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Understanding ROS 2 services

Goal: Learn about services in ROS 2 using command line tools.

Tutorial level: Beginner

Time: 10 minutes

Background

Services are another method of communication for nodes on the ROS graph. Services are based on a call-and-response model, versus topics’ publisher-subscriber model. 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.

Prerequisites

Some concepts mentioned in this tutorial, like nodes and topics, were covered in previous tutorials in the series.

You will need the turtlesim package

As always, don’t forget to source ROS 2 in every new terminal you open.

Tasks

1 Setup

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

2 ros2 service list

Running the ros2 service list command in a new terminal will return a list of all the topics 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 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 “Introducing turtlesim and rqt” tutorial.

3 ros2 service type

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:

ros2 service type /clear

Which should return:

std_srvs/srv/Empty

The Empty type means the service call sends no data when making a request and receives no data when receiving a response.

3.1 ros2 service list -t

To see the types of all the active services at the same time, you can append the --show-types option, abbreviated as -t, to the list command:

ros2 service list -t

Which will return:

/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]
...

4 ros2 service find

If you want to find all the services of a specific type, you can use the command:

ros2 service find <type_name>

For example, you can find all the Empty typed services like this:

ros2 service find std_srvs/srv/Empty

Which will return:

/clear
/reset

5 ros2 interface show

You can call services from the command line, but first you need to know the structure of the input arguments.

ros2 interface show <type_name>.srv

To run this command on the /clear service’s type, Empty:

ros2 interface show std_srvs/srv/Empty.srv

Which will return:

---

The --- separates the request structure (above) from the response structure (below). But, as you learned earlier, the Empty type doesn’t send or receive any data. So, naturally, its structure is blank.

Let’s introspect a service with a type that sends and receives data, like /spawn. From the results of ros2 service list -t, we know /spawn’s type is turtlesim/srv/Spawn.

To see the arguments in a /spawn call-and-request, run the command:

ros2 interface show turtlesim/srv/Spawn.srv

Which will return:

float32 x
float32 y
float32 theta
string name # Optional.  A unique name will be created and returned if this is empty
---
string name

The information above the --- line tells us the arguments needed to call /spawn. x, y and theta determine the location of the spawned turtle, and name is clearly optional.

The information below the line isn’t something you need to know in this case, but it can help you understand the data type of the response you get from the call.

6 ros2 service call

Now that you know what a service type is, how to find a service’s type, and how to find the structure of that type’s arguments, you can call a service using:

ros2 service call <service_name> <service_type> <arguments>

The <arguments> part is optional. For example, you know that Empty typed services don’t have any arguments:

ros2 service call /clear std_srvs/srv/Empty

This command will clear the turtlesim window of any lines your turtle has drawn.

../../../_images/clear.png

Now let’s spawn a new turtle by calling /spawn and inputting arguments. Input <arguments> in a service call from the command-line need to be in YAML syntax.

Enter the command:

ros2 service call /spawn turtlesim/srv/Spawn "{x: 2, y: 2, theta: 0.2, name: ''}"

You will get this method-style view of what’s happening, and then the service response:

waiting for service to become available...
requester: making request: turtlesim.srv.Spawn_Request(x=2.0, y=2.0, theta=0.2, name='None')

response:
turtlesim.srv.Spawn_Response(name='None')

Your turtlesim window will update with the newly spawned turtle right away:

../../../_images/spawn.png

Summary

Nodes can communicate using services in ROS 2. Services only pass information to a node if that node specifically requests it, and will only do so once per request (not in a continuous stream). You generally don’t want to use a service for continuous calls; topics or even actions would be better suited.

In this tutorial you used command line tools to identify, elaborate on, and call services.

Next steps

In the next tutorial, Understanding ROS 2 parameters, you will learn about configuring node settings.