Using roslaunch to run distributed systems over ssh

edit

I'm just trying to make my distributed ROS system run with one single command for the first time. I went through roslaunch/Architecture and roslaunch tips for larger projects which helped me a lot, but not enough.

My system is disributed over 2 machines, say Alpha and Beta, the second being localhost (authorized_keys defined, hostnames checked, communication works flawlessly). So my launchfile looks like this:

<launch>
  <machine name="alpha" address="alpha" user="alpha_user" 
    ros-root="/correct/path/on/alpha/machine" 
    ros-package-path="/very/true/package/path" default="true" />
  <machine name="beta" address="beta" user="beta_user" 
    ros-root="$(env ROS_ROOT)" 
    ros-package-path="$(env ROS_PACKAGE_PATH)" default="false" />

  <node pkg="hokuyo_node" type="hokuyo_node" name="hokuyo_node" output="screen">
    <param name="frame_id" value="/laser"/>
    <param name="port" value="/dev/ttyACM0"/>
  </node>

  <node pkg="laser_scan_matcher" type="laser_scan_matcher_node"
    name="laser_scan_matcher_node" output="screen">
  </node>

  <node pkg="tf" type="static_transform_publisher" name="laser_link" 
    args="0.0 0.0 0.10 0.0 0.0 0.0 /base_link /laser 40" />

  <node pkg="gmapping" type="slam_gmapping" name="gmapping" 
    output="screen" machine="beta">
    <param name="odom_frame" value="/world" />
  </node>
</launch>

Running this on beta yields:

started roslaunch server http://beta:59751/
remote[alpha-0] starting roslaunch
remote[alpha-0]: creating ssh connection to alpha:22, user[alpha_user]
remote[alpha-0]: ssh connection created

SUMMARY
========

PARAMETERS
 * /gmapping/odom_frame
 * /rosdistro
 * /laser_scan_matcher_node/max_iterations
 * /laser_scan_matcher_node/use_odom
 * /hokuyo_node/max_ang
 * /rosversion
 * /hokuyo_node/port
 * /laser_scan_matcher_node/use_imu
 * /laser_scan_matcher_node/fixed_frame
 * /hokuyo_node/frame_id
 * /laser_scan_matcher_node/use_alpha_beta
 * /hokuyo_node/min_ang

MACHINES
 * beta
 * alpha

NODES
  /
    hokuyo_node (hokuyo_node/hokuyo_node)
    laser_scan_matcher_node (laser_scan_matcher/laser_scan_matcher_node)
    laser_link (tf/static_transform_publisher)
    gmapping (gmapping/slam_gmapping)

auto-starting new master
process[master]: started with pid [3002]
ROS_MASTER_URI=http://localhost:11311

setting /run_id to 3bf5a6de-307c-11e1-ba58-00216a2f5558
process[rosout-1]: started with pid [3017]
started core service [/rosout]
process[gmapping-2]: started with pid [3020]
[alpha-0]: launching nodes...
[alpha-0]: auto-starting new master
[alpha-0]: process[master]: started with pid [1433]
[alpha-0]: ROS_MASTER_URI=http://localhost:11311
[alpha-0]: setting /run_id to 3bf5a6de-307c-11e1-ba58-00216a2f5558
[alpha-0]: process[hokuyo_node-1]: started with pid [1452]
[alpha-0]: process[laser_scan_matcher_node-2]: started with pid [1453]
[alpha-0]: process[laser_link-3]: started with pid [1454]
[alpha-0]: ... done launching nodes

I think there are two roscore's being spawned which don't know about each other. I'm guessing it, because running rosgraph locally after changing:

export ROS_MASTER_URI=http://alpha:11311
export ROS_MASTER_URI=http://beta:11311

returns only nodes run on the particular computer, and because of the suspicious lines above:

[alpha-0]: launching nodes...
[alpha-0]: auto-starting new master
[alpha-0]: process[master]: started with pid [1433]
[alpha-0]: ROS_MASTER_URI=http://localhost:11311

Testing what processes are really being spawned on both machines with:

ps aux | grep ros

yields: ps_alpha ps_beta.

Where do I tell ROS to only spawn one roscore (preferably on the remote machine, i.e. beta)? Or how do I else configure the system correctly?