Publish-subscribe communication model in ROS - Step-by-Step Execution

Concept Flow - Publish-subscribe communication model

Publisher Node

↓

Subscriber Node(s)

↓

Process Message

The publisher sends messages to the ROS Master, which then forwards them to all subscriber nodes that have registered interest.

Execution Sample

ROS

import rospy
from std_msgs.msg import String

def callback(msg):
    rospy.loginfo(f"Received: {msg.data}")

rospy.init_node('listener')
rospy.Subscriber('chatter', String, callback)
rospy.spin()

This subscriber node listens to the 'chatter' topic and prints received messages.

Execution Table

Step	Action	Publisher State	ROS Master State	Subscriber State	Message Flow
1	Publisher node starts and initializes	Ready to publish	Idle	Idle	No messages
2	Subscriber node starts and subscribes to 'chatter'	Ready to publish	Registers subscriber for 'chatter'	Subscribed to 'chatter'	No messages
3	Publisher publishes message 'Hello'	Message 'Hello' sent	Receives message 'Hello' on 'chatter'	Waiting for message	Message queued for subscribers
4	ROS Master forwards message to subscriber	Message sent	Forwards 'Hello' to subscriber	Receives message 'Hello'	Message delivered
5	Subscriber callback processes message	Idle	Idle	Processes and logs 'Received: Hello'	Message processed
6	No new messages	Idle	Idle	Idle	No messages
Exit	No more messages to publish	Publisher stops or waits	No messages to forward	Subscriber waits	Execution stops

💡 Publisher stops sending messages; system waits idle.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
Publisher State	Not started	Ready to publish	Message 'Hello' sent	Message sent	Idle	Idle
ROS Master State	Idle	Registers subscriber	Receives message	Forwards message	Idle	Idle
Subscriber State	Idle	Subscribed	Waiting for message	Receives message	Processes message	Idle
Message Flow	No messages	No messages	Message queued	Message delivered	Message processed	No messages

Key Moments - 3 Insights

Why does the subscriber receive messages only after subscribing?

Because in the execution_table at Step 2, the subscriber registers interest with the ROS Master. Before that, the ROS Master does not forward messages to it.

What role does the ROS Master play in message delivery?

As shown in Steps 3 and 4, the ROS Master acts as a broker that receives messages from publishers and forwards them to all subscribers registered for that topic.

Why does the subscriber process messages asynchronously?

In Step 5, the subscriber processes messages via a callback function triggered when a message arrives, allowing it to handle messages as they come without blocking.

Visual Quiz - 3 Questions

Look at the execution_table, what is the ROS Master state at Step 3?

A Registers subscriber

B Idle

C Receives message 'Hello' on 'chatter'

D Forwards 'Hello' to subscriber

💡 Hint

Check the 'ROS Master State' column in execution_table row for Step 3.

At which step does the subscriber first receive the message?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Subscriber State' and 'Message Flow' columns in execution_table.

If the subscriber never subscribes, what happens to the messages?

A Messages are not forwarded to subscriber

B ROS Master forwards messages anyway

C Messages are queued for subscriber

D Publisher stops publishing

💡 Hint

Refer to Step 2 and Step 4 in execution_table about subscriber registration.

Concept Snapshot

Publish-subscribe model in ROS:
- Publisher sends messages to a topic via ROS Master
- Subscribers register interest to topics with ROS Master
- ROS Master forwards messages from publishers to all subscribers
- Subscribers receive messages asynchronously via callbacks
- Decouples sender and receiver for flexible communication

Full Transcript

In the ROS publish-subscribe communication model, a publisher node sends messages to a topic registered with the ROS Master. Subscriber nodes register their interest in topics with the ROS Master. When a publisher sends a message, the ROS Master receives it and forwards it to all subscribers of that topic. Subscribers process messages asynchronously using callback functions. This model allows nodes to communicate without knowing about each other directly, enabling flexible and scalable message passing.

Practice

1. What is the main role of a publisher in the ROS publish-subscribe model?

easy

A. To create new topics dynamically

B. To receive messages from a topic

C. To store messages in a database

D. To send messages to a topic

Solution

1. Step 1: Understand the publish-subscribe roles

   In ROS, publishers send messages to topics, while subscribers receive messages from topics.

2. Step 2: Identify the publisher's function

   The publisher's job is to send data to a topic so subscribers can receive it.

