Data Flow Diagrams

A structured analysis technique that employs a set of visual representations of the data that moves through the organization, the paths through which the data moves, and the processes that produce, use, and transform data.

2

Why Data Flow Diagrams?

• Can diagram the organization or the system

• Can diagram the current or proposed situation

• Can facilitate analysis or design

• Provides a good bridge from analysis to design

• Facilitates communication with the user at all stages

Types of DFDs

• Current - how data flows now

• Proposed - how we’d like it to flow

Levels of Detail

• Context level diagram - shows just the inputs and outputs of the system

• Level 0 diagram - decomposes the process into the major subprocesses and identifies what data flows between them

• Child diagrams - increasing levels of detail

• Primitive diagrams - lowest level of decomposition

• Write your Narrative of Existing System/Process Description

• Identify important elements• Create Level 0 Diagram (Existing)• Create Context Level Diagram

(Existing)• Create Level 0 Diagram (Proposed)• Create Context Level Diagram

(Proposed)

Recommended Progression

Four Basic Symbols

Source/Sink

Data Flow

#

Process# Data Store

Context Level Diagram

• Just one process

• All sources and sinks that provide data to or receive data from the process

• Major data flows between the process and all sources/sinks

• No data stores

Running Example

Course Registration: Context level Diagram

0

CourseRegistration

System

Student

Registrar

Professor

Class Request

Payment

Receipt

Student Schedule

Class roster

EnrollmentStatistics

Level 0 Diagram

• Process is “exploded”

