ROS2 : How to call a service from the callback function of a subscriber ?

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The problem is, that you can't use the spin_node_until_future_complete() command inside the outer callback. The solution is to use an MultiThreadedExecutor and different callback groups. Then you can call the inner service by giving it a callback function (instead of waiting for the return code with spin_node_until_future_complete).

If you want to wait for the result from the inner service call before the outer service callback finishes you need to block the thread with a while(return_from_inner_service == false) loop. This is important if your outer callback is from a service, because then you want to receive the inner service result and use it to compute the result of the outer service. But this is bad design, because you block the spin() call from the outer service. Luckily we can still handle other callbacks with the MultiThreadedExecutor as long as we assign them to a different callback group. If the outer callback is from a topic you don't want to block the callback, just handle the inner service call inside the assigned callback function. It will execute asynchronous.

Here is some code example in C++ (assuming all necessary headers included and ROS services created)

Create the MultiThreadedExecuter and an instance of the Node in main:

int main(int argc, char *argv[])
{
    rclcpp::init(argc, argv);
    rclcpp::executors::MultiThreadedExecutor executor;
    auto node = std::make_shared<Communication>();
    executor.add_node(node);
    executor.spin();
    rclcpp::shutdown();
    return 0;
}

Create seperate Callback group and assign it to the inner service client (inside the Node/class constructor):

callback_group_input_ = this->create_callback_group(rclcpp::callback_group::CallbackGroupType::MutuallyExclusive);
get_input_client_ = this->create_client<petra_core::srv::GetInput>("GetInput", rmw_qos_profile_services_default, callback_group_input_);

Inside the outer callback from a topic or service, send the inner_request and define a callback for it:

int choice = -1;
    auto inner_client_callback = [&,this](rclcpp::Client<petra_core::srv::GetInput>::SharedFuture inner_future)
        { 
            auto result = inner_future.get();
            choice = stoi(result->input);
            RCLCPP_INFO(this->get_logger(), "[inner service] callback executed");
        };
    auto inner_future_result = get_input_client_->async_send_request(inner_request, inner_client_callback);

If it is necessary to wait inside the outer callback for the result block the thread:

// Wait for the result.
    while (choice < 0 && rclcpp::ok())
    {
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }

I hope it helps you, or whoever has the same problem. I was stuck with this for weeks...

I was stuck on this for whole day, so I'm adding my solution in case anyone else runs into a similar problem. The previously mentioned solutions are on the right track, but they are incomplete. There is an official solution in the ros docs, which I am pasting here just in case the link breaks.

Note that this problem is not unique to calling a service client from within a subscriber callback. This problem occurs whenever you have to call a callback from within a callback. Therefore the example of calling a service from within the timer callback is effectively the same as calling a service from within a subscription callback, or calling a service from within a service callback.

The solution is to put your nested callbacks in different callback groups (see the link above) and to use std::future::wait_for() instead of rclcpp::spin_until_future_complete().

#include <chrono>
#include <memory>
#include "rclcpp/rclcpp.hpp"
#include "std_srvs/srv/empty.hpp"

using namespace std::chrono_literals;

namespace cb_group_demo
{
class DemoNode : public rclcpp::Node
{
public:
    DemoNode() : Node("client_node")
    {
        client_cb_group_ = this->create_callback_group(rclcpp::CallbackGroupType::MutuallyExclusive);
        timer_cb_group_ = this->create_callback_group(rclcpp::CallbackGroupType::MutuallyExclusive);
        client_ptr_ = this->create_client<std_srvs::srv::Empty>("test_service", rmw_qos_profile_services_default,
                                                                client_cb_group_);
        timer_ptr_ = this->create_wall_timer(1s, std::bind(&DemoNode::timer_callback, this),
                                            timer_cb_group_);
    }

private:
    rclcpp::CallbackGroup::SharedPtr client_cb_group_;
    rclcpp::CallbackGroup::SharedPtr timer_cb_group_;
    rclcpp::Client<std_srvs::srv::Empty>::SharedPtr client_ptr_;
    rclcpp::TimerBase::SharedPtr timer_ptr_;

    void timer_callback()
    {
        RCLCPP_INFO(this->get_logger(), "Sending request");
        auto request = std::make_shared<std_srvs::srv::Empty::Request>();
        auto result_future = client_ptr_->async_send_request(request);

        // Do this instead of rclcpp::spin_until_future_complete()
        std::future_status status = result_future.wait_for(10s);  // timeout to guarantee a graceful finish
        if (status == std::future_status::ready) {
            RCLCPP_INFO(this->get_logger(), "Received response");
            std::shared_ptr<ServiceT::Response> response = result_future.get();
            // Do something with response 
        }
    }
};  // class DemoNode
}   // namespace cb_group_demo

int main(int argc, char* argv[])
{
    rclcpp::init(argc, argv);
    auto client_node = std::make_shared<cb_group_demo::DemoNode>();
    rclcpp::executors::MultiThreadedExecutor executor;
    executor.add_node(client_node);

    RCLCPP_INFO(client_node->get_logger(), "Starting client node, shut down with CTRL-C");
    executor.spin();
    RCLCPP_INFO(client_node->get_logger(), "Keyboard interrupt, shutting down.\n");

    rclcpp::shutdown();
    return 0;
}

Dear ROS developers,

If you want a solution in C++, check out THIS. Pieces of code for illustrating the solution:

struct ActivationFuture
{
  std::shared_future<std::shared_ptr<bica_msgs::srv::ActivateComponent_Response_<std::allocator<void>>>> future;
  std::string component;
};

class MyNode : public rclcpp::Node
{
  rclcpp::Service<bica_msgs::srv::ActivateComponent>::SharedPtr activation_service_; 
  std::vector<ActivationFuture> pending_act_futures_;

  void
  MyNode::activateDependency(const std::string & dep)
  {
    auto client = this->create_client<bica_msgs::srv::ActivateComponent>("/" + dep + "/activate");

    if (!client->wait_for_service(100ms)) {
      RCLCPP_ERROR(get_logger(), "Service %s is not available.",
        client->get_service_name());
      return;
    }

    auto request = std::make_shared<bica_msgs::srv::ActivateComponent::Request>();
    request->activator = get_name();

    auto future = client->async_send_request(request);
    pending_act_futures_.push_back(ActivationFuture{future, dep});
  }

  bool
  MyNode::ok()
  {
    if (rclcpp::ok()) {
      while(!pending_act_futures_.empty()) {
        auto pending_future = pending_act_futures_.back();
        if ((rclcpp::spin_until_future_complete(this->get_node_base_interface(), pending_future.future)) !=
          rclcpp::executor::FutureReturnCode::SUCCESS)
        {
          RCLCPP_WARN(get_logger(), "Component [%s] failed to activate", pending_future.component.c_str());
        }
        pending_act_futures_.pop_back();
      }
    }

    return rclcpp::ok();
  }

  void
  MyNode::execute()
  {
    while (this->ok()) {
      rclcpp::spin_some(this->get_node_base_interface());
      rate_.sleep();
    }
}

The bad thing is that you need a different vector for each type of future. Any ideas?

I hope it helps!!!