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

asked 2018-08-30 04:38:04 -0600

Marc Testier gravatar image

updated 2018-08-30 04:49:45 -0600


This question is actually related to :

But I wrote a minimal example and wanted to have your feedback on how to fix this.

So I'm using ROS2 Bouncy, built from source on Ubuntu 18.04. I have a node that subscribe to a topic and in the callback function of the sub, I get stuck if I try to rclpy.spin_once(self).

I wrote a minimal package to show the problem, which can be found here :

I was actually trying to call a service from the callback function but got stuck when doing rclpy.spin_until_future_complete(node, future) to get the answer form the service.

So any idea how to call a service from the callback function of a subscriber ?? Maybe threads ??


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answered 2018-08-30 21:12:20 -0600

Marc Testier gravatar image

Here is the solution proposed by @codebot to call a service from the callback of a sub :

Here is what he said about the solution though :

The key issue is that calling a service synchronously from within a subscriber callback is usually not a good design, because it's possible that the inbound messages will be coming more quickly than the service can process them, which could result in an uncontrolled message queue forming. The general pattern is that topic callbacks should be "fast".

Anyway, if you still want to try :

To accomplish this in rclpy, we can asynchronously issue the service request. Then in the main spin() loop, we can look at the array of futures and, whenever one is completed, we can read its result.

So the way I threw that together will also result in an unbounded request queue if the inbound message stream is coming really fast and the service is slow but you could control that by looking at the length of self.client_futures and not adding to it if it's already larger than some threshold.

#!/usr/bin/env python3
import sys

import rclpy
from rclpy.node import Node
from std_msgs.msg import String
from test_msgs.srv import Empty

class SubscriberClientAsync(Node):

    def __init__(self):
        self.client = self.create_client(Empty, 'service')
        self.sub = self.create_subscription(String, 'topic', self.topic_cb)
        self.client_futures = []

    def topic_cb(self, msg):

    def spin(self):
        while rclpy.ok():
            incomplete_futures = []
            for f in self.client_futures:
                if f.done():
                    res = f.result()
                    print("received service result: {}".format(res))
            self.client_futures = incomplete_futures

if __name__ == '__main__':
    node = SubscriberClientAsync()
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@Marc Testier

The key issue is that calling a service synchronously from within a subscriber callback is usually not a good design

what should I do if I wanna call a service synchronously due to the msg from a subscriber callback? Can you give me an idea what I should look into?

lin404 gravatar image lin404  ( 2020-01-28 02:33:08 -0600 )edit

answered 2020-05-18 05:56:35 -0600

Andreas Z gravatar image

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>();
    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())

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

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answered 2019-12-13 13:16:23 -0600

fmrico gravatar image

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_;

  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.",

    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});

    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_WARN(get_logger(), "Component [%s] failed to activate", pending_future.component.c_str());

    return rclcpp::ok();

    while (this->ok()) {

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

I hope it helps!!!

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Asked: 2018-08-30 04:38:04 -0600

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Last updated: May 18