is ROS nondeterministic ?

asked 2018-12-19 09:58:55 -0600

Hamza gravatar image

updated 2018-12-20 02:56:31 -0600

Hello Everyone,

I'm developing a code to control a follower rover aiming at keeping a reference distance with a leader rover. I 'm using ROS nodes that communicate using topics. I define a scenario Node which initiate the positions of the two rover for a specific period of time. After that I initiate the positions and the thing is i get different output for the same input. So I was wondering if ROS is nondeterministic or my code is wrong ?

In my case, I associate to each node a C++ class containing two methods init() and run. I have a Leader Node (uncontrolledRobot.cpp), a follower Node (controlledRobot.cpp) and a follower Controller Node along side with a scenario node. This latter determine the simulation duration and then order the two node (leader and follower to start from initial position).

//Start of user code User includes
//End of user code

//global variables
std_msgs::Float64 posX;
std_msgs::Float64 posY;
std_msgs::Bool startLeader;
std_msgs::Bool stopComputation;
std_msgs::Int32 trajectory;
//Callback function
void myCallbackLeader(const std_msgs::Bool& message_holder)
    ROS_INFO("received leader restart %d",;;

void myCallbacktrajectory(const std_msgs::Int32& message_holder)
    //check for data on topic "trajectory"
    ROS_INFO("trajectory value is:%f",;;

//main program
int main(int argc, char **argv)
    ros::init(argc,argv,"LeaderNode");//name this node
    ros::NodeHandle nh;//node handle

    //create subscribers
    ros::Subscriber my_subscriber_object1=nh.subscribe("startLeader",1,myCallbackLeader);
    ros::Subscriber my_subscriber_object2=nh.subscribe("trajectory",1,myCallbacktrajectory);
    //publish outputs;
    ros::Publisher my_publisher_object1=nh.advertise<std_msgs::Float64>("posXLeader",1);
    ros::Publisher my_publisher_object2=nh.advertise<std_msgs::Float64>("posYLeader",1);
    ros::Publisher my_publisher_object3=nh.advertise<std_msgs::Bool>("stopComputation",1);
    double sample_rate=1.0;//computing the corresponding update frequency
    ros::Rate naptime(sample_rate);//use to regulate loop rate;;
//Start of user code User declaration
    UncontrolledRobot theLeader;
//End of user code
    while (ros::ok())
//Start of user code User code
        if ( {;  
//End of user code

        ROS_INFO("posX command =%f",;
        ROS_INFO("posY command =%f",;

        ros::spinOnce();//allow data update from callback
        naptime.sleep();//wait for remainder of specified period
    return 0;//should never get here, unless roscore dies

scenario Node

//Variable declaration
std_msgs::Float64 Kv;
using namespace std;
std_msgs::Bool startLeader;
std_msgs::Bool startFollower;
std_msgs::Bool startObserver;
std_msgs::Bool endObserver;
std_msgs::Bool startComputeD;
int simulationDuration=3900;
std_msgs::Int32 trajectory;
//main program
int main(int argc, char **argv)
    ros::init(argc,argv,"scenarioNode");//name this node
    ros::NodeHandle nh; // node Handle

   //Publish  set point
    ros::Publisher my_publisher_object1=nh.advertise ...
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Can you describe in more detail how you are setting the positrons of your rovers. From you question or sounds like you're initializing them twice. Can tell us where the positions are stored and how you're updating and reading these positions.

PeteBlackerThe3rd gravatar image PeteBlackerThe3rd  ( 2018-12-19 11:18:00 -0600 )edit

I updated the question and putted some code i developed. thanks in advance for your help.

Hamza gravatar image Hamza  ( 2018-12-19 11:58:11 -0600 )edit

ROS does not guarantee message delivery and does not guarantee ordering between messages delivered on different topics. I cannot tell if either of these will affect your code or not.

ahendrix gravatar image ahendrix  ( 2018-12-19 15:29:16 -0600 )edit

I've had a quick look at your code, and I'm afraid it doesn't make any sense to me. You're using ROS in a very strange way that it was never intended to be used, and I'm honestly not surprised it isn't working. I recommend working through the turtle bot tutorials before continuing.

PeteBlackerThe3rd gravatar image PeteBlackerThe3rd  ( 2018-12-19 18:26:19 -0600 )edit

I've refrained from commenting myself as the code is indeed rather convoluted. Perhaps @Hamza can clarify a bit more what it is supposed to do.

Don't explain the code that's there. Describe what you're trying to achieve.

gvdhoorn gravatar image gvdhoorn  ( 2018-12-20 02:34:17 -0600 )edit

I tried to describe what i want to do above. I hope it is clear. Thanks again for your help.

Hamza gravatar image Hamza  ( 2018-12-20 02:58:53 -0600 )edit