ROS Namespaces

In ROS, Namespaces are used to allow multiple topics with the same name to exists. It works similar to a folder structure in any operating system where inside one folder no duplicate names are allowed but in a different folder a file can have the exact same name. We therefore use Namespaces to have multiple topics with the same name e. g. when launching multiple Turtlebots we need two different “cmd_vel” topics to control each of the robots.

Inside a launch file you can use the following code to launch a node inside a namespace. Don’t forget to replace the in uppercase written names.

import launch
import launch_ros

def generate_launch_description():
    namespace_node = launch_ros.actions.Node(
        package='PACKAGE_NAME',
        executable='EXECUTABLE_NAME',
        namespace='NAMESPACE_NAME',
        name='NODE_NAME'
    )
    
    return launch.LaunchDescription([
        namespace_node,
    ])

When referencing topic names inside a Namespace be aware of the syntax. Defining the name with a leading “/” means that I will define the full name with all Namespaces e.g. “/tb3_0/odom”. If you don’t put the leading “/” you are defining the name from the Namespace you are in. If I launch my ROS Node inside the Namespace “tb3_0” I can then reference the same topic as before with just “odom”.

Troubleshooting

Here are a view terminal commands that can be used for troubleshooting:

Listing all running ros topics:
```
ros2 topic list
```
Inspecting the details of one topic:
```
ros2 topic info -v TOPIC-NAME
```
Note: TOPIC-NAME should be replaced with the name of the topic shown in the ros2 topic list command.
Print out what is published on a topic:
```
ros2 topic echo TOPIC-NAME
```
Note: TOPIC-NAME should be replaced with the name of the topic shown in the ros2 topic list command.
A visual representation of all running ros nodes, their namespace and how they are connected to each other:
```
ros2 run rqt_graph rqt_graph
```

Exercise

To goal of this exercise is to create a robot_handler node that is launched twice (once in each namespace of the two turtlebots) and a leader that is launched outside any namespace. The robot_handler node subscribes to the lidar topic of the turtlebot in the same namespace and should publish the value at 180 degrees to a topic called namespace_test. The leader should subscribe to both namespace_test topics and print out what it has received with an indicator from which robot that information comes. In the python scripts you will find TODO comments that specify exactly what needs to be done.

Inside the given ROS package multi_robot_challenge_23 do the following steps:

In the ros2_students_25/namespace_exercise repository you will find the following files: robot_handler.py and leader.py. Copy them to ros2_ws/src/multi_robot_challenge_23/multi_robot_challenge_23.
Make sure that both robot_handler.py and leader.py are configured as console scripts in setup.py similar to what you have done to set up the braitenberg_vehicle assignment.
Create a launch file which launches the robot_handler node two times in the same Namespaces as the two Turtlebots and the leader node once outside any namespace.
Note: you can use the code example in the beginning of this webpage for that.