According to this thread ,since  the  session will keep the state likes facts or timer ,   I use "one process instance per session"  architecture in which each process instance is created and manutiplated  by their own dedicated session.

For the human task , I use LocalTaskService which is defined as singleton Spring bean.   So , after creating the session from an application scoped Knowledge base , I registered the work item handler using the following code .

StatefulKnowledgeSession session = JPAKnowledgeService.newStatefulKnowledgeSession(kbase, null, env);
TaskService localTaskService =  (TaskService)springContext.getBean("localTaskService"); 
LocalHTWorkItemHandler humanTaskHandler = new LocalHTWorkItemHandler(localTaskService, session);
session.getWorkItemManager().registerWorkItemHandler("Human Task", humanTaskHandler);

All the session use the same localTaskService instance to call the related API to do the human task operations . Is it okay or should I create the localTaskService for each session ?  I ask because it seems that calling the method  of localTaskService instance will somehow invoke the method on StatefulKnowledgeSession instance .