1. Data Flow Diagrams

2. Context Diagram
Order
Company
Customer
Rejected Order
Despatch Note + Goods

3. A Complete DFD 1
Sales
Order
Check Credit Rating
Customer
Rejected Order D1
Credit Rating
Dispatch Note
Accounts 3
Maintain Credit Ratings 2
Warehouse
Dispatch Goods

4. Data Flow Diagrams (DFDs) show:
The processes within the system
The datastores (files) supporting its operation
The information flows within it
The system boundary
Interactions with external entities

5. A Data Flow
Customer
DFD Symbols
An External Entity
Sales 3
A Process
Check Credit Rating D1
Credit Rating
A Data Store

6. The Process Symbol
Unique Number
Where it’s done 3
Sales
Input Data
Check Credit Rating
Output Data
What’s done

7. The data store symbol D1
Credit Rating
Writing/Updating
Accounts 3
Maintain Credit Rating
Reading 1
Sales
M = Manual data Store
D = Computer File
Check Credit Rating

8. The Data Flow Symbol
Use Meaningful names
Customer Balance
Self Evident Data Flows need
not be named D1
Credit Rating
Sales 3
Check Credit Rating

9. Physical Resource Flow
Customer
Returned
Library Book

10. System Boundary
The External Entity Symbol
Customer
A Source and/or sink of data outside the system boundry

11. Levelling – to avoid complex DFDs, processes can be grouped into higher level processes2
Dispatch Goods
Process
2.1
Validated
Order 1
Dispatch
Note 2

12. A Complete DFD 1
Sales
Order
Check Credit Rating
Customer
Rejected Order D1
Credit Rating
Dispatch Note
Accounts 3
Maintain Credit Ratings 2
Warehouse
Dispatch Goods

13. Repeated Symbols – to avoid crossing lines, data
stores and external entities can be repeated:
Customer
Customer
A Duplicated External Entity D1
Credit Rating
Duplicated Data Store

14. DFD Levels Elementary Functional Description Level 2 E F D Level 1
Get cust dets
If order>cred
refer
Else
raise order
…..
E F D
Level 1

15. A wines and spirits wholesaler stocks a range of approximately 400 different lines and delivers by van on regular weekly runs to customers who are mainly off-licenses and clubs. The majority of orders are received by telephone and any one customer may place several orders during the week which must be accumulated and delivered in one consignment. When a customer telephones he expects to be advised on the availability of an item requested and, if not in stock, the alternatives that are available. At present, sales order forms are filled in by the telephone salesmen and sent to the warehouse where they are used for the despatch of the items. Hand-written despatch notes are used as the basis of the invoices which are typed and sent by post sometime later.
Prepare a dataflow diagram (DFD) which represents the above system.

16. Developing Dataflow Diagrams
1. Identify external entities
2. Draw context diagram
3. Who/Where does what
4. Identify datastores
5. Draw diagram

17. External Entities
Customer
Context Diagram
order
advice
customer
company
des. note + goods
invoice

18. What Where/who
Receive orders Sales
Accumulate orders Sales
Advise availability Sales
Complete Sales Order Form Sales
Despatch Items Warehouse
Write despatch notes Warehouse
Type Invoices Warehouse
Send Invoices Warehouse
Datastore
Stock file

19. Draw DFD customer Stock file customer Order Advice 1 Sales M1
Accumulate orders
Advise availability
Complete forms
Sales Order Forms
Warehouse 2
Produce despatch notes
Produce Invoices
Despatch note + goods
Invoice
customer

20. Extensions
Maintenance of Stock File
Customer file – address, credit etc
Payments
Accounts dept – invoices, payments etc
Accumulate orders – temporary file