• Sources, sinks, and data flows repeated from context diagram

• Process broken down into subprocesses, numbered sequentially

• Lower-level data flows and data stores added

Running Example

Course Registration: Current Level 0 Diagram

Student RegistrarProfessor

1.0

RegisterStudent for

Course

D1 Student Class Records

D2 Student Payments

2.0

CollectStudent Fee

Payment

3.0

ProduceStudent

Schedule

4.0

ProduceClassRoster

5.0

ProduceEnrollment

Report

PaymentInformation

Student andCourse Data

StudentClass Record

Student Class Record Student Class Record

Student Class Record

Student Schedule Class RosterEnrollmentStatistics

StudentClass Request

Receipt

Payment

Child Diagrams

• “Explode” one process in level 0 diagram

• Break down into lower-level processes, using numbering scheme

• Must include all data flow into and out of “parent” process in level 0 diagram

• Don’t include sources and sinks

• May add lower-level data flows and data stores

Running Example

Course Registration: Current Child Diagram

1.2

Checkfor

Availability

1.1

CheckPrerequisites

Met

1.3

EnrollStudentin Class

D1 Student Class RecordsD5 Course CatalogueD4 Student Transcripts

D3 Semester Schedule

Class Request Valid ClassRequest

Feasible ClassRequest

Available Seats

Available Seats

StudentRecord

Course RecordStudent

and CourseData

Another Example

Perfect Pizza: Context Level Diagram

0

CustomerOrder

System

Customer

Cook

Management

Phone Number

Customer Order

Customer Info

DeliveryInformation

WeeklyReport

OrderInformation

DeliveryPerson

Another Example

Perfect Pizza: Current Level 0 Diagram

1.0Find

CustomerRecord

7.0Print

WeeklyTotals

5.0Add

CustomerRecord

2.0Take

CustomerOrder

3.0Print

DeliveryOrder

Customer

CustomerInfo

PhoneNumber

Customer Order

D1 Customer Master

CustomerRecord

CustomerRecord

CustomerInformation

D2 Customer HistoryD3 Sales Records

SalesInformation

OrderInformation

CustomerHistory

DeliveryInformation

Cook

Order Information

Management

Sales Information details

Weekly Report

DeliveryPerson

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

1.0

GatherData

2.0

CompileStatistics

Demographic Data

3.0

AnalyzeResponses

SurveyResponses

FinalReport

2.0

VisaAuthorization

2.0

TotalRecords

2.0

QAProcess

2.0

CheckCustomer

Credit

2.0

ComputeTotalSales

2.0

InspectFinishedProducts

BETTER

BETTER

BETTER

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

2.1

StoreCustomer

Data

CustomerInformation

D1 Customer Data

D2 Customer Preferences

CustomerData

CustomerPreferences

2.1

StoreCustomer

Data

CustomerInformation

D1 Customer Data

D2 Customer Preferences

CustomerData Customer

Preferences

2.1

StoreCustomer

Data

CustomerInformation

D1 Customer Data

D2 Customer Preferences

CustomerData

CustomerPreferences

2.1

StoreCustomer

Data

CustomerInformation

D1 Customer Data

D2 Customer Preferences

CustomerData

CustomerPreferences

2.2

ExtractCustomer

Preferences

CustomerData

D1 Customer Data

CustomerData

2.0

StoreCustomer

Data

D1 Customer Data

CustomerData

Customer

CustomerInformation

Customer

0

MedicalBillingSystem

Doctor

Patient

Diagnosis

ServiceInformation

Bill

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

2.0

TotalDailySales

1.0

TakeCustomer

Order

3.0

PrintDelivery

Instructions

CustomerOrder

OrderInformation

OrderTotal

2.0

TotalDailySales

1.0

TakeCustomer

Order

3.0

PrintDelivery

Instructions

OrderInformation

OrderTotal

2.0

LookupCustomer

Record

1.0

TakeCustomer

Order

3.0

PrintDelivery

Instructions

CustomerOrder

CustomerAddress

CustomerInformation

2.0

LookupCustomer

Record

1.0

TakeCustomer

Order

3.0

PrintDelivery

Instructions

CustomerOrder

CustomerAddress

1.0

CalculateWeekly

Sales

DailySales

CumulativeTo-Date

Sales

Data Flow Diagramming Guidelines

• The inputs to a process are different from the outputs

• Every object in a DFD has a unique name

1.0

ValidateCustomer

Data

1.0

ValidateCustomer

Data

CustomerData

CustomerData

ValidCustomer

Data

CustomerData

2.0

TakeCustomer

Order

1.0

GetCustomer

Data

3.0

ProcessCustomer

Order

CustomerData

CustomerData

Order

2.0

TakeCustomer

Order

1.0

GetCustomer

Data

3.0

ProcessCustomer

Order

CustomerData

Order

1.0

GetCustomer

Data

2.0

TakeCustomer

Order

3.0

ValidateCustomer

Data

CustomerData

Only if these are exactly the same

CustomerData

Data Flow Diagramming Guidelines

• A data flow at one level may be decomposed at a lower level

• All data coming into and out of a process must be accounted for

• On low-level DFDs, new data flows can be added to represent exceptional situations

1.0

GenerateReport

SalesInformation

SalesReport

1.2

PrintInventory

Report

1.1

PrintSales

Report

SalesInformation

InventoryInformation

SalesReport

InventoryReport

InventoryInformation

InventroyReport

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

1.0

CalculateGrossPay

D2 Employee Time File

Employee

Employee

D1 Employee Master

D3 Check Reconciliation

D1 Employee Master

2.0

CalculateWithholding

Amount

3.0

CalculateNetPay

4.0

PrintEmployeePaycheck

HoursWorked

EmployeeTime

Record

GrossPay

Withholding

NetPay

EmployeeRecord

EmployeeRecord

CheckReconciliation

Record EmployeePaycheck

1.0

CalculateGrossPay

D2 Employee Time File

Employee

Employee

D1 Employee Master

D3 Check Reconciliation

D1 Employee Master

2.0

CalculateWithholding

Amount

3.0

CalculateNetPay

4.0

PrintEmployeePaycheck

HoursWorked

EmployeeTime

Record

GrossPay

WithholdingAmount

NetPay

EmployeeRecord

EmployeeRecord

CheckReconciliation

Record

EmployeePaycheck

5.0

CreateTime

RecordEmployee

Time Record

Number ofDependents

GrossPay

D4Withholding Tables

WithholdingRates

6.0

ReconcilePay

Check

PaycheckInformation

GrossPay

WithholdingAmount

DFDs and ERDs

• DFDs and ERDs are both used to model systems, but they show two very different perspectives on the system

• A DFD shows what the system does as well as the data that the system manipulates

• An ERD shows only the data that the system manipulates.

