Using multimaster with 2 identical robots and a ground station

Hi, I’d like to use multimaster for a setup with 3 machines, each running their own ROS master. One of the machines is a ground station, while the other 2 are identical vehicles with many nodes and topics. I would only like a small subset of topics from each of the vehicles to be shared with the base station alone. On each of the vehicles, I launch:

<launch>
  <arg name="robot_name" default="robot0" />
  <node name="master_discovery" pkg="master_discovery_fkie" type="master_discovery" >
  </node>
  <!-- Launch the relay node -->
  <node pkg="topic_tools" type="relay" name="robot_relay" args="/shared_topic /$(arg robot_name)/shared_topic" />
</launch>

with a different "robot_name" argument for each since I want the base station to differentiate between this shared_topic. On the base station, I launch:

<launch>
  <node name="master_discovery" pkg="master_discovery_fkie" type="master_discovery" output="screen">
  </node>
  <node name="master_sync" pkg="master_sync_fkie" type="master_sync" output="screen">
    <rosparam param="sync_topics"> ['/robot0/shared_topic', '/robot1/shared_topic'] </rosparam>
  </node>
</launch>

which allows the base station to successfully receive /robot0/shared_topic OR /robot1/shared_topic when only one of the vehicle’s has launched the first launch file. When I try to run both, I get the following warning before the nodes on the vehicle shutdown:

Reason given for shutdown: [new node registered with same name].

Each vehicle and the base station have their own ROS Master with their own local IP address as the ROS_MASTER_URI. I was under the impression that master_discovery simply makes the machines aware of one another, and only master_sync will cause collisions like this. Is there some argument that master_discovery needs like "ignore_hosts" or "ignore_nodes"? I only want the ground station to be aware of a few topics from each vehicle, but the fact that the vehicles have a large number of nodes with the same name prevents this. I realize I could namespace each vehicle so that the topics and nodes all have different names, but I was wondering if there was another solution with just arguments to the launch files I have here, or if I’m misunderstanding the default behaviour of master_discovery. Thanks!

[mattgiamou]

Ahhh thank you so much, that did the trick! I had read elsewhere that for single ROS master setups I should avoid setting ROS_MASTER_URI explicitly, but that made it become localhost. It works great now!

[sunil1349]

how can give my shared topics to sync node from launch file