This is a short question with a really LOOONG answer - several pages long.

The answer depends on the database you're using, what transaction isolation 
you've specified, and, most importantly, the specific concurrency scenario 
you're thinking of.

To see what I mean about the scenario, consider the case you presented.  The 
only record modified was the token.  Under what conditions would another 
transaction be concurrently modifying the same token?  Very few - the only one 
I can think of is some sort of intervention, like someone killing the process.  
I'm not sure, but I think I remember seeing specific code to deal with this 
concurrency scenario in JBPM.

So there's no general answer - only specific ones, which have to be considered 
in the context of the database and configuration.

There are some specific areas that are believed to be buggy (at least in some 
db/transaction/container configurations), dealing with:
 - race conditions at join nodes
 - race conditions at return from a subprocess
 - race conditions with asynchronous agents trying to interact with a process 
instance soon after start - typically an MDB trying to interact with the 
process.

I believe these are being worked on.

-Ed Staub

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