Part 1: Using Nodes

Nodes are essential to the ROS ecosystem. Used as the primary building blocks of the architecture, we can add nodes in to encompass anything we need. Before we get to building them, let’s first see how they work.

Before we can begin though, we need to install the nodes we will be using by running the following commands in one of your temrinals:

apt update
apt install ros-humble-demo-nodes-{cpp,py}

In the first terminal, run the following command:

ros2 run demo_nodes_cpp talker

This will launch a simple C++ node who’s job is to publish a message to the network, roughly every second. The message will be print to the terminal each time it is sent, with the amount of seconds since the node was started.

In the second one, run the following command:

ros2 run demo_nodes_cpp listener

This launches a simple C++ node who’s job is to listen for a “talker node” on the network which is publishing messages. You should see this node start to respond to the previous node we started, printing out all of the messages as they’re sent.

Finally, let’s test something a little bit more difficult. Run the following command in the third terminal:

ros2 run demo_nodes_py listener

This launches a simple Python node which also listens for “talker nodes” on the network. You should see this node start printing out everything that our other listener node is printing. However, this node is in Python. In ROS, we’re able to seamlessly communicate between nodes in Python and C++ without issue; ROS handles the logistics of sending and recieving messages, and all we need to do is write the code to handle the messages that we’re sending and recieving.

Lastly, let’s do one last test. Jump over to the first terminal window you created (with the other two listeners still running), and press Control + C in the terminal running the talker node. Then, run the following code:

ros2 run demo_nodes_py talker

A new Python node will be created, publishing messages to the network. Similar to before, you should see both of the listener nodes start printing out messages again. As mentioned before, ROS allows for seamless communication between languages, with only a small amount of overhead.