ROS Session

By - Mookesh Dash

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ROS programmes work in the form of nodes

Now what are nodes?

Let's understand it with a simple example...

• Sachin and Carryminati are two users signed in to Youtube.
• Sachin wants to receive Carryminati's videos in his feed and therefore, he subscribes to Carryminati.
• Now Carryminati publishes the video so that Sachin, who has subscribed to him can watch it.
• Therefore,
  • Sachin is a subscriber,
  • Carryminati is a publisher , and,
  • YouTube is the medium which allows them to communicate.

Now coming to ROS terms,

• rosnode - Sachin & Carryminati. The publishers and subscribers. They are programmes running inside ROS.
• roscore - YouTube. The medium that allows nodes to communicate. We can call it the master node which every node registers itself to.
• rostopic - Vines. The topic on which two nodes communicate.
• rosmsg - Video or '.mp4'. The type of message communicated between the nodes.

Enough talk! Let's see a real pubsub programme...

First we have to initialize the roscore, so that nodes can communicate.

$ roscore

Go into your catkin_ws, scripts folder and create a new package.

$ cd ~/catkin_ws/src
$ catkin_create_pkg pubsub std_msgs rospy roscpp

catkin_create_pkg creates a package named pubsub that depends on std_msgs, rospy and roscpp.

Then, we have to build the packages in the catkin_ws and source the generated setup file to add our workspace to the ROS environment.

$ cd ~/catkin_ws
$ catkin_make
$ source devel/setup.bash

Make a script folder, (I have named it src) inside your pubsub package and move into it. Create two python files named carry.py and sachin.py

$ cd src/pubsub/
$ cd src/
$ touch carry.py sachin.py

Open your carry.py file in a text editor, and paste the following code:-

#!/usr/bin/python3

import rospy # Importing the ROS client library for python
from std_msgs.msg import String # Importing the String message type

def publish_video():
    pub = rospy.Publisher('vines', String, queue_size=1) # Subscribing to a topic
    rospy.init_node('carryminati', anonymous=True) # Intitializing the node
    rate = rospy.Rate(1) # Frequency of sent messages. Here 1 means 1Hz
    iteration = 1
    while not rospy.is_shutdown(): 
        video = f"Carry's video number {iteration}"
        pub.publish(video) # Publishing the message
        print(video)
        iteration += 1
        rate.sleep()

if __name__ == '__main__':
    try:
        publish_video()
    except rospy.ROSInterruptException:
        pass

Next open your sachin.py file in a text editor, and paste the following code:-

#!/usr/bin/python3

import rospy # Importing the ROS client library for python
from std_msgs.msg import String # Importing the String message type

def callback(data): 
    # Declaring a callback function. data is the message we receive from the publisher
    print(f"I saw {data.data}") # Printing it to console
    
def viewer():
    rospy.init_node('sachin', anonymous=True) # Intitializing the node
    rospy.Subscriber('vines', String, callback) # Subscribing to a topic
    rospy.spin() # keeps your node from exiting until the node has been shutdown

if __name__ == '__main__':
    viewer()

Save both the files, and in your bash make both the files executable by running:-

$ chmod +x carry.py sachin.py

Now go back to your catkin_ws and build your packages again.

$ cd ~/catkin_ws && catkin_make && source devel/setup.bash

Next, run your scripts using

$ rosrun pubsub carry.py
$ rosrun pubsub sachin.py

Our nodes are up and running!

Let's inspect our nodes and topics...

$ rosnode list
$ rostopic list

There's a nice way to check how the nodes and topics are working in a graphical way

$ rosrun rqt_graph rqt_graph

And indeed! carryminati and sachin are our two nodes, and the topic they are interacting with each other on is about vines.

That's it for today's sesh. I hope y'all have understood how nodes interact with each other via topics and if any doubts at all feel free to contact me anytime!