the Robot Construction Kit

Using Avahi to find the tasks


In this tutorial you will learn how to use the Avahi name service that is available with Rock. This will allow you to distribute your components over multiple physical systems without having to rely on a centralized name service. The goal of this tutorial is thus:

  • distributing two components, i.e. message_producer and message_consumer will be started independently
  • connecting two components using the Avahi name service

More details on this topic you find in the general documentation.

For this tutorial you will reuse the components that you created during the basics tutorials. You can also retrieve them directly by installing the tutorial package set

IMPORTANT: For this tutorial to work you need to have an +active+ network device, otherwise Avahi will not publish any information.

Ruby scripts

Until now you might not have questioned how the Ruby scripts establish a connection in details to a task that runs in a deployment. Orocos.rb used CORBA as one of its transport layers, and it also uses the CORBA name service for finding components. For that reason, an orocos.rb script has an initialization section, i.e. the following statement are essential.

require 'orocos'

By default this initialization sets up the CORBA communication layer, and if no other statements are made, the CORBA name server on the localhost is used.

You rely on this name service as soon as you call Orocos.name_service.get. The call will query all available name services to find the task of the given name. 'message_producer_deployment' do 
    message_producer = Orocos.name_service.get 'message_producer'

If the task cannot be found, an exception will be raised, e.g. if you misspelled the task name and used ‘message_producr’ instead of the correct name

The Avahi name service

Before you can use the Avahi name service you have to make sure the tools/service_discovery package is installed, since the support for service_discovery is optional. Only when this package is installed orogen components will be generated with support for service discovery via Avahi. The minimal Rock installation already provides this package for you, so you can continue right away.

After being sure service_discovery has been installed and your component has been build with service discovery support you can instantiate an Avahi name service in your ruby script with a specific search domain as parameter. For the search domain you can use a pattern of ‘name’.’suffix’, where the suffix must be either ‘._tcp’ or ‘._udp’. Here, use ‘_robot._tcp’.

require 'orocos'

# remove the default CORBA name service
# as we do not want to use it for finding
# components
Orocos.name_service << '_robot._tcp'

For the name resolution via Avahi to work, you have to start your deployment within a service discovery domain, and since there exists a command line option on the deployment to set the service discovery domain (sd-domain), you just forward the command line option to the deployment. Be reminded, that this option is available to deployments +after+ the tools/service_discovery package has been installed and +after+ (re)building components with the available tools/service_discovery package. 'message_producer_deployment', :cmdline_args => 
	{ 'sd-domain' => '_robot._tcp' }, :wait => 3 do |p|

Since the publishing of the service in the Avahi domain happens with delay once you start the deployment, you have to allow for a waiting time, before querying the name service for the name. By setting the wait option you can pass a time in second that the script should wait, until executing the block. Select 3 seconds until the actual block of the Ruby script starts to run.

After enabling the message producer startup to use Avahi, also activate the Avahi name service for the message consumer. The message producer will be started separately, thus remove the message_producer_deployment from the start.rb in the message consumer component.

require 'orocos'
require 'readline'

Orocos.name_service << '_robot._tcp' 'message_consumer_deployment', :cmdline_args => 
	{ 'sd-domain' => '_robot._tcp'}, :wait => 3 do  

     message_producer = Orocos.name_service.get 'message_producer'
     message_consumer = Orocos.name_service.get 'message_consumer'


     message_producer.messages.connect_to message_consumer.messages

     Readline::readline("Press ENTER to exit\n") do

Run it

First start the message producer, then the message consumer. The message consumer will output the message in the same way a before, however, this time both component found each other using the Avahi naming service.

ruby message_producer/scripts/start.rb &
ruby message_consumer/scripts/start.rb

NOTE: You need to have a proper configuration of Avahi, and there are some known issue with using IPv4. Enabling IPv6 for the daemon in /etc/avahi/avahi-daemon.conf will help to make service discovery more robust.




In this tutorial you have learned to:

  • enable the Avahi name service for finding running tasks
  • apply options to the Ruby script
  • connect two components using the Avahi name service