• Practice data flow diagramming as a tool for structured system programming (process modelling)

DATA FLOW DIAGRAMs

Key Definitions

• A process model is a formal way of representing business processes– Illustrates processes/activities and how data

moves among them

• Data flow diagramming is a technique for creating a process model.– The primary output of data flow diagramming

is a data flow diagram (DFD)

DFD Elements

External Entities

• External people, organizations, systems and data stores

• Reside outside the system, but interact with system

• Either receive info from system (“sink”) or provide data to the system (“source”)

• Examples: Customers, managers

ExternalEntities

Data Stores

• Internal to the system• Data at rest• Include in system if the system

processes transform the data– Create, Update, Delete

• Every data store on DFD should correspond to an entity on an ERD

• Must have at least one input data flow (or else they never contain any data)

• Usually have at least one output data flow

• Data can only enter a data store from a process and can only leave a data store to a process

Data StoresD1

Data Flows

• Data in motion– From external entity (“source”)

to system

– From system to external entity (“sink”)

Data Flow

Context Diagram

• Shows the context into which the business process fits

• Shows the overall business process as just one process

• Shows all the external entities and the data flows into and out of the system from them

Context Diagram Example

DFD Fragment Example

DFD level 0 Example

PatientPatient

Find patient

1

Update patient

3

Add new patient

4

Delete patient

2

New patient information

Changes topatient information

Patientname

D1 Patient Information

Patientinformation

Patientinformation to delete

Patientinfo to be updated

Updatedpatientinformation

Patientinformation

Deletedpatient

Key Definition

• Decomposition is the process of modeling the system and its components in increasing levels of detail.– Ideally 3-9 processes per DFD

• Balancing involves insuring that information presented at one level of a DFD is accurately represented in the next level DFD.

Are these in balance?0

GradingSystem

ProfessorStudent

assignment info

student info

grade report

grade info

completed assignment

GradesD1

Maintainstudentsstudent info

Maintainassignments

7

Producegrade report

grade info

Professor

5

Gradeassignment

D2Assignments

D3Students

assignment info

Student

1

grade info

student info

assignment info

grade report

3

studentinfocompleted

assignment

grade info

assignmentinfo

Context

Level 0

Hierarchical Consistency

• Balancing Data Flows– An input (output) data flow on a PARENT

diagram must appear on a CHILD diagram as input (output).

– Conversely, an input (output) data flow on a CHILD diagram must appear on a PARENT diagram as input (output).

– A set of data flows on a child diagram that were split from a data flow on a parent diagram must match the parent data flow's composition.

Problem statement

Al Burns is the professor for the class. He manually creates a spreadsheet with all the student information based on a course roster he gets from Campus Connect. He also creates a list of assignments, a description for each assignment, and a grading key for each one.

Students complete the assignment and give them to Al Burns. Al Burns collects the assignments and gives them to Ms. Grader for grading. Ms. Grader grades the assignments based on a key provided by Al Burns, and assigns a score (grade) from 0 to 5. She then records the scores for the students on the spreadsheet.

Grades are reported back to students via a course web page created from the spreadsheet.

Assignment: Draw a context diagram and level 0 diagram for the Grading System.

Grading System DFD Context

0

GradingSystem

ProfessorStudent

assignment info

student info

grade report

grade info

completed assignment

Grading System DFD Level 0

GradesD1

Maintainstudentsstudent info

Maintainassignments

7

Producegrade report

grade info

Professor

5

Gradeassignment

D2Assignments

D3Students

assignment info

Student

1

grade info

student info

assignment info

grade report

3

studentinfocompleted

assignment

grade info

assignmentinfo