3. Final Answer:

   To send messages to a topic -> Option D

4. Quick Check:

   Publisher = sends messages [OK]

Hint: Publisher sends, subscriber receives messages [OK]

Common Mistakes:

• Confusing publisher with subscriber
• Thinking publisher stores messages
• Assuming publisher creates topics

2. Which of the following is the correct way to create a ROS publisher in Python?

easy

A. pub = rospy.Publisher('topic_name', callback, queue_size=10)

B. pub = rospy.Subscriber('topic_name', String, callback)

C. pub = rospy.Publisher('topic_name', String, queue_size=10)

D. pub = rospy.Subscriber('topic_name', String)

Solution

1. Step 1: Recall publisher syntax in ROS Python

   The correct syntax uses rospy.Publisher with topic name, message type, and queue size.

2. Step 2: Check each option

   pub = rospy.Publisher('topic_name', String, queue_size=10) matches the correct syntax. Using Subscriber instead of Publisher is incorrect. pub = rospy.Publisher('topic_name', callback, queue_size=10) incorrectly uses callback instead of message type.

3. Final Answer:

   pub = rospy.Publisher('topic_name', String, queue_size=10) -> Option C

4. Quick Check:

   Publisher syntax = rospy.Publisher(topic, type, queue_size) [OK]

Hint: Publisher uses rospy.Publisher(topic, type, queue_size) [OK]

Common Mistakes:

• Using Subscriber instead of Publisher
• Passing callback instead of message type
• Omitting queue_size parameter

3. Given the following ROS Python code snippet, what will be printed when a message with data 'Hello' is published?

import rospy
from std_msgs.msg import String

def callback(msg):
    print(f'Received: {msg.data}')

rospy.init_node('listener')
sub = rospy.Subscriber('chatter', String, callback)
rospy.spin()

medium

A. Received: Hello

B. Hello

C. Received: msg

D. No output

Solution

1. Step 1: Understand the callback function

   The callback prints 'Received: ' followed by the message's data field.

2. Step 2: Analyze the message content

   The message data is 'Hello', so the print statement outputs 'Received: Hello'.

3. Final Answer:

   Received: Hello -> Option A

4. Quick Check:

   Callback prints 'Received: ' + msg.data [OK]

Hint: Callback prints 'Received: ' plus message data [OK]

Common Mistakes:

• Printing message object instead of data
• Expecting just the message data without prefix
• Thinking no output occurs without explicit print

4. Identify the error in this ROS Python subscriber code:

import rospy
from std_msgs.msg import String

def callback(msg):
    print(msg.data)

rospy.init_node('listener')
sub = rospy.Subscriber('chatter', String)
rospy.spin()

medium

A. Incorrect message type used

B. Missing callback function in Subscriber

C. Node not initialized

D. No import for rospy

Solution

1. Step 1: Review Subscriber syntax

   Subscriber requires three arguments: topic name, message type, and callback function.

2. Step 2: Check the given code

   The Subscriber call misses the callback argument, causing an error.

3. Final Answer:

   Missing callback function in Subscriber -> Option B

4. Quick Check:

   Subscriber needs callback argument [OK]

Hint: Subscriber must include callback function [OK]

Common Mistakes:

• Omitting callback in Subscriber
• Confusing Publisher and Subscriber arguments
• Forgetting to initialize node

5. You want to create a ROS system where multiple nodes publish sensor data to the same topic, and one node subscribes to process all data. Which feature of the publish-subscribe model makes this possible?

hard

A. Topics allow multiple publishers and subscribers to communicate independently

B. Each topic can have only one publisher and one subscriber

C. Publishers must know all subscribers to send messages

D. Subscribers can only listen to one topic at a time

Solution

1. Step 1: Understand topic communication in ROS

   Topics support multiple publishers and subscribers simultaneously, enabling flexible communication.

2. Step 2: Analyze the scenario

   Multiple nodes publishing to the same topic and one subscriber processing all data is possible because topics allow many publishers and subscribers.

3. Final Answer:

   Topics allow multiple publishers and subscribers to communicate independently -> Option A

4. Quick Check:

   Topics support many-to-many communication [OK]

Hint: Topics support many publishers and subscribers [OK]

Common Mistakes:

• Thinking topics allow only one publisher or subscriber
• Believing publishers must know subscribers
• Assuming subscribers can listen to only one topic