Names and Namespaces

Namespaces

The concept of names and namespaces is explained in detail in the ROS Documentation.

You can start nodes in namespaces (you can also have nested namespaces). This means that the namespace gets prepended to all relative and private topic/action/service names. We already used this in the launch file tutorial. Every node (and also every node in included launch files) inside a GroupAction with a PushRosNamespace action is launched inside a namespace.

Names

If you have a node subscribing or publishing to/from a topic, you have to specify the topic name. You can do this in three different ways:

Global

self.pose_pub = self.create_publisher(PoseStamped, "/my_robot/pose", 1)

A topic name with a leading / will be resolved globally. This means that it does not matter if the node was launched in a namespace or not. The resulting topic name will be exactly /my_robot/pose.

Hint

Usually, we do not want to use global names. Avoid them if you do not have a good reason to use them.

Relative

self.pose_pub = self.create_publisher(PoseStamped, "pose", 1)

A topic without leading / will be relative. This means that if the node was launched in a namespace, the namespace will get prepended. So, for example if the node was launched in the namespace my_robot, the resolved topic name will become /my_robot/pose. In case the node was not launched inside any namespace, nothing will get prependended to the topic name. It will be just /pose. If the node is pushed into multiple namespaces, they all get prepended to the resolved topic name.

Hint

This will probably the most common way of specifying the topic name during this class.

Private

class MyNode(Node):

   def __init__(self):
      super().__init__('my_controller')
      self.debug_pub = self.create_publisher(ControlDebugMessage, '~/debug', 1)

Private topics are similar to relative ones. The topic name starts with ~. The namespace will get prepended (if one has been specified), just as it will for relative topics. Additionally, the name of the node will also be prepended. So, if a node with the name my_controller has been started in the namespace my_robot, the resolved topic name will be /my_robot/my_controller/debug. Without a namespace, it would be /my_controller/debug.

Hint

This way of specifying topic names is less common, but it is especially useful for debug messages and other messages that are closely tied to a specific node.

BlueROV

You will only work with a single robot. Still, it is nice to keep things clean and tidy. We will start everything in the bluerov00 namespace.

Note

If you have a controller subscribing to a setpoint topic, it might be a good idea to use a private name '~/setpoint'. This way you avoid topic name conflicts in case you have another controller also subscribing to a setpoint topic.