Services are another method of communication for nodes in the ROS graph. Services are based on a call-and-response model versus the publisher-subscriber model of topics. While topics allow nodes to subscribe to data streams and get continual updates, services only provide data when they are specifically called by a client.
Start up the two turtlesim nodes, /turtlesim and /teleop_turtle.
Open a new terminal and run:
ros2 run turtlesim turtlesim_node
Open another terminal and run:
ros2 run turtlesim turtle_teleop_key
Running the ros2 service list command in a new terminal will return a list of all the services currently active in the system:
/clear
/kill
/reset
/spawn
/teleop_turtle/describe_parameters
/teleop_turtle/get_parameter_types
/teleop_turtle/get_parameters
/teleop_turtle/list_parameters
/teleop_turtle/set_parameters
/teleop_turtle/set_parameters_atomically
/turtle1/set_pen
/turtle1/teleport_absolute
/turtle1/teleport_relative
/turtlesim/describe_parameters
/turtlesim/get_parameter_types
/turtlesim/get_parameters
/turtlesim/list_parameters
/turtlesim/set_parameters
/turtlesim/set_parameters_atomically
You will see that both nodes have the same six services with parameters in their names. Nearly every node in ROS 2 has these infrastructure services that parameters are built off of. There will be more about parameters in the next tutorial. In this tutorial, the parameter services will be omitted from the discussion.
For now, let’s focus on the turtlesim-specific services, /clear, /kill, /reset, /spawn, /turtle1/set_pen, /turtle1/teleport_absolute, and /turtle1/teleport_relative. You may recall interacting with some of these services using rqt in the Use turtlesim, ros2, and rqt tutorial.
Services have types that describe how the request and response data of a service is structured. Service types are defined similarly to topic types, except service types have two parts: one message for the request and another for the response.
To find out the type of a service, use the command:
ros2 service type <service_name>
Let’s take a look at turtlesim’s /clear service. In a new terminal, enter the command: