[rules-users] Possibly race condition in a persisted Drools + JBPM during a StatefulKnowledgeSession

[Mauricio Salatino]

Ok, so what is happening is that two or more threads are fighting for the
resources, causing that two or more transactions wants to be completed, but
just one can win. All the other must be retried. Are you seeing rolled back
exceptions in your stack trace?
My question to you is if you really need to handle everything in just one
big persistent session. I've solved similar issues in the past using more
than one session, for example one session to run business
processes(persistent) and one or more for receiving and reacting to events.
If you have an async way to communicate both sessions there should be no
problem.
Hope it helps!
Cheers

On Tue, May 29, 2012 at 7:30 AM, Alberto R. Galdo <argaldo at gmail.com> wrote:

> Hi all,
>
>    We've been for long time now developing a complex event processing
> system that ( simplified version ahead !! ) involves a set of rules that in
> turn activates a set processes that fulfill human tasks and other kind of
> tasks.. This all is running in a StatefulKnowledgeSession with JPA
> persistence configured both for Drools and JBPM. We are running an stack of
> Drools, JBPM, Drools Integration, Drools fussion, etc..
>
>    We've been able to persist Drools sessions & JBPM processes in the same
> Persistence Context  ( not without pain :( ) using different JTA
> implementations including ( but not limited to ) Bitronix & Atomikos.
>
>     What we are observing here is what seems to be some kind of race
> condition between Drools and JBPM when running the knowledge session when
> JPA&JTA persistence is configured. Very often, as soon as after 2-3
> processes get created as rule's consequences are fired in response of
> events inside the session we see how JBPM finds its instance nullified by
> Drools when it tries to end a process and persist it.
>
>    We've been able to find where Drools decides to delete an instance of
> the process ... at a given time Drools executes
> JPAProcessInstanceManager.clearProcessInstances() [1] when it finalizes a
> SingleSessionCommand wich in turn calls disconnect() for *all* the
> local-stored processinstances ( wich gets populated with instances of
> processes every time a process is started in the knowledge session ):
>
>     public void clearProcessInstances() {
>
>
>         for (ProcessInstance processInstance: new ArrayList<ProcessInstance>(processInstances.values())) {
>
>
>             ((ProcessInstanceImpl) processInstance).disconnect();
>
>         }
>     }
>
>
>     So, Drools decides to disconnect all process instances in it's JPA
> context without taking in account the state the process is in, and when an
> processinstance that is not stopped gets removed then JBPM finds it's
> NullPointerException...
>
>     We've modified the code to make Drools aware of the state of the
> process before wiping it from the context   ( no problem here, there will
> be no leak as a running processinstance will be removed in future calls of
> "clearProcessInstances" given the process is closed ). But unfortunatelly
> this seems to resolve this problem, but lots of other problems ( wich seems
> also race conditions arise :  for instance: Drools closes connections to
> the database and JBPM finds the connection closed,
>
>
>     So, we are really worried about using Drools & JBPM in a persisted
> environment. Maybe our asumptions are wrong...  Is it possible to have an
> scenario like ours given the current Drools & JBPM integration status for a
> persistent statefulKnowledge Session?  Did anyone build a complex event
> processing system like ours in a unaltered persistence environment such as
> provided in Drools and JBPM by default?
>
>
> Greets,
>
> [1]
> https://github.com/droolsjbpm/jbpm/blob/master/jbpm-persistence-jpa/src/main/java/org/jbpm/persistence/processinstance/JPAProcessInstanceManager.java
>
>
>
>
>
>
> Alberto R. Galdo
> argaldo at gmail.co <argaldo at gmail.com>
>
> _______________________________________________
> rules-users mailing list
> rules-users at lists.jboss.org
> https://lists.jboss.org/mailman/listinfo/rules-users
>
>


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