ROS2 Services - Basic Service Server

🧠 Concept: Services (Request → Response)

Topics are fire-and-forget — the publisher doesn't know if anyone received the message. Services are different: they're synchronous request/response calls, like a function call between nodes.

[client node] ──request──▶ /add_two_ints ──response──▶ [client node]
                                ▲
                         [server node handles it]

Key API (server side):

self.srv = self.create_service(ServiceType, '/service_name', self.callback)

def callback(self, request, response):
    response.sum = request.a + request.b
    return response  # must return the response object

When to use services vs topics:

• Topics: continuous data streams (sensor readings, robot state)
• Services: one-time requests that need a response (compute path, take photo, open gripper)

Problem Statement

Services provide a synchronous Request/Response communication pattern. Create a service server that adds two integers.

Requirements

• Create a node named add_two_ints_server
• Create a service named add_two_ints
• Service Type: example_interfaces/AddTwoInts
• Callback: Sums request.a and request.b into response.sum
• Log "Incoming request: a=[a] b=[b]" inside the callback

Input/Output Format

Input:

• Service Request: a=2, b=3

Output:

• Service Response: sum=5
• Console Log: "Incoming request: a=2 b=3"

⚠️ Common Pitfalls

• Returning None instead of response.
• Forgetting to import the service type definition.

📚 Helpful Resources

• ROS2 Services Tutorial