Suppose you are given the details of a small mail order catalogue system that allows people to shop from home. When a customer receives the catalogue and wants to buy something, they can telephone, fax or email their order to the company. The company gets the order and sends the goods and an invoice. When the customer receives the goods with a delivery note, they send payment and receive a receipt for their payment. The first thing we must do is model the main outputs and sources of data in the scenario above. Then we draw the system box and name the system. Next we identify the information that is flowing to the system and from the system.
Here is the Level 0 (also known as the context diagram) DFD for the Mail Order system:
You will see that the Customer (a source of information) has sent in an order.
- The system has sent out an invoice data flow, a delivery note data flow and goods ('goods' is not information; it is shown as a double-headed arrow because it is a physical or resource data flow).
- Payment comes in - ie a payment data flow is received, and a receipt data flow goes out to the customer.
- Catalogue goes out to the customer - this is also a physical data flow.
We now have a top-level view of the information flow in and out of the system.
Next: Activity 4