DFDs and ERDs (cont.)

• Entities on an ERD often (but not always) correspond to data stores on a DFD

• Attributes on an ERD usually correspond to data elements (listed in the data dictionary) that make up the data store and data flows on a DFD

• Relationships on an ERD do not correspond to processes on a DFD.

• Sources and sinks on a DFD usually do not show up as entities on an ERD

Example DFD and ERD

Customer

CookInventory

Processing

1.0

TakeOrder

2.0

Convert Orderto CookingInstructions

3.0

Convert Orderto Ingredient

List

ProcessedOrder

D1 Order Log

CookingInstructions

Ingredients

Correct ERD

DFDOrder

Item

Ingredient

CookingInstructions

Contains

Includes

Requires

OrderId

Date

Time

ItemId ItemName

StepId Description

Description

ItemQuantity

IngredientQuantity

Index

Create an ERD and Map the ERD into RelationPerfect Pizza: Current Level 0 Diagram

1.0Find

CustomerRecord

7.0Print

WeeklyTotals

5.0Add

CustomerRecord

2.0Take

CustomerOrder

3.0Print

DeliveryOrder

Customer

CustomerInfo

PhoneNumber

Customer Order

D1 Customer Master

CustomerRecord

CustomerRecord

CustomerInformation

D2 Customer HistoryD3 Sales Records

SalesInformation

OrderInformation

CustomerHistory

DeliveryInformation

Cook

Order Information

Management

Sales Information details

Weekly Report

DeliveryPerson

1.0

CalculateGrossPay

D2 Employee Time File

Employee

Employee

D1 Employee Master

D3 Check Reconciliation

D1 Employee Master

2.0

CalculateWithholding

Amount

3.0

CalculateNetPay

4.0

PrintEmployeePaycheck

HoursWorked

EmployeeTime

Record

GrossPay

WithholdingAmount

NetPay

EmployeeRecord

EmployeeRecord

CheckReconciliation

Record

EmployeePaycheck

5.0

CreateTime

RecordEmployee

Time Record

Number ofDependents

GrossPay

D4Withholding Tables

WithholdingRates

6.0

ReconcilePay

Check

PaycheckInformation

GrossPay

WithholdingAmount

Create an ERD and Map the ERD into RelationPayroll Management System: Current Level 0 Diagram

Create an ERD and Map the ERD into RelationsCourse Registration: Current Level 0 Diagram

Student RegistrarProfessor

1.0

RegisterStudent for

Course

D1 Student Class Records

D2 Student Payments

2.0

CollectStudent Fee

Payment

3.0

ProduceStudent

Schedule

4.0

ProduceClassRoster

5.0

ProduceEnrollment

Report

PaymentInformation

Student andCourse Data

StudentClass Record

Student Class Record Student Class Record

Student Class Record

Student Schedule Class RosterEnrollmentStatistics

StudentClass Request

Receipt

Payment

Course Registration: Current Child Diagram of Process 1.0

1.2

Checkfor

Availability

1.1

CheckPrerequisites

Met

1.3

EnrollStudentin Class

D1 Student Class RecordsD5 Course CatalogueD4 Student Transcripts

D3 Semester Schedule

Class Request Valid ClassRequest

Feasible ClassRequest

Available Seats

Available Seats

StudentRecord

Course RecordStudent

and CourseData