Nodes, Messages, and Topics

Opening challenge: A camera publishes obstacles, but the safety monitor receives nothing. Is the camera broken—or is the graph disconnected?

The mental model

A node is a running ROS 2 participant with a focused responsibility. A camera node might detect obstacles; a safety node might watch them; a navigation node might plan around them. Nodes are not required to know each other's implementation.

A message is a strongly typed data sample. Its fields and their meanings form a contract. A topic is a named asynchronous stream. Publishers write messages to the stream; any number of compatible subscribers may receive them.

Trace one evidence path

Assume /camera_node publishes ObstacleArray messages on /camera/obstacles. Both /safety_monitor and /navigation_node subscribe:

1. The camera creates one ObstacleArray sample.
2. It publishes the sample to /camera/obstacles.
3. ROS 2 discovers compatible endpoints.
4. Each compatible subscriber receives the stream independently.
5. A recorder or debugger can join as another subscriber without changing the camera.

The endpoints need the same topic name and a compatible message type. Quality-of-service settings can also affect compatibility; this lesson keeps QoS at defaults and returns to delivery policy later.

Guided inspection

Use a ROS 2 environment with turtlesim, or study the sample output below:

ros2 node list
ros2 node info /turtlesim
ros2 topic list -t
ros2 topic info /turtle1/cmd_vel
ros2 interface show geometry_msgs/msg/Twist
ros2 topic echo /turtle1/pose

For each command, ask: Which node? Which topic name? Which message type? Which direction? ros2 topic echo behaves as an additional subscriber; it does not interrupt the existing flow.

Diagnose the silent subscriber

Misconception checks

• “A topic stores messages like a mailbox.” Not necessarily. Treat it as a live named stream; durability depends on QoS.
• “One topic connects exactly two nodes.” A topic may have zero or more publishers and subscribers.
• “A topic name alone defines the data.” The message type and semantics are part of the contract.
• “ROS 2 is the robot's desktop operating system.” ROS 2 is middleware, libraries, tools, and conventions for robot software.

Retrieval pause

Without looking back, explain why a recorder can subscribe to a sensor topic without modifying the sensor node. Then name three checks for a subscriber that receives no data.

Summary

Nodes divide responsibilities. Messages define typed samples. Topics provide named asynchronous streams. Diagnose the graph from evidence—node, topic name, type, direction, and delivery compatibility—rather than guessing which program is broken.

Further learning

• ROS 2 Topics, Open Robotics, retrieved 2026-08-19. Supports publish/subscribe, anonymous endpoints, strong typing, and recorder-as-subscriber claims; link-and-cite use.
• ROS 2 Interfaces, Open Robotics, retrieved 2026-08-19. Supports topics as asynchronous continuous streams; link-and-cite use.
• Understanding ROS 2 Nodes, Open Robotics, retrieved 2026-08-19. Supports focused node responsibility and graph inspection commands; link-and-cite use.