Complete Workflow step through code - using WorkflowSession not working in AEM6.5

[deepalakshmip35]

I am trying to complete a workflow step through code in same step workflow processor as below.
workflowSession.complete(workItem, workflowSession.getRoutes(workItem, false).get(0));

here, it throws the exception:
Caused by: com.adobe.granite.workflow.WorkflowException: Unable to retrieve workflow instance: "/var/workflow/instances/...."

AEM version : AEM6.5, this working in AEM6.2
Is there anything need to be configured to WorkflowException?

I tried with system user provided all privileges and added it as a workflow super user but same the issue occurs on completing the workItem.
Please share solution if anyone faced this issue, advance thanks.

[deepalakshmip35 · Accepted solution]

I found it..

We cannot take ownership on workflow instance of transient workflow. Because these are executed in memory. 

To have workItem workflow should write nodes under /var/workflow/instances/server0/

With a transient workflow it is executed in memory.