Hi Mike,

Think of topics like connecting to a live video feed -- whenever you
connect, you receive the most recent feed. The examples you give
demonstrate the latency/race condition of subscribers attempting to
get that first message from the publisher. A publisher can have
arbitrarily many subscribers, and not all of those subscribers are
guaranteed to connect, so this is the intended behavior.

There are several remedies that ROS supports:

1) latch=True: the last message you published is saved and is
delivered to any new subscriber
2) subscriber callbacks (see rospy_tutorials/004_listener_subscribe_notify)

Hope this helps,
Ken




On Tue, Jun 1, 2010 at 5:39 PM, Mike Purvis
<mpurvis@clearpathrobotics.com> wrote:
> Hey everyone,
> I'm experiencing a curious issue with regards to a node which sends a
> message on startup. The following works:
> pub = rospy.Publisher("/clearpath/announce", Announce, latch=False)
> rospy.init_node('controller')
> pub.publish(action="rollcall")
> However, if I initialize the node first before setting up the publisher, the
> message send silently fails. No error, but the message never arrives, and
> rostopic shows no activity:
> rospy.init_node('controller')
> pub = rospy.Publisher("/clearpath/announce", Announce, latch=False)
> pub.publish(action="rollcall")
> But, when I insert some delays, it works again!
> rospy.init_node('controller')
> rospy.sleep(0.5)
> pub = rospy.Publisher("/clearpath/announce", Announce, latch=False)
> rospy.sleep(0.5)
> pub.publish(action="rollcall")
> I assume that there's some initialization going on in a background thread;
> is this a known/documented behavious? Is there some better way to handle it
> than just these delays?
> Thanks,
> Mike Purvis
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