff Course Offerings 1.3 Course to delete Delete course Add new course Course to delete Existing Course information 1.4 Course modifications Course modifications Modify existing courses Level 1 DFD (1 of 3): Registration
Produce course offering list D1 D3 D2 Fees Enrollments Course Offerings Delete course from schedule Add course to schedule Obtain current schedule Level 1 DFD (2 of 3): Registration Available Course Request 2.1 Available courses 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
Obtain report type D3 D2 3.1 Report Request Report type Course offering information Dept. Staff Enrollments Course Offerings 3.2 Generate requested report Enrollment information Requested report Level 1 DFD (3 of 3): Registration
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!
Logical & Physical DFDs • Logical • focuses on modeling the business • system independent • Physical • focuses on modeling the system • system dependent
Logical DFDs - Current System • Good for communication & understanding • Focus on business activities • User view of the current system • Model of the business
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”
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
Partitioning the Physical DFD • Defining • manual procedures • automated procedures • circled using a dotted line • Batch versus On-Line Procedures
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 Receipt Tenant Info Create Receipt