In a recent consulting, I found the need to attach conditions to transitions 
going out of task nodes. A sample scenario is easy to imagine: you buy an air 
ticket and the airline gives you the option to cancel it until some days before 
the plane departs. When this line is crossed, all you can do is check in. So 
you would attach a condition to the cancel transition that checks whether the 
current date is after the cancel deadline.

In order for this to be useful, two premises must hold:
a) The UI should be aware of conditions and don't even show disabled 
transitions. Getting an error message after taking a given transition just 
doesn't make sense to users.
b) Ronald's point: at least one transition must stand alive. A token placed in 
a node with all outgoing transitions transitions is effectively in deadlock, 
which implies that the process is badly designed.

I think we should allow conditions in all transitions, but jBPM should think of 
them only as assertions. Just like in JDK 1.4+, the developer should be able to 
turn them on (throwing an exception) and off (silently ignoring, possibly with 
a log entry).

The application using jBPM should be responsible for giving truly meaningful 
semantics to conditioned transitions, as in the scenario portrayed above.

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http://www.jboss.com/index.html?module=bb&op=viewtopic&p=3910206#3910206

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