ROS services provide a call-and-response model for communication between nodes, unlike topics which provide continual updates. ros2 service list returns a list of active services, while ros2 service type and ros2 service find provide information on service types. ros2 service call can be used to call a service from the command line.

1. What are Services?

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.

2. ros2 service list

<aside> 💡 Check if /turtlesim & /teleop_turtle nodes are active, if not run ros2 run turtlesim turtlesim_node & ros2 run turtlesim turtle_teleop_key in separate terminal respectively. If no node is active, you will not get any output!

</aside>

Running the ros2 service list command in a new terminal will return a list of all the services currently active in the system. In this example, the currently running nodes are the turtlesim_node and the turtle_teleop_key nodes.

The output of the ros2 service list command is below:

/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.

Adding the additional --show-types argument, abbreviated to -t, to the ros2 service list command will return the message type of the service.

/clear [std_srvs/srv/Empty]
/kill [turtlesim/srv/Kill]
/reset [std_srvs/srv/Empty]
/spawn [turtlesim/srv/Spawn]
...
/turtle1/set_pen [turtlesim/srv/SetPen]
/turtle1/teleport_absolute [turtlesim/srv/TeleportAbsolute]
/turtle1/teleport_relative [turtlesim/srv/TeleportRelative]
...

3. ros2 service type

<aside> 💡 This needs atleast one service running, Check if /turtlesim or /teleop_turtle nodes are active, if not run ros2 run turtlesim turtlesim_node orros2 run turtlesim turtle_teleop_key in separate terminal respectively. If no service is active, you will not get any output!

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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.