Filter
Top Products
Chapter 3. Black-box thinking: Defining the system context 45
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 it—finding use cases and
operations.
An example of a context diagram with I/O entities is shown in Figure 3-2.