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Enabling topic statistics (C++)

Goal: Enable ROS 2 Topic Statistics and view the output statistics data.

Tutorial level: Advanced

Time: 10 minutes

Table of Contents

Background
Prerequisites
Tasks
Summary
Related content

Background 

This is a short tutorial of how to enable Topic Statistics in ROS 2 and view the published statistics output using command line tools (ros2topic).

ROS 2 provides the integrated measurement of statistics for messages received by any subscription, called Topic Statistics. With Topic Statistics enabled for your subscription, you can characterize the performance of your system or use the data to help diagnose any present issues.

For more details please see the Topic Statistics Concepts Page.

Prerequisites 

An installation from either binaries or source.

In previous tutorials, you learned how to create a workspace, create a package, and create a C++ publisher and subscriber.

This tutorial assumes that you still have your cpp_pubsub package from the C++ tutorial.

Tasks 

1 Write the subscriber node with statistics enabled 

Navigate into the ros2_ws/src/cpp_pubsub/src folder, created in the previous tutorial, and download the example talker code by entering the following command:

wget -O member_function_with_topic_statistics.cpp https://raw.githubusercontent.com/ros2/examples/foxy/rclcpp/topics/minimal_subscriber/member_function_with_topic_statistics.cpp

wget -O member_function_with_topic_statistics.cpp https://raw.githubusercontent.com/ros2/examples/foxy/rclcpp/topics/minimal_subscriber/member_function_with_topic_statistics.cpp

Right click this link and select Save As publisher_member_function.cpp:

https://raw.githubusercontent.com/ros2/examples/foxy/rclcpp/topics/minimal_subscriber/member_function_with_topic_statistics.cpp

Now there will be a new file named member_function_with_topic_statistics.cpp. Open the file using your preferred text editor.

#include <chrono>
#include <memory>

#include "rclcpp/rclcpp.hpp"
#include "rclcpp/subscription_options.hpp"

#include "std_msgs/msg/string.hpp"

class MinimalSubscriberWithTopicStatistics : public rclcpp::Node
{
public:
  MinimalSubscriberWithTopicStatistics()
  : Node("minimal_subscriber_with_topic_statistics")
  {
    // manually enable topic statistics via options
    auto options = rclcpp::SubscriptionOptions();
    options.topic_stats_options.state = rclcpp::TopicStatisticsState::Enable;

    // configure the collection window and publish period (default 1s)
    options.topic_stats_options.publish_period = std::chrono::seconds(10);

    // configure the topic name (default '/statistics')
    // options.topic_stats_options.publish_topic = "/topic_statistics"

    auto callback = [this](std_msgs::msg::String::SharedPtr msg) {
        this->topic_callback(msg);
      };

    subscription_ = this->create_subscription<std_msgs::msg::String>(
      "topic", 10, callback, options);
  }

private:
  void topic_callback(const std_msgs::msg::String::SharedPtr msg) const
  {
    RCLCPP_INFO(this->get_logger(), "I heard: '%s'", msg->data.c_str());
  }
  rclcpp::Subscription<std_msgs::msg::String>::SharedPtr subscription_;
};

int main(int argc, char * argv[])
{
  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared<MinimalSubscriberWithTopicStatistics>());
  rclcpp::shutdown();
  return 0;
}

1.1 Examine the code 

As in the C++ tutorial, we have a subscriber node which receives string messages from the topic topic from the topic_callback function. However, we’ve now added options to configure the subscription to enable topic statistics with the rclcpp::SubscriptionOptions() options struct.

// manually enable topic statistics via options
auto options = rclcpp::SubscriptionOptions();
options.topic_stats_options.state = rclcpp::TopicStatisticsState::Enable;

Optionally, fields such as the statistics collection/publish period and the topic used to publish statistics can be configured as well.

// configure the collection window and publish period (default 1s)
options.topic_stats_options.publish_period = std::chrono::seconds(10);

// configure the topic name (default '/statistics')
// options.topic_stats_options.publish_topic = "/my_topic"

The configurable fields are described in the following table:

Subscription Config Field	Purpose
topic_stats_options.state	Enable or disable topic statistics (default `rclcpp::TopicStatisticsState::Disable`)
topic_stats_options.publish_period	The period in which to collect statistics data and publish a statistics message (default `1s`)
topic_stats_options.publish_topic	The topic to use when publishing statistics data (default `/statistics`)

1.2 CMakeLists.txt 

Now open the CMakeLists.txt file.

