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Chapter 3. Black-box thinking: Defining the system context 63
To determine an entity’s operations on a sequence diagram, note the arrows
pointing into the lifeline and originating from another lifeline only. When working
with paper models—such as those on flip charts—we often circle these
arrowheads in a bright color to emphasize their importance. Each arrow pointing
in toward the enterprise element’s lifeline represents a candidate operation for
the enterprise. Why? An arrow represents a message that carries a request—a
request being made of the entity. If the system is to work, the entity must be
capable of responding to that request when it is initiated. Thus the entity must
have an operation that corresponds to the request.
To put it simply, if the entity is at some point requested, by any actor, to unlock,
then it is required that the entity have an operation called
unlock. It is as simple
as that. So, we can read the operations for the entity right off of the sequence
diagram we have just drawn, by simply noting the arrows that point in towards its
lifeline (Figure 3-9).
Figure 3-9 Sequence diagram with arrowheads circled in red
At this point it is often asked why arrows that point out (away) from the element’s
lifeline do not represent operations of the element; after all, they seem to be
something the element must do. Indeed, the system must issue the requests
represented by those arrows, but the system does not just make these requests
at any time. Because we have modeled the actual sequence of operations, we
know when the system must take such an action, and it is as a part of fulfilling the
previously requested operation.