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Can a node be a subscriber and client at the same time?

asked 2014-11-20 09:24:45 -0600

Clancy gravatar image

I figured out how to do this, but couldn't find any documentation anywhere so I figured I'd help out anyone that is trying to do the same thing.

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What do you mean with client? Are you mean that you want to call a server service? If so, yes you could do both, subscribe to regular topics and also call services from a server (being a client) in one node / nodelet.

Torsten gravatar imageTorsten ( 2014-11-20 10:11:38 -0600 )edit

I'll have to check out the nodelet solution. My answer will go through a couple more revisions, but I wanted to put up our immediate solution so that others can see a way of doing it. Thanks.

Clancy gravatar imageClancy ( 2014-11-20 11:00:48 -0600 )edit

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answered 2014-11-20 10:55:51 -0600

Clancy gravatar image

updated 2014-11-20 11:01:56 -0600

Yes. A node can be a subscriber and a client at the same time.

We needed a node that could subscribe to a topic and call a service request from another node at any given time. I couldn't find any solutions (probably because it is a relatively simple concept)... but I think there should be some documentation for other people stuck with the same problem.

The problem we were having: Most of the service request examples have the service request being called from inside the main function. When we tried to do this, we were only able to request a service once and then the node would be caught in a spin loop. To avoid this error, we decided to move the service request into the subscriber callback function. This way it would be called whenever the subscriber heard a message.

The basic idea: The client is instantiated in the main function, but it needs to be used in the callback function. I made a ros::ServiceClient pointer at a global level (w.r.t. the node) and then gave it the address of the client. In the callback function I dereference the client pointer and use it to request a service.

 void SUBSCRIBE_CALLBACK_FUNCTION (const PACKAGE_NAME::MESSAGE_NAME::ConstPtr& msg);
    ros::ServiceClient *clientPtr; //pointer for a client

    int main (int argc, char **argv)
    {

        ros::init(argc, argv, "MY_NODE");
        ros::NodeHandle n;
        ros::ServiceClient client = n.serviceClient<PACKAGE_NAME::SERVICE_NAME>("SERVICE_NAME"); //create the client

        clientPtr = &client; //give the address of the client to the clientPtr

        ros::Subscriber sub = n.subscribe<PACKAGE_NAME::MESSAGE_NAME>("TOPIC_NAME",1000,SUBSCRIBE_CALLBACK_FUNCTION); //subscribing

        ros::spin();

        return 0;

    }

    void SUBSCRIBE_CALLBACK_FUNCTION (const PACKAGE_NAME::MESSAGE_NAME::ConstPtr& msg)
    {
        PACKAGE_NAME::SERVICE_NAME srv;
        srv.request.someBool= false; //just some variables to give the service some structure.
        srv.request.someFloat = 4;
        srv.request.someFloat = 30;

        ros::ServiceClient client = (ros::ServiceClient)*clientPtr; //dereference the clientPtr

        if(client.call(srv)) //request service from the client
        {
            ROS_INFO("Success = %d", srv.response.success);
        }   
        else
        {
            ROS_ERROR("Failed to call service from motor_control_node");    
        }

    }
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At least accept your own answer ;)

AReimann gravatar imageAReimann ( 2014-11-23 19:36:21 -0600 )edit

Ha! Unfortunately I don't have enough points yet.

Clancy gravatar imageClancy ( 2014-11-23 23:16:58 -0600 )edit
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answered 2014-11-20 11:18:36 -0600

Torsten gravatar image

updated 2014-11-20 11:43:36 -0600

I think the answer to your primary question is clear, Yes they can have / be both!

There are several tutorials which describe the background, I dont think a all in one tutorial is needed.

Look at this tutorial for the ros main-loop behaviour: http://wiki.ros.org/ROS/Tutorials/Wri...

Look at this tutorial for the server-client stuff: http://wiki.ros.org/ROS/Tutorials/Wri...

If putting both together you will have a basic understanding to find the solution for your problem. Of course you can also use Nodelet's but they are more C++ object oriented style, so there you don't have a main-loop anymore, you can only use callbacks which includes timer callbacks desribed here:

http://wiki.ros.org/roscpp/Overview/T...

And now some notes regarding your code:

  1. Why dereference pointer, you can also do clientPtr->call(srv). You don't have to use a ptr at all.
  2. You don't have to call ros::spin(), you can also use ros::spinOnce(), so you will have your main loop to call the service.
  3. Make sure that you know when and how often you want to call the service. Everytime you got a incoming message (subscribed topic callback), every frame in your main loop (inside spinOnce main-loop), or maybe every x seconds (Timer callbacks). And this are only the synchronious ones.
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answered 2014-11-20 09:54:38 -0600

Erwan R. gravatar image

I don't really get your goal : to answer the question, yes, a node can subscribe to a topic and be client for a service.

If you want to share a working solution, please post it.

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Sorry for the ambiguity, hopefully the answer clears things up.

Clancy gravatar imageClancy ( 2014-11-20 10:56:25 -0600 )edit

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Asked: 2014-11-20 09:24:45 -0600

Seen: 1,858 times

Last updated: Nov 20 '14