creates unit_of_work and inserts it into the DB
            :raise DuplicateKeyError: if unit_of_work with given parameters already exists

def _insert_uow(self, process_name, timeperiod, start_timeperiod, end_timeperiod, start_id, end_id):
        """creates unit_of_work and inserts it into the DB
            :raise DuplicateKeyError: if unit_of_work with given parameters already exists """
        uow = UnitOfWork()
        uow.process_name = process_name
        uow.timeperiod = timeperiod
        uow.start_id = str(start_id)
        uow.end_id = str(end_id)
        uow.start_timeperiod = start_timeperiod
        uow.end_timeperiod = end_timeperiod
        uow.created_at = datetime.utcnow()
        uow.submitted_at = datetime.utcnow()
        uow.source = context.process_context[process_name].source
        uow.sink = context.process_context[process_name].sink
        uow.state = unit_of_work.STATE_REQUESTED
        uow.unit_of_work_type = unit_of_work.TYPE_MANAGED
        uow.number_of_retries = 0
        uow.arguments = context.process_context[process_name].arguments
        uow.db_id = self.uow_dao.insert(uow)

        msg = 'Created: UOW {0} for {1}@{2}.'.format(uow.db_id, process_name, start_timeperiod)
        self._log_message(INFO, process_name, start_timeperiod, msg)
        return uow