I/O entities
Here, we consider I/O entities and how they can be identified.
MDSD Step 4: Finding I/O entities
As actors are identified and placed on the context diagram, I/O entities can begin to be identified. An I/O entity is something that is exchanged between an actor and the system under consideration. It can be information or a physical item, and can be either sent or received by the system to or from the actor. Each I/O entity is associated with an actor and is designated as either sent or received (or both) by that actor.
If the system under consideration were an online bookstore, I/O entities would include books (received by actor) and money (sent by actor). I/O entities are drawn on the context diagram with associations to actors.
I/O entities are useful in several ways. In the early stages of the model, they are used to more fully understand actors and the nature and purpose of their interaction with the system. As the model develops, I/O entities are also used as parameters to fully specify operations, and also form the basis for a domain model that can be created later. I/O entities are often simply identified in the early stages of the model and are later elaborated with attributes as the model develops.
With the addition of I/O entities, the static portion of the context model is complete, and we move on to the behavioral aspects of
An example of a context diagram with I/O entities is shown in Figure
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