[ROS2][service][topic]service and topic cooperate

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The existing system has topics cmd_vel,light_sensors and a service motor_power. I created a node that it will take the msg from light_sensors, then publish msg into cmd_vel. While, I wanna call service motor_power to turn on/off the motor when the msg from light_sensors meets some conditions.

I found many examples that executes topics and services separately, but there is no example that combine the service and topic. Also, I found an article about actions, but I am not quite sure if it is what I want.

What is the right way/design to do so? Calling a service from the callback function of a subscriber? Or there is more proper way? I have tried this, but it did not work for me, and the post is in 2018, a bit old?

#!/usr/bin/env python3
import sys
import rclpy
from rclpy.node import Node
from std_msgs.msg import String
from std_srvs.srv import SetBool

class SubscriberClientAsync(Node):

    def __init__(self):
        super().__init__('sub_client_node')
        self.client = self.create_client(SetBool, 'motor_power')
        self.sub = self.create_subscription(String, 'topic', self.topic_cb)

    def topic_cb(self, msg):
        // How should I implement here to just spin once self.client ?
        if msg.data == 'stop':
            req = SetBool.Request()
            req.data = False
            self.future = self.client.call_async(req)

if __name__ == '__main__':
    rclpy.init()
    node = SubscriberClientAsync()
    try:
        rclpy.spin(node)
    except KeyboardInterrupt:
        pass

    node.destroy_node()