Service Server & Service Client

The first type of ROS communication that we explored was a one-way interaction called messages which are sent over channels called topics. Now we are going to explore a different communication type, which is a two-way interaction via a request from one node to another and a response from that node to the first.

After completing this exercise, you will have the answers to the following questions:

• How to create a custom messages/services?
• How to use message_generation?
• How to implement a simple service server?

1. First, read through the ROS tutorials section on Creating a ROS msg and srv.

2. Next, navigate to the root of your fake_sonar_driver package and create a subdirectory named srv for our custom service. Our service will enable changing the sonar’s field of view (FOV), which is published as part of the Range message.

3. In the srv directory, create a service file named SetSonarFOV.srv with the following request and response parts:

   # Sets .
   float64 fov_to_set
   ---
   float64 set_fov
   

4. Make changes in your package.xml and CMakeLists.txt to include message_generation for this service.

5. Test it out:

   rossrv show fake_sonar_driver SetSonarFOV
   

6. Good Job! It’s now time to learn the steps needed to create a service server (waits for request and comes up with response) and client (makes request for info then waits for response).

7. Complete the Writing a Simple Service and Client (C++) tutorial.

8. Now that you know how service server and client are implemented in nodes, let’s add a service server to fake_sonar_driver_node.

9. Add the following functionality to our fake sonar node:

   • the node advertises a service on topic /set_sonar_fov
   • the service type is fake_sonar_driver/SetSonarFOV
   • the server will check fov_to_set field and responds back with the same value in set_fov field.

10. Test the server with rossrv call /sonar_distance 1.4 and show the result to the instructor.

11. Improve the fake_sonar_driver_node such that the fov_to_set value would be updated into the /sonar_distance topic. The easies way to do this is by defining field_of_view as a global variable:

    #include <...>
    
    double field_of_view;
    
    bool callback_function_for_server(...)
    {
      // set field_of_view here
    }
    
    int main(...)
    {
      // use field_of_view here
    }
    ...
    

12. Would it be awesome to visualize the sonar range and its field of view graphically? In ROS, this is peace of cake:

    • Open a new terminal and type: rviz
    • In RViz, click the add button on the Displays panel (the one on the left side).
    • Select the By topic tab and double-click on the /sonar_distance topic.

13. Open another terminal and change the field of view with the rossrv call command. See if you notice any changes in RViz.

14. Present your solution to the instructor. Congratulations, you are now ready to face the challenges in day 3!

Bonus task

Currently our fake sonar node has publishers and subscribers laying next to main(). However, to make our code modular, we should restructure it and place the publisher, service server, and callback functions into a separate FakeSonarDriver class.

1. Create the src/fake_sonar_driver.cpp and include/fake_sonar_driver/fake_sonar_driver.h files.
2. Replace the existing while loop with a timer that would update the sine() value in the class.
3. The main() function should now simply create the FakeSonarDriver object and spin forever using ros::spin().