Understanding ROS Services and Parameters

Description: This tutorial introduces ROS services, and parameters as well as using the rosservice and rosparam commandline tools.

Tutorial Level: BEGINNER

Next Tutorial: Writing a simple publisher and subscriber (python) (c++)

Assuming your turtlesim node is still running from the last tutorial, let's look at what services the turtlesim provides:

Using rosservice

rosservice can easily attach to ROS's client/service framework with services. rosservice has many commands that can be used on topics, as shown below:

Usage:

rosservice list print information about active topics
rosservice call call the service with the provided args
rosservice type print service type
rosservice uri  print service ROSRPC uri

rosservice list

$ rosservice list

The list command shows us that the turtlesim node provides five services, reset, clear, set_pen, turtlesim/get_loggers, and turtlesim/set_logger_level:

• /turtlesim/get_loggers
  /turtlesim/set_logger_level
  /reset
  /rosout/get_loggers
  /set_pen
  /clear
  /rosout/set_logger_level

Let's look more closely at the reset service using rosservice type:

rosservice type

Usage:

rosservice type [service]

Let's find out what type the clear service is:

$ rosservice type reset

• std_srvs/Empty

This service is empty, this means when the service call is made it takes no arguments. Let's call this service using rosservice call:

rosservice call

Usage:

rosservice call [service] [args]

Here we'll call with no arguments because the service is of type empty:

rosservice call reset

This does what we expect, it resets the turtlesim node to it's starting state.

Let's look at the case where the service has arguments, let's look at the srv for set_pen

$ rosservice type set_pen| rossrv show

• uint8 r
  uint8 g
  uint8 b
  uint8 width
  uint8 off
  ---

This service let's us change the line drawn behind our turtle as it moves, let's change the line color and make our turtle move:

$ rosservice call set_pen 0 255 0 5 0
$ rostopic pub command_velocity turtlesim/Velocity -r 1 -- 90.0  -1.8

Now our turtle should look like this:

Using rosparam

rosparam allows you to store and manipulate data on the ROS Parameter Server. The Parameter Server can store integers, floats, boolean, dictionaries, and lists. rosparam uses the YAML markup language for syntax. In simple cases, YAML looks very natural: 1 is an integer, 1.0 is a float, one is a string, true is a boolean, [1, 2, 3] is a list of integers, and {a: b, c: d} is a dictionary. rosparam has many commands that can be used on topics, as shown below:

Usage:

rosparam set    set parameter
rosparam get    get parameter
rosparam load   load parameters from file
rosparam dump   dump parameters to file
rosparam delete delete parameter
rosparam list   list parameter names

Let's look at what parameters are currently on the param server:

rosparam list

$ rosparam list

Here we can see that the turtlesim node has three parameters on the param server for background color:

• /background_b
  /background_g
  /background_r
  /roslaunch/uris/aqy:51932
  /run_id

Let's change one of the parameter values using rosparam set:

rosparam set and rosparam set

Usage:

rosparam set [param_name]
rosparam get [param_name]

Here will change the red channel of the background color:

$ rosparam set background_r 255

This changes the parameter value, now we have to call the clear service for the parameter change to take effect:

$ rosservice call clear

Now our turtlesim look like this:

Now let's look at the values of other parameters on the param server. Let's get the value of the green background channel:

$ rosparam get background_g 

• 144

We can also use rosparam get / to show us the contents of the entire Parameter Server.

$ rosparam get /

• background_b: 255
  background_g: 144
  background_r: 255
  roslaunch:
    uris: {'aqy:51932': 'http://aqy:51932/'}
  run_id: e07ea71e-98df-11de-8875-001b21201aa8

You may wish to store this in a file so that you can reload it at another time. This is easy using rosparam:

rosparam dump and rosparam load

Usage:

rosparam dump [file_name]
rosparam load [file_name] [namespace]

Here we write all the parameters to the file params.yaml

$ rosparam dump params.yaml

You can even load these yaml files into new namespaces, e.g. copy:

$ rosparam load params.yaml copy
$ rosparam get copy/background_b

• 255

Now that you understand how ROS topics, services, and params work, let's look at how to write a simple publisher and subscriber (python) (c++).