Yaml file loaded in the parameter server but disappears after.

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Parmaters are not like topics, as you said they are loaded into the parameter server. In the introduction of the wiki :

A parameter server is a shared, multi-variate dictionary that is accessible via network APIs. Nodes use this server to store and retrieve parameters at runtime. As it is not designed for high-performance, it is best used for static, non-binary data such as configuration parameters. It is meant to be globally viewable so that tools can easily inspect the configuration state of the system and modify if necessary.

The Parameter Server is implemented using XMLRPC and runs inside of the ROS Master, which means that its API is accessible via normal XMLRPC libraries.

So it's not a node that stores the parameters and publish them to the other nodes. The parameters can be read/set via your code or with the tool rosparam .

So instead of rostopic list you can see the parameters with rosparam list. For example if you want to check the value of all your parameters at once you can use rosparam get /structure/ that will return {angle: 120, bras: 3, longueur: 550}