There exist two simple nodes, one as a server and the other as a client. The server node just gives service to the client node request. The client node has a timer that calls the request and it has to shows the response of the request.

class MinimalService(Node):

def __init__(self):
    super().__init__('minimal_service')
    self.srv = self.create_service(AddTwoInts, 'add_two_ints', self.add_two_ints_callback)

def add_two_ints_callback(self, request, response):
    response.sum = request.a + request.b
    self.get_logger().info('Incoming request\na: %d b: %d' % (request.a, request.b))

    return response

def main(args=None):
    rclpy.init(args=args)
    minimal_service = MinimalService()
    rclpy.spin(minimal_service)
    rclpy.shutdown()

and the client code:

class MinimalClientAsync(Node):

def __init__(self):
    super().__init__('minimal_client_async')
    self.cli = self.create_client(AddTwoInts, 'add_two_ints')
    while not self.cli.wait_for_service(timeout_sec=1.0):
        self.get_logger().info('service not available, waiting again...')
    self.req = AddTwoInts.Request()
    self.grp_timer = MutuallyExclusiveCallbackGroup()
    self.create_timer(1.0, self.send_request, callback_group=self.grp_timer)

def send_request(self):
    self.req.a = 2
    self.req.b = 3
    self.future = self.cli.call_async(self.req)

    rclpy.spin_until_future_complete(self, self.future)

    try:
        response = self.future.result()
    except Exception as e:
        self.get_logger().info(
            'Service call failed %r' % (e,))
    else:
        self.get_logger().info(
            'Result of add_two_ints: for %d + %d = %d' %
            (self.req.a, self.req.b, response.sum))


def main(args=None):
    rclpy.init(args=args)
    minimal_client = MinimalClientAsync()

    while rclpy.ok():
        rclpy.spin_once(minimal_client)

    minimal_client.destroy_node()
    rclpy.shutdown()

These two are working now, but the problem is that I do not know exactly why? I appreciate if someone gives me some resources about callback group and executers. When I try to use the MultiThreadExecuter by creating the service instance in the client's main function the code stuck in a deadlock. Can somebody shows how I can use a multithread executer for these two codes?