Add the executable and name it listener_with_topic_statistics so you can run your node using ros2 run:

add_executable(listener_with_topic_statistics member_function_with_topic_statistics.cpp)
ament_target_dependencies(listener_with_topic_statistics rclcpp std_msgs)

install(TARGETS
  talker
  listener
  listener_with_topic_statistics
  DESTINATION lib/${PROJECT_NAME})

Make sure to save the file, and then your pub/sub system, with topic statistics enabled, should be ready for use.

2 Build and run 

To build, see the Build and run section in the pub/sub tutorial.

Run the subscriber with statistics enabled node:

ros2 run cpp_pubsub listener_with_topic_statistics

Now run the talker node:

ros2 run cpp_pubsub talker

The terminal should start publishing info messages every 0.5 seconds, like so:

[INFO] [minimal_publisher]: Publishing: "Hello World: 0"
[INFO] [minimal_publisher]: Publishing: "Hello World: 1"
[INFO] [minimal_publisher]: Publishing: "Hello World: 2"
[INFO] [minimal_publisher]: Publishing: "Hello World: 3"
[INFO] [minimal_publisher]: Publishing: "Hello World: 4"

The listener will start printing messages to the console, starting at whatever message count the publisher is on at that time, like so:

[INFO] [minimal_subscriber_with_topic_statistics]: I heard: "Hello World: 10"
[INFO] [minimal_subscriber_with_topic_statistics]: I heard: "Hello World: 11"
[INFO] [minimal_subscriber_with_topic_statistics]: I heard: "Hello World: 12"
[INFO] [minimal_subscriber_with_topic_statistics]: I heard: "Hello World: 13"
[INFO] [minimal_subscriber_with_topic_statistics]: I heard: "Hello World: 14"

Now that the subscriber node is receiving messages, it will periodically publish statistics messages. We will observe these messages in the next section.

3 Observe published statistic data 

While the nodes are running, open a new terminal window. Execute the following command:

ros2 topic list

This will list all currently active topics. You should see the following:

/parameter_events
/rosout
/statistics
/topic

If you optionally changed the topic_stats_options.publish_topic field earlier in the tutorial, then you will see that name instead of /statistics.

The subscriber node you created is publishing statistics, for the topic topic, to the output topic /statistics.

We can visualize this using RQt

Now we can view the statistics data published to this topic with the following command:

ros2 topic echo /statistics

The terminal should start publishing statistics messages every 10 seconds, because the topic_stats_options.publish_period subscription configuration was optionally changed earlier in the tutorial.

---
measurement_source_name: minimal_subscriber_with_topic_statistics
metrics_source: message_age
unit: ms
window_start:
  sec: 1594856666
  nanosec: 931527366
window_stop:
  sec: 1594856676
  nanosec: 930797670
statistics:
- data_type: 1
  data: .nan
- data_type: 3
  data: .nan
- data_type: 2
  data: .nan
- data_type: 5
  data: 0.0
- data_type: 4
  data: .nan
---
measurement_source_name: minimal_subscriber_with_topic_statistics
metrics_source: message_period
unit: ms
window_start:
  sec: 1594856666
  nanosec: 931527366
window_stop:
  sec: 1594856676
  nanosec: 930797670
statistics:
- data_type: 1
  data: 499.2746365105009
- data_type: 3
  data: 500.0
- data_type: 2
  data: 499.0
- data_type: 5
  data: 619.0
- data_type: 4
  data: 0.4463309283488427
---

From the message definition the data_types are as follows

data_type value	statistics
1	average
2	minimum
3	maximum
4	standard deviation
5	sample count

Here we see the two currently possible calculated statistics for the std_msgs::msg::String message published to /topic by the minimal_publisher. Because the std_msgs::msg::String does not have a message header, the message_age calculation cannot be performed, so NaNs are returned. However, the message_period can be calculated and we see the statistics populated in the message above.

Summary 

You created a subscriber node with topic statistics enabled, which published statistics data from the C++”s publisher node. You were able to compile and run this node. While running, you were able to observe the statistics data.