Call-and-response model

Services only provide data when they are specifically called by a client.

Features of ROS2 Services

1. Client/Server system: The server (node) responds to the client (node) only when the client requests for any data.
   1. Synchronous or Asynchronous
   2. One message type for Request, one message type for Response
2. Can be written in Python, C++, … directly inside ROS nodes
3. A service server can only exist once, but can have many clients.

So, the question is, when to use Topics, and when to use Services?

Services offer nice complementarity with Topics. Topics will be used for unidirectional data streams, and Services will be used when you need a client-server architecture.

Illustration

Now let’s suppose we want the robot to do something specific. Something that’s synchronous and guaranteed. That’s a service call.

Similar to the other ROS commands we’ve used, there’s a ros2 service list command to show which services are available. The following services are available for the turtlesim:

Calling a service in ROS 2 requires three arguments: the service, the service type and the message data to send to the service. After retrieving the list of service names, the ros2 service type [service] command shows the service type. By examining the turtlesim \\spawn that’s used to add a new turtle to the simulator, we find the service type is turtlesim/srv/Spawn.

The message data is defined by the service interface. Use the command

ros2 interface show turtlesim/srv/Spawn

to display the message format required to make the service call: