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Similarly to fork
nodes, join nodes are denoted as black bars. The effect is a synchronization (i.e.,
the joined fl ow won ??™ t start before all incoming fl ows have arrived). This is subject
to the following rules:
1. If control tokens arrive at all incoming edges, then all tokens except one
are deleted. The remaining token is made available at the outgoing edge.
2. All control tokens are deleted if both object tokens and control tokens
arrive.
3. If several object tokens arrive, then all of them are made available at the
outgoing edge, because a join node does not delete any data from the
fl ow. An exception occurs when several object tokens refer to an identical
object. In this case, only one object token is forwarded.
Figure 3.52 shows a simple example. The object token trip from the action
plan trip is duplicated at the fork node. The concurrent actions add a booked
FIGURE 3-52
Example for fork and join nodes.
act Booking a trip
plan
trip
Trip
Book
hotel
Trip
Trip
[with hotel]
Trip
[with flight]
Book
flight
Trip
Confirm
trip
Trip
189
hotel and a fl ight to the trip.


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