Tools and Techniques for Requirements Engineering and Management - Requirements Engineering and Management for Software Development Projects - page 178

Fig. 13.5 DFD for a purchase order process

10. The purchase department raises a purchase order on the selected vendor.

Let us model this process in a DFD. Figure 13.5 depicts this process pictorially.
DFDs for real life systems would span across multiple sheets. To make a large
system comprehensible, it is normally divided into multiple levels. The top level
DFD is normally a context diagram described in the subsequent sections. Then for
each subsequent level a DFD is prepared. The lowest level DFD would be for a
software unit. If a totally granular DFD set is prepared, it can almost supplement a
design document. But more often than not, DFDs would not be prepared to the
lowest unit level. Each organization decides at what level of granularity, the DFDs
would be stopped in its projects.

13.2.12 Context Diagram

Context diagrams are used to show the context in which the proposed system
operates. It also shows the context of the modules within the system. In the context
diagrams, circles are used to represent entities and arrows to show the relationship
with arrows pointing in the direction of the flow of information between the
entities.

An arrow head only at one of the ends depicts a unidirectional