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rqt_graph

rqt_graph is a graphical tool for visualizing the ROS 2 computation graph. It helps you see which nodes are running, which topics they publish or subscribe to, and how those connections are related.

Overview​

ROS 2 systems are composed of nodes that communicate through topics, services, and actions. While command-line tools such as ros2 node list and ros2 topic info show this information textually, rqt_graph provides a visual overview of the node/topic graph.

It is especially useful when:

  • checking whether expected nodes are running
  • confirming publisher/subscriber connections
  • understanding namespaces and remapped topic names
  • identifying unexpected or missing connections
  • explaining the structure of a ROS 2 system during development or debugging

Why It Matters​

As a ROS 2 system grows, it becomes difficult to understand the communication structure from terminal output alone.

For example, the Publisher/Subscriber tutorial creates this relationship:

simple_publisher
      |
      | /tutorial_chatter
      v
simple_subscriber

rqt_graph lets you inspect this relationship visually and verify that the nodes and topic are present in the ROS 2 graph.

See Publisher/Subscriber for the example used below.

Core Concepts​

ROS 2 Graph​

The ROS 2 graph describes the currently discovered communication entities in the system, including:

  • nodes
  • topics
  • publishers
  • subscriptions
  • services
  • actions

rqt_graph focuses primarily on visualizing node and topic relationships.

Graph Visibility Does Not Guarantee Data Flow​

Seeing a publisher and subscriber in rqt_graph confirms that ROS 2 has discovered the graph entities, but it does not guarantee that messages are actually being delivered.

For example, communication can still fail because of:

  • incompatible QoS settings
  • type mismatches
  • timestamp or TF problems in downstream processing
  • application-level logic

Use rqt_graph together with command-line inspection tools when debugging.

Hands-on Steps​

1. Start the Publisher and Subscriber​

Use the nodes created in Publisher/Subscriber.

In one sourced terminal:

ros2 run my_ros2_tutorial simple_publisher

In a second sourced terminal:

ros2 run my_ros2_tutorial simple_subscriber

2. Start rqt_graph​

In a third sourced terminal:

ros2 run rqt_graph rqt_graph

3. Inspect the Graph​

You should see the publisher and subscriber connected through the /tutorial_chatter topic.

Depending on the selected view options, ROS 2 system topics or internal graph entities may also be shown.

Use the available filtering options to simplify the graph when many nodes are running.

Expected Result​

For the Publisher/Subscriber example, the graph should show a relationship similar to:

/simple_publisher
       |
       | /tutorial_chatter
       v
/simple_subscriber

This confirms that both nodes and the topic have been discovered in the ROS 2 graph.

To verify actual communication, also check:

ros2 topic echo /tutorial_chatter

or:

ros2 topic info /tutorial_chatter -v

Useful Commands​

# Start rqt_graph
ros2 run rqt_graph rqt_graph

# List discovered nodes
ros2 node list

# List topics and their types
ros2 topic list -t

# Inspect publishers, subscribers, and QoS
ros2 topic info <topic_name> -v

# Inspect one node
ros2 node info <node_name>

Common Problems​

  • Expected node does not appear — confirm that the node is actually running and that the terminal running rqt_graph uses the same ROS 2 environment and ROS_DOMAIN_ID.
  • Expected topic connection does not appear — verify the fully resolved topic name and namespace with ros2 topic list and inspect the node with ros2 node info.
  • The graph is difficult to read — use the filtering and view options in rqt_graph to hide unnecessary system topics or nodes.
  • The connection appears but no messages are received — rqt_graph shows graph discovery, not guaranteed message delivery. Check message type and QoS compatibility with ros2 topic info <topic_name> -v. See Troubleshooting.
  • rqt_graph does not start — confirm that the package is installed and that the ROS 2 environment is sourced.

Key Takeaways​

  • rqt_graph visualizes the ROS 2 node/topic graph.
  • It is useful for understanding communication structure and discovering missing or unexpected connections.
  • A visible graph connection does not by itself prove that messages are being delivered successfully.
  • Combine rqt_graph with ros2 node, ros2 topic, and troubleshooting tools for deeper diagnosis.

Next​

Continue to rqt_console to inspect and filter ROS 2 log messages, or return to Troubleshooting when a graph connection is missing or communication is not working as expected.