System design
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1 A company raises a purchase order (PO) and sends this to a supplier. This creates an entry on a computer system. 2 When the goods inwards department of the company physically receives the goods from the supplier, they create the goods received note (GRN) manually. This formally records the fact that goods have been received. The GRNs are sent to the purchasing department where they are filed. 3 When an invoice from a supplier is received, this is matched to the GRN (manual file) and the original PO (computer records). If these match (e.g. if the quantity of goods is the same on the GRN, PO and invoice), then the purchasing clerk will make an entry into a computer system to authorize payment. If the information does not match, a letter querying the invoice is sent to the supplier. 4 Once a week, a payments clerk will print cheques. This is based on the computer file of authorized invoices. As a result of this process, the supplier will be sent a cheque. The process above can be represented more formally as the DFD shown in Figure 6.5.
Figure 6.5
Example of a DFD
6.7.3 Decomposition of DFDs In practice, systems may be far too complex to be represented on a single DFD. It is possible therefore for a process to be functionally decomposed and the system represented hierarchically. For example, in the DFD example in Figure 6.5, it is possible to functionally decompose process 1. In this process, after information matching, a transient file of query letters is created. These are sent to suppliers