Inspect Topic Endpoints

echo answers "what messages are arriving?"

info answers "who is connected?"

Run:

ros2 topic info /turtle1/cmd_vel

The beginner tutorial reports:

• topic type;
• publisher count;
• subscription count.

For more detail:

ros2 topic info /turtle1/cmd_vel --verbose

The verbose form can show endpoint node names, namespaces, endpoint type, and QoS profile information.

Stay at beginner depth

You will see QoS fields such as reliability, durability, and history.

For this course, recognize them as communication policy metadata. Do not detour into advanced QoS tuning.

The questions you need to answer are simpler:

• Which node publishes?
• Which node subscribes?
• What message type must they agree on?
• How many endpoints are currently present?

Add an observer

While ros2 topic echo /turtle1/cmd_vel is running, execute ros2 topic info again.

Did the subscriber count change?

Explain why.

Checkpoint

Prove one edge from Unit 3 using only ros2 topic info --verbose evidence.