1. Modelling Complex Systems
Data Flow Diagrams
Modelling Complex Systems

2. Why Data Flow Diagrams? What are They For?
Pictures are easier to understand than words
They provide a standard way of representing systems
We can use them for all sorts of different systems:
E-commerce system – modelling customer order
College application – process of being accepted
Insurance quote – contacting head office from regional branch
And so on…
Most Importantly: They provide a structure which forces us to really think about how a system works, and figure out which parts we are unclear on

3. Our First Example – Insurance Company
An insurance company has regional offices which communicate with the head office for the purpose of generating insurance quotes. When the Actuarial Services at head office receive a request for a quote they check 1) current operating costs and 2) previously issued quotes to determine the appropriate quote. As well as passing this information to the regional office they also save a copy of all issued quotes on their server under “Issued Quotes”. Draw the DFD.

4. D1
Current Costs
Regional Office
Enquiry 1
Actuarial Services
Quotation
Prepare Quotation
Quotation
Similar Previously Issued Policies D2
Previous Policies D3
Issued Quotes

5. DFD Symbols

6. Process 1
Actuarial Services
Prepare Quotation

7. Data Flows
Quotation

8. Data Store D2
Previous Policies

9. External Entity
Regional Office

10. Wholesalers Example
A wholesaler receives orders by phone from its customers. These are processed by a sales clerk, who adds the order to an “Orders” file, and also passes them to the shipping clerk. The shipping clerk dispatches the goods and creates a dispatch note. A copy of this note is placed in the “Dispatch Notes” file, and another copy is sent to the customer.

11. 1
Sales Clerk 2
Shipping Clerk
Order
Order
Customer
Dispatch Goods
Create Dispatch Notice
Create Order
Dispatch Note D1
Orders D2
Dispatch Notes
Customer

12. Important Notes
The numbering on processes and data stores doesn’t indicate order – it is however important that we be able to refer to them by number
The shapes of the various elements ARE important
The order or arrangement of shapes is up to you
Both in the exam and in a business setting the important thing is that whatever layout you choose is both sensible and relatively easy for someone else to follow
Data flows don’t have to be named if the contents is implied (as in the previous example)
If in doubt, add a label

13. Important Notes
Data cannot move directly from one data store to another data store - it must be moved by a process
Data cannot move directly from an outside source to a data store - it must be moved by a process
Data cannot move directly to an outside sink from a data store - it must be moved by a process
All processes must have outputs and inputs

14. Food Ordering System
The Food Order System Data Flow Diagram example contains three processes, four external entities and two data stores.
A Customer can place an Order. The Order Food process receives the Order, forwards it to the Kitchen, store it in the Order data store, and store the updated Inventory details in the Inventory data store. The process also deliver a Bill to the Customer.
Manager can receive Reports through the Generate Reports process, which takes Inventory details and Orders as input from the Inventory and Order data store respectively.
Manager can also initiate the Order Inventory process by providing Inventory order. The process forwards the Inventory order to the Supplier and stores the updated Inventory details in the Inventory data store.
Write out the processes, external entities and data stores and then draw the DFD. Finally, list the data flows.

16. Online Order System
There are 3 processes, 2 data stores and 2 external entities in this system
A customer places an order with the ecommerce system, which processes the order, sends the customer an acknowledgement and writes to the customer database
A cybercheck process reads data from the database and sends it to the credit card company for approval/rejection, and writes the result back to the customer database
A shipping process reads data from the customer database and sends a shipping confirmation to the customer, updates the customer database and also updates the inventory
Write out the processes, data stores, external entities and then draw the DFD. Finally, list the data stores.

18. Sample Exam Question
XYZ Limited bills clients for software projects. The personnel department is responsible for employment and records employee details. When hired, each employee is assigned a specific job description (the associated salary determines the chare out per hour spent on a client project). Each employee is eligible to work on multiple projects simultaneously.
The Sales Director is responsible for accepting initial client projects and ensuring unique naming, however the Project Director assigns employees to projects and records the commencement date. An employee is selected as project leader.
As employees work on the project they inform the Project Director’s secretary weekly of the hours spent on a project. This information is recorded for that project.
When the project is completed the finance department bills the client. The fee is calculated from the employees hours recorded for the project.
The finance department provides a monthly billing report to the Financial Director.
List the Processes, Data Stores, External Entities and Data Flows.
Draw a Level 1 Data Flow Diagram
Draw a Level 0 Data Flow Diagram (NOTE: We haven’t discussed this yet)

19. Processes Data Stores Process New Employee Record Client and Project
Assign Employee to Project
Assign Project Leader
Record Employee Project Hours
Send Invoice to Client
Produce Monthly Report
Employee
Client
Project

20. External Entities Data Flows Employee Client Finance Director
Project Director
Employee Details
ID & Job Description
Client Details and Project
Project Detail (Hours)
Invoice
Report

21. 1
Personnel
Details
Employee
Record Employee Details
Job Information D1
Employees

22. 2
Sales Director
Details
Client
Record Client/Project D2
Client D3
Project

23. 3
Project Director D1
Employees D3
Project
Assign Employee to Project 4
Project Director
Assign Project Leader

24. Proj Dir 6
Finance Dept 5
Secretary
Invoice
Employee
Bill Client
Client
Record Hours D1
Client D3
Project 7
Finance
Invoice
Assign Project Leader
Fin Report