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A fork node is denoted as a black bar.
187
A token waits at the fork node bar when the target is not ready to accept it.
This can cause a situation, for example, where, at two outgoing edges, one fl ow
already starts while the other fl ow is still waiting. The waiting token is subject to
the FIFO 14 principle if more tokens arrive at the fork node. In Figure 3.51 , action A
is not ready if no token is present at the edge coming from the far right. The token
has to wait at the fork node bar. Action B is ready and can be started. This action ??™ s
14 First In, First Out.
3.6 The Activity Diagram
FIGURE 3-51
Example for fork nodes.
act Fork node (section)
A B
FIGURE 3-50
Example for merge nodes.
act Check usage right (section)
Request usage right
[Usage right o.k.]
[usage not authorized]
Check emergency
driving
188 CHAPTER 3 UML??”Unifi ed Modeling Language
token doesn ??™ t have to wait. The fork node bar is the only control node in the activity
model where tokens can wait.
A join node joins concurrent fl ows to form one single fl ow.


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