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Advertising & Pulling

Overview

Pulling is the inverse operation of flip. Whereas a flip is initiated by the sender, a pull is initiated by the receiver of a connection.

It is actually a two step process. First a remote gateway has to publicly make available a topic (i.e. advertise). Once available, other gateways can freely pull and register that connection on their own ros system.

Workflow

The usual workflow for a pull is as follows (note that some of these steps are internal, but presented here for illustration):

• Remote gateway creates an advertisement with the following data:
  • Connection name (e.g. '/chatter')
  • Connection type (e.g. 'publisher')
  • Connection node name (e.g. '/talker')

Note that each of the fields except for type may also be a python regex pattern, e.g. '/chat.*'.

• The local gateway creates a pull rule (like a flip rule).
• If a match is found on the specified remote gateway, the local gateway registers the remote connection on the local master.
• Pull rules can be activated and are periodically checked for new/lost connections (same as flips).

Code

Some example python code. Advertisements on ROS_MASTER_URI=http://localhost:11312:

   1 req = AdvertiseRequest()
   2 rule = Rule()
   3 rule.name = '/chatter'
   4 rule.type = gateway_msgs.ConnectionType.PUBLISHER
   5 rule.node = '/talker'
   6 rospy.loginfo("Advertise : [%s,%s,%s]."%(rule.type, rule.name, rule.node))
   7 req.rules.append(self.rule)
   8 resp = advertise_service(self.req)
   9 if resp.result != 0:
  10     rospy.logerr("Advertise : %s"%resp.error_message)

Then pulling on ROS_MASTER_URI=http://localhost:11313:

   1 gateway = 'Pirate Gateway'
   2 pull_service = rospy.ServiceProxy('/gateway/pull',Remote)
   3 req = RemoteRequest()
   4 req.cancel = False
   5 req.remotes = []
   6 rule = gateway_msgs.msg.Rule()
   7 rule.name = '/chatter'
   8 gateway_msgs.ConnectionType.PUBLISHER
   9 rule.node = '/talker'
  10 req.remotes.append(RemoteRule(gateway, rule))
  11 rospy.loginfo("Pull : [%s,%s,%s,%s]."%(self.gateway,
  12                                        rule.type,
  13                                        rule.name,
  14                                        rule.node or 'None'))
  15 resp = pull_service(req)
  16 if resp.result != 0:
  17     rospy.logerr("Pull : %s"%resp.error_message)
  18 req.remotes = []

Examples

In three separate shells, launch the hub, a gateway system with tutorials running, and an empty gateway system.

> roslaunch --port=11311 rocon_gateway_tutorials pirate_hub.launch

> roslaunch --port=11312 rocon_gateway_tutorials pirate_gateway_tutorials.launch

> roslaunch --port=11313 rocon_gateway_tutorials pirate_gateway.launch

To advertise the publisher /chatter only in the first gateway(ROS_MASTER_URI=http://localhost:11312):

> rosrun rocon_gateway_tutorials advertise_tutorials --pubonly

To pull the advertised connection into the second gateway, (ROS_MASTER_URI=http://localhost:11313):

> rosrun rocon_gateway_tutorials pull_tutorials --pubonly

Run pull_tutorials with --help to find the other target operations. For more code samples, check the source code of the advertise/pull scripts in rocon_gateway_tutorials.

Limitations

Same as those for Flipping