3. ROS Concepts: Understanding Topics, Nodes, Messages, and Services

Introduction

ROS (Robot Operating System) is a powerful framework for building robotic systems. In order to effectively work with ROS, it’s important to understand some key concepts that are central to the system. This includes topics, nodes, messages, and services.

Topics

A topic can be thought of as a bulletin board. Just as a person can post an article on a bulletin board for others to read, a node can publish data to a topic for other nodes to receive. Each topic has a specific name, and nodes can subscribe to a topic to receive the data that is being published to it. Topics are used for a variety of purposes, such as sending sensor data, sending control commands, and more.

To see a list of active topics in the system, you can use the command:

$ rostopic list

Make sure to run roscore beforehand. To see more detailed information about a specific topic, you can use the command:

$ rostopic info <topic_name>

Nodes

A node can be thought of as a person. Just as a person can post an article on a bulletin board or read an article on a bulletin board, a node can publish data to a topic or subscribe to a topic to receive data. Each node performs a specific function, such as processing sensor data or sending control commands. Nodes communicate with each other through topics and services.

To see a list of currently running nodes, you can use the command:

$ rosnode list

To see more detailed information about a specific node, you can use the command:

$ rosnode info <node_name>

Messages

A message can be thought of as the article that is posted on the bulletin board. Just as an article has a specific format, such as a title, author, and content, a message also has a specific format. The format of a message can include various data types, such as integers, floating point numbers, and strings.

To see a list of available message types in the system, you can use the command:

$ rosmsg list

To see more detailed information about a specific message type, you can use the command:

$ rosmsg show <message_type>

Services

A service is a way for nodes to request a specific action or receive a specific piece of information from another node, using a specific request-response format defined by a service definition file.

To see a list of available services in the system, you can use the command:

$ rosservice list

To see more detailed information about a specific service, you can use the command:

$ rosservice info <service_name>

Conclusion

In conclusion, understanding the concepts of topics, nodes, messages, and services is essential for working effectively with ROS. By understanding these concepts, you can more easily navigate and work within the ROS ecosystem. The analogy of a person and a bulletin board can help to illustrate the relationships between topics and nodes, and make the concepts more relatable and intuitive.

Check out my next article about creating a publisher and a subscriber HERE.