how to call services periodically using a wall_timer

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Dear Community,

I'm an inexperienced coder and new to ROS, please understand...

I want to figure out why I cannot call a service (by sending a request and waiting for a response) from a node's wall_timer. I've attached a text file outlining two simple methods that yielded different, undesirable results. I hope they sufficiently describe the error I'm having. If not please let me know. I am not necessarily looking for a solution, rather for an explanation of what could be causing this issue from occurring. My knowledge of ROS and C++ is limited so maybe it is really obvious to you...

Thanks to anyone taking the time to help me out!

The issue:

Calling a service periodically, using a rclcpp::wall_timer:

Wall Timer Callback Function:

void connection::timer_callback() { // 1. create a request auto test_request = network_library::create_request(1,1,1,1);

// 2. print a confirmation and the contents of the request to cmd line
network_library::print_string(" 2. sending a request asynchronously");
network_library::print_message(test_request);

// 3. send the request asynchronously, create a shared_future to store response
auto result = test_client->async_send_request(test_request);

//4. print a confirmation that the request has been sent
network_library::print_string(" 4. the request has been sent...");

// 5.1 use the shared_future::get() function to "get" the resulting response from server
//network_library::print_message(result.get()); // 6 print result

// 5.2 use the rclcpp::spin_until_future_complete() function to receive the result
if (rclcpp::spin_until_future_complete(shared_from_this(), result) == rclcpp::executor::FutureReturnCode::SUCCESS) {
    network_library::print_message(result); // 6 print result
}

}

Generates the following results at runtime (copied from command line):

Results from CLIENT:

Result using method 5.1:

ostfalia@ros:~/dev_ws/src$ ros2 run client main //NOTE: nothing else happens. No other processes continue to function on the client node, no errors are returned.

Result using method 5.2:

ostfalia@ros:~/dev_ws/src$ ros2 run client main terminate called after throwing an instance of 'std::runtime_error' what(): Node has already been added to an executor.

Results from the SERVER:

Result using method 5.1/5.2 (both are identical):

ostfalia@ros:~/dev_ws/src$ ros2 run server main ^C[INFO] [1667311554.420049898] [rclcpp]: signal_handler(signal_value=2) ostfalia@ros:~/dev_ws/src$ ros2 run server main Essentially it shows that calling the spin_until_future_complete() function (5.2) results in a runtime error and in the other configuration (5.1) the .get() never returns and the client node doesn't get anywhere.