ROS Topics

Create an ament_cmake built-type package called demo_interfaces

ros2 pkg create --build-type ament_cmake demo_interfaces

Topics are:

• Anonymous
• Strongly-typed
• Allow:
  • intra-process
  • inter-process

For details about topics names: check Names in ROS 2 .

Publishers/Subscribers

Topics use a asynchronous communication paradigms refer as publication/subscription:

• publishers don’t know if anyone’s listening
• subscribers are event-triggered (by incoming messages)
• messages may be dropped (depending on the QoS, but more on that later.)

Anonymous

Topics are anonymous: “This means that when a subscriber gets a piece of data, it doesn’t generally know or care which publisher originally sent it (though it can find out if it wants). ” from Official ROS docs ↗ .

Strongly-typed

The communication using topics relies in the concept of Strongly-typed definitions called messages.

Messages are a way for a ROS 2 node to send data on the network to other ROS nodes, with no response expected. Messages are described and defined in .msg files in the msg/ directory of a ROS package.

Message Definition Structure

Structure:

• Field type.
  • Any Built-in-types ↗ .
  • Other message as the type to define a field (nesting messages).
• Field name
• Field with constant value. Fields with constant values have to be UPPERCASE.
• Field with default value

With in the demo_interfaces package, let’s create a folder called msg.

Inside msg directory create a file called Counter.msg with the following content:

int64 count

Add the rosidl_default_generators and “friends” to the package.xml of the demo_interfaces package.

<buildtool_depend>rosidl_default_generators</buildtool_depend>
<exec_depend>rosidl_default_runtime</exec_depend>
<member_of_group>rosidl_interface_packages</member_of_group>

Add the following code to the CMakeLists.txt:

find_package(rosidl_default_generators REQUIRED)

rosidl_generate_interfaces(${PROJECT_NAME}
  "msg/Counter.msg"
)

Build the message definition:

Let’s create a minimal publisher that publishes our custom message definition called Counter.msg.

import rclpy
from rclpy.node import Node
from demo_interfaces.msg import Counter

class MinimalPublisher(Node):

    def __init__(self):
        super().__init__("publisher_node")

        self.__count = 0

        self.__pub = self.create_publisher(Counter, "/count", 10)

        timer = self.create_timer(0.5, self.callback)

    def callback(self):
        msg = Counter()
        msg.count = self.__count
        self.__pub.publish(msg)
        self.get_logger().info(f"Publishing count: {self.__count}")
        self.__count += 1

def main(args=None) -> None:

    rclpy.init(args=args)

    minimal_publisher = MinimalPublisher()

    rclpy.spin(minimal_publisher)

    minimal_publisher.destroy_node()

    rclpy.shutdown()

if __name__ == "__main__":
    main()

Let’s create a minimal subscriber that subscribes our custom message definition called Counter.msg.

import rclpy
from rclpy.node import Node
from demo_interfaces.msg import Counter

class MinimalSubscriber(Node):

    def __init__(self):
        super().__init__("subscriber_node")

        self.__sub = self.create_subscription(Counter, "/count", self.callback, 10)

    def callback(self, msg) -> None:
        self.get_logger().info(f"Got count: {msg.count}")


def main(args=None) -> None:

    rclpy.init(args=args)

    minimal_subscriber = MinimalSubscriber()

    rclpy.spin(minimal_subscriber)

    minimal_subscriber.destroy_node()

    rclpy.shutdown()

if __name__ == "__main__":
    main()

“ROS 2 offers a rich variety of Quality of Service (QoS) policies that allow you to tune communication between nodes” from Quality of Service settings ↗ .

“A set of QoS policies combine to form a QoS profile. Given the complexity of choosing the correct QoS policies for a given scenario, ROS 2 provides a set of predefined QoS profiles for common use cases (e.g. sensor data). At the same time, developers are given the flexibility to control specific policies of the QoS profiles.”

Check Quality of Service settings ↗ for more details.