ROS distributed operation -- launch file starts nodes on multiple machines

ROS Distributed operations –launch File starts nodes on multiple machines

Write it at the front ： When we need to start multiple nodes , Will be used $lanch$ File to batch start , At the same time, it can be in $launch$ Set and load parameters in the file , Avoid the trouble of starting nodes one by one , But when multiple nodes are distributed on different hosts , You still need to start the nodes on the corresponding host one by one . Actually $launch$ The file also provides the function of starting the host in batch , After all $ROS$ It's a distributed system in itself , This function is not unexpected .

About $launch$ How to use the document , I won't go into details here , Mainly record today's use $launch$ File starts the operation of nodes in multiple hosts in the network .

machine label

$launch$ There is one in the label of the file $machine$ label , The function is to identify the host under the same network , Corresponding ,node There is one in the label $machine$ attribute , Used to set the host of node operation . adopt $machine$ Property settings , It can be opened remotely and automatically on other hosts $launch$ process , And let the node run on the specified host .ros In the official documents, the machine label Description of the property , The common ones are as follows ：

• name: Host name ,node The label specifies the host with this name
• address: Set up the IP Address , have access to IP Address 、HosrName( namely /etc/hosts Set in the ip Address mapping ) Or domain name
• env-loader: Used to specify the configuration of environment variables on the remote host shell Script , The script must have the configuration of all environment variables required by the runtime , And with the last line exec "[email protected]" ending . This attribute is somewhat complicated , Configure it to bypass the road , I'll tell you more later .
• user: The user name to use on the remote host
• password: The login password of the corresponding user

In this way, we can node The label specifies machine Tag to start the remote host

eg:

<launch>
  <!-- here env.bash The file name of the file can be specified by yourself -->
  <machine name="foo" address="foo-address" user="someone" env-loader="/home/user_name/path/env.bash">
    <env name="LUCKY_NUMBER" value="13" />
  </machine>
  <!-- adopt machine Attribute assignment , It can make talker The node runs on the host foo On -->
  <node machine="foo" name="footalker" pkg="test_ros" type="talker.py" />

</launch>

matters needing attention

If nothing happens , In fact, it's over here , The above configuration can achieve our goal , But the reality is cruel , Problem stratification , In general, it took more than half a day to solve .

problem 1： function roslaunch An error occurred during startup ,ssh Can't connect

The first thing to say is this ,roslaunch Start this function remotely , In fact, with the help of ssh To achieve , But it uses ssh The client is different from what we usually use , add ros Some condition checks will be carried out , Any slight difference in the pool will lead to startup failure .

• The most difficult problem to solve is that the most direct error message cannot be connected , The log indicates that the reason is that the target host failed to known_hosts Found... In the file

  Commonly used ssh We should all know ,ssh When connecting a host for the first time, it will connect the other host xxxx Add to known_hosts In file , It's strange to have such an error warning . The reason is the above ,ros Used ssh The client is bored , It can only recognize rsa Algorithm generated hostkey, So before using ssh And what is saved is for ros It is invalid .

The solution is simple , Just regenerate . The specific way is ：

1. Delete the original known_hosts file ,rm ~/.ssh/known_hosts
2. Reuse ssh Connect to the target host once , And designate HostKey Generation algorithm of ,ssh [email protected]_address -oHostKeyAlgorithms=“ssh-rsa”
3. The prompt will add hostKey when , Input yes Just go back and
4. At this time, you will be prompted to enter some parameters , It doesn't matter , Just return all the way , After connecting, the goal is achieved , At this time, you can launch ssh Connected to .
5. Run it again roslaunch You can see ssh The connection can be successfully created

• Secondly, there is a small problem , It has something to do with the above problems .

  Specifically , When the previous problem is solved , Sometimes an error is reported xxx The host cannot ping Through this kind of error reporting . The reason is that machine It seems that hostname To specify the address attribute ,ros Check the smoothness of the network before remote startup , One of the operations is ping Initiator host and itself , And this one ping The address parameter used in the operation is address Value .

  This is the time , If hosts It is not done well in the document ip and hostname The mapping of ,ros There is no way to pass the designated hostname To find your address , This will lead to failure ping Through your own mistakes .

So master and slave hosts Files need to be mapped accordingly , The two sides of the /etc/hosts Two statements should be appended to the file ：

master_ip master_hostname
slave_ip slave_hostname

problem 2 ：env-loader designated bash The script doesn't exist ( I just can't find )

After successful connection , Can't find env-loader designated bash Script , Report an error and say it doesn't exist

Originally, according to the description of documents and tutorials , We just need to write our own shell The full path of the script is given to env-loader Attribute is enough ,

But after practice, it is found that only /opt/ros/melodic/setup.bash, This error will appear for other values .

Checked the documents and communities for a long time , The reason for this problem has not been found , Even many other successful bloggers haven't mentioned this problem at all . It's troublesome to find the cause of the problem and solve it . In order not to delay time , I can only bypass this problem , Turn to other solutions to solve this problem .

It's also simple , since /opt/ros/melodic/setup.bash It can be used legally , Then we will configure it ourselves shell Just include the script . open /opt/ros/melodic/setup.bash, Add a line at the end ：

# shell in source Statement equivalent C/C++ Medium include sentence 
source your_script_path

In this way , The user-defined script is indirectly executed and the environment variables are set , Thus bypassing env-loader The problem of not finding the file .

If a big man reads here and knows the direct solution to this problem , Please also comment , Thank you very much ！

Reference resources ：http://answers.ros.org/question/244060/roslaunch-ssh-known_host-errors-cannot-launch-remote-nodes/?answer=244064#post-id-244064

https://blog.csdn.net/heroacool/article/details/75228749