act Check usage right
Usage right Make usage
request
Customer
Customer
... Usage right
176 CHAPTER 3 UML??”Unifi ed Modeling Language
2. The destination has to be ready to accept the marble. An action is only ready
if marbles are available at all incoming edges of the action (AND semantics).
In Figure 3.38 , there is one marble ready on the left side after action 1. It
cannot start rolling, because the target node, action 3, is not ready yet. For
this to happen, action 2 also has to make a marble available. On the righthand
side, there are marbles available at both incoming edges, and action 3
can be started.
The AND semantics can easily lead to misunderstandings in practice. In other
fl ow notations (e.g., activity diagrams in UML 1) the OR semantics holds when there
are several incoming edges.
FIGURE 3-37
Example for the fl ow property of an activity parameter.
act Mail parcel (section)
Parcel Put stamp on
parcel
Parcel Parcel
...
{stream} {stream}
Parcel
FIGURE 3-38
Token fl ow rule AND semantics.
act AND semantics act AND semantics
Action 1 Action 2
Action 3
Action 1 Action 2
Action 3
Action 3 is not started Action 3 is started
177
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