A context diagram is, in essence, a data flow diagram. In its simplest
form the system that is to be designed and developed is represented by
a circle, with the name of that system written in the circle. Actors, which
can be human roles, other systems and organisations with which the
new system will interact, are written as text statements outside of the
circle – beyond the system boundaries. Interaction is represented as a
data flow, that is the flow of exchange of data, either between an actor
and the new system, or between actors beyond the system boundary.
The arrow head indicates the direction of the flow of the information –
the arrowhead is closest to the receiving actor or system. Sometimes,
the arrows are labelled to describe the information that is flowing.
Context diagrams are deceptively simple to draw. This is a good thing.
You want to be able to put together a context diagram very quickly.
What is more challenging is deciding what is inside and outside the
boundaries, and what the data flows between actors are. The simple
modelling technique enables you to focus on the complex questions
about the boundaries and scope of the system.
In RESCUE we use context diagrams as a starting point for developing
i* SD models that we will cover later in this lecture. Hence, see them as
a means to an end – producing the richer and more powerful SD
Reference: Neil Maiden (2011) Requirements Engineering Lecture Notes