Parallel gateway does not activate

[mahavirj]

Since <inclusiveGateway> is not supported as yet, a combination of divergent parallelGateway + exclusiveGateway has to be used to get that functionality. In one specific scenario, however, a convergent parallelGateway does not activate. I could reproduce this issue by adding this scenario to the SubProcessTest.fixSystemFailureProcess.bpmn20.xml that comes with the examples in activiti R5.2.  Does this scenario violate BPMN2, or is this a bug?

See the process flow below. The sequence between the "Begin/End POC Sequence" comments is the scenario added for this test. "subProcessJoin" does not activate (all intermediate tasks were completed)

    <subProcess id="subProcess">
   
      <startEvent id="subProcessStart" />
      <sequenceFlow id="flow2" sourceRef="subProcessStart" targetRef="subProcessFork" />
     
      <parallelGateway id="subProcessFork" />
      <sequenceFlow id="flow3" sourceRef="subProcessFork" targetRef="task1" />
      <sequenceFlow id="flow4" sourceRef="subProcessFork" targetRef="task2" />
     
      <userTask id="task1" name="Investigate hardware" activiti:assignee="kermit" />
      <sequenceFlow id="flow5" sourceRef="task1" targetRef="subProcessJoin" />
     
      <userTask id="task2" name="Investigate software" activiti:assignee="kermit" />
      <sequenceFlow id="flow6" sourceRef="task2" targetRef="subProcessJoin" />
     
      <!– Begin POC Sequence –>     
      <userTask id="poc_task3" name="Task Foo" activiti:assignee="kermit" />
      <userTask id="poc_task4" name="Task Moo" activiti:assignee="kermit" />
      <exclusiveGateway id="poc_eg1" />
      <exclusiveGateway id="poc_eg2" />
      <parallelGateway id="poc_pg1" />
                 
      <sequenceFlow id="sp3_flow29" name="" sourceRef="task2"  targetRef="poc_eg1" ></sequenceFlow>
      <sequenceFlow id="sp3_flow301" name="" sourceRef="poc_eg1" targetRef="poc_pg1" >
            <conditionExpression xsi:type="tFormalExpression"><![CDATA[${1 == 1}]]></conditionExpression>
      </sequenceFlow>     
      <sequenceFlow id="sp3_flow302" name="" sourceRef="poc_eg1" targetRef="subProcessJoin" >
            <conditionExpression xsi:type="tFormalExpression"><![CDATA[${1 == 2}]]></conditionExpression>
      </sequenceFlow>     
      <sequenceFlow id="sp3_flow303" name="" sourceRef="poc_pg1"  targetRef="poc_task3" ></sequenceFlow>
      <sequenceFlow id="sp3_flow304" name="" sourceRef="poc_pg1"  targetRef="poc_eg2" ></sequenceFlow>
      <sequenceFlow id="sp3_flow305" name="" sourceRef="poc_eg2" targetRef="poc_task4">
               <conditionExpression xsi:type="tFormalExpression"><![CDATA[${ 1 == 1 }]]></conditionExpression>
      </sequenceFlow>
      <sequenceFlow id="sp3_flow306" name="" sourceRef="poc_eg2" targetRef="subProcessJoin">
               <conditionExpression xsi:type="tFormalExpression"><![CDATA[${ 1 == 2 }]]></conditionExpression>
      </sequenceFlow> 
      <sequenceFlow id="sp3_flow34" name="" sourceRef="poc_task3" targetRef="subProcessJoin"></sequenceFlow>
      <sequenceFlow id="sp3_flow35" name="" sourceRef="poc_task4" targetRef="subProcessJoin"></sequenceFlow>                  
     
      <!– End POC Sequence –>
     
      <parallelGateway id="subProcessJoin" />
      <sequenceFlow id="flow7" sourceRef="subProcessJoin" targetRef="subProcessEnd" />
      <endEvent id="subProcessEnd" />
   
    </subProcess>

[jbarrez]

I'm not quite following what you are trying to acomplish here. Could you provide a visual diagram to make it more clear?

[mahavirj]

Here's the subprocess flow. PG = parallel gateway, EG = exclusive gateway.  The issue is PG2 does not activate

[attachment=0]Update External Systems.jpg[/attachment]

[jbarrez]

The parallelGW will activate only if the number of incoming executions is equal to the number of incoming sequence flow.

In this case, you have 4 incoming sequence flow, but PG2 can maximum receive 3 executions in the best case (if I'm not mistaken).
So the parallel gateway will indeed never activate. Placing a converging exclusive gateway in between should do the trick, I think.

[mahavirj]

Yes, if I change PG2 to an exclusive gateway, say EGX, it works. However, according to BPMN2, a converging exclusive gateway does not synchronize flows, which means in this case it will always get triggered since task1 is always being completed regardless of what happens in EG2.  i.e. I don't want the subprocess to end just based on task1 completion.

So, if I leave task1->PG2 flow untouched and converge the other 4 flows to EGX and connect EGX to PG2 it works conditionally.

i.e. if EG1 evaluates to true EG1->EGX->PG2 does not work. But, if EG1 evaluates to false and EG2 evaluates to true, it works since the number of flows = number of executions @ PG2.

[mahavirj]

Got it to work by adding a some 'do nothing' servicetasks when there is no need for an execution instance for e.g. between eg1->egx->pg2.  This should not be necessary though. Would it not be possible to count the execution instance of task2 when applying that rule (#of flows=#of executions) for activating paralell gateway?

[jbarrez]

Could you attach your process xml, because it is hard to follow.

Would it not be possible to count the execution instance of task2 when applying that rule (#of flows=#of executions) for activating paralell gateway

I'm not following here, why would task2 have influence on the count of the parallel gateway?

[mahavirj]

In the diagram, there are 5 inflows to parallel gateway PG2.  Three of them have executions (task1, task4, task3) and two don't (EG1-to-PG2 and EG2-to-PG2).  Since the number of executions != number of flow,PG2 does not activate.

But, this is a valid requirement i.e. there is really nothing to do in EG1-to-PG2 and EG2-to-PG2.  Changing PG2 to an exclusive gateway will not help because completion of task1 will always activate it i.e. it wont wait for the result of the other 4 flows

So, the only way out is leave PG2 as a parallel gateway and define a do nothing <servicetask> for each of the EG1-to-PG2 and EG2-to-PG2 flows. This will make the number of executions == number of flows in all cases and will activate PG2 correctly.

However, these do-nothing <servicetask> are unnecessary. It should just be a matter of counting the # of executions…that's why i was suggesting to use task2 execution-id as the execution token for the flows which do not have an execution (in this case EG1-to-PG2 and EG2-to-PG2) i.e task2 is the preceding task for those do-nothing flows

The subprocess XML was in the first post. However, let me know if you still need the entire process xml and the unit tests