Choose a topic, service or action

What you will understand

• Choose the right interface.
• Call a simulation service.
• Read an action contract.

Prerequisites: Publish and subscribe with meaningful messages and its stated environment.

The idea, made clear.

Topics suit ongoing data streams. Services suit bounded requests with one response. Actions suit longer tasks that need progress and cancellation, such as navigation. Do not substitute a blocking service for a mission that needs supervision.

An interface is a contract. Service definitions separate request and response with ---. Action definitions have goal, result and feedback sections. Document units and ranges. Custom ROS interfaces need generation metadata and rosidl dependencies in an appropriate package.

Clients must handle unavailability and timeouts. Executors and callbacks affect responsiveness; merely choosing an action interface does not implement cancellation in the server. Inspect actual behavior and failure responses.

Try it, step by step.

ros2 run turtlesim turtlesim_node
# Other terminal:
ros2 service list
ros2 interface show turtlesim/srv/Spawn

ros2 service call /spawn turtlesim/srv/Spawn "{x: 3.0, y: 3.0, theta: 0.0, name: learner}"

float64 distance_m
---
bool reached
---
float64 remaining_m

If something goes wrong

Service waits

Check that the server is running and the name matches.

Action does not cancel

Inspect server cancellation handling.

Check your understanding

Make it yours

Classify a laser stream, reset request and docking task.

Go to the source documentation

• ROS 2 Jazzy documentation ↗

Commands are educational examples for the stated environment, not a transcript of local ROS execution. Verify actual behavior on your machine.