Services and Actions

Beyond Publish-Subscribe

Topics handle continuous data streams well — sensor data, state updates, command velocities. But some interactions need a request-response pattern:

• "Compute a navigation path from A to B" (takes time, returns a result)
• "Calibrate the IMU" (one-shot operation)
• "Pick up the object at position (x, y, z)" (long-running with progress feedback)

ROS 2 provides two mechanisms for this: Services and Actions.

Services

A service is a synchronous request-response call. The client sends a request and blocks until the server responds.

from example_interfaces.srv import AddTwoInts
import rclpy
from rclpy.node import Node

class MinimalService(Node):
    def __init__(self):
        super().__init__('minimal_service')
        self.srv = self.create_service(
            AddTwoInts,
            'add_two_ints',
            self.add_callback
        )

    def add_callback(self, request, response):
        response.sum = request.a + request.b
        self.get_logger().info(f'{request.a} + {request.b} = {response.sum}')
        return response

Use services when the operation is fast and the caller can wait.

Actions

An action is an asynchronous goal-based interface. The client sends a goal, receives periodic feedback, and eventually gets a result. Actions can be cancelled mid-execution.

Client sends goal → Server accepts → Server sends feedback (0%... 50%... 90%) → Server sends result

Actions are the right choice for long-running robot tasks: navigation, manipulation, and perception pipelines.

When to Use What

Pattern	Use Case	Blocking?	Feedback?
Topic	Continuous data streams	No	N/A
Service	Fast request-response	Yes	No
Action	Long-running tasks	No	Yes

Next Steps

With nodes, topics, services, and actions, you have the full ROS 2 communication toolkit. Next, we model the robot itself using URDF — the Unified Robot Description Format.