Hi arash,
the process engine does the persisting for you. As long as you use the service interfaces and your jbpm.cfg.xml is set up right, you don't need to worry about persistence. You resume a process by looking up a task with the taskService and complete it, see the user doc: http://docs.jboss.com/jbpm/v4/userguide/html_single/#taskservice
JBPM4 is a community project, feel free to improve the documentation if you think you can do better.