[ros-users] First Message Missing

[David Lu!!]

Thanks Ken, for the help and the quick even-on-a-Sunday response.

Latching seems to work, although I'm not sure why it's applicable.
Spinning at the end did not.

What I was doing was 'rostopic echo chatter' then running the code I
pasted above, with the two lines switched and a spin at the end.
rostopic still doesn't print out the message, unless I add latch=True.
I could understand it not working if I started rostopic after the
code, but saving the message on the server seems irrelevant here.

-David!!

On Sun, Sep 12, 2010 at 5:56 PM, Ken Conley <kwc at willowgarage.com> wrote:
> Hi David,
>
> There are a couple of issues here:
>
>  1) your program starts and then exits, which means that only
> subscribers that are really fast will get the message. Your node needs
> to stay up if you want to guarantee that others receive its message
>  2) you probably want to enable 'latching', described below
>
> ROS topics are like tuning into a streaming broadcast -- if you tune
> in 15 minutes late, you don't get to see the stat. Imagine you are
> streaming data from the camera -- you want to start receiving messages
> the most recent one available.
>
> The fact that you're getting slightly different results is generally
> initialization latency. If you put the Publisher init before the call
> to init_node(), then the rospy node is able to send this data
> immediately when it communicates with the master. If you put the
> Publisher init call after init_node(), it has to go back to the
> master, which incurs more latency.
>
> There is a different behavior you can enable with ROS called
> 'latching', where a publisher will store the most recently published
> message indefinitely and send it to any new subscriber. We often use
> this behavior for data that can be static or slowly changing, like
> maps.
>
> See:
> http://www.ros.org/wiki/rospy/Overview/Publishers%20and%20Subscribers#rospy.Publisher_initialization
>
> You probably want to add a rospy.spin() or other sort of delay at the
> end as well.
>
> HTH,
> Ken
>
> On Sun, Sep 12, 2010 at 3:34 PM, David Lu!! <davidlu at wustl.edu> wrote:
>> Hey all,
>> I'm not quite sure if this is a bug, or if it is, where to file it,
>> but it took me forever to track down the problem.
>>
>> Here's my test program based on rospy tutorials. (I'm using C-Turtle
>> binaries on 10.04)
>> #!/usr/bin/python
>> import roslib; roslib.load_manifest('crazy')
>> import rospy
>> from std_msgs.msg import String
>>
>> def talker():
>>    pub = rospy.Publisher('chatter', String)
>>    rospy.init_node('talker', anonymous=True)
>>    str = "hello world"
>>    pub.publish(str)
>>
>> if __name__ == '__main__':
>>    try:
>>        talker()
>>    except rospy.ROSInterruptException: pass
>>
>> Fairly standard, and it works like you would expect. However, if you
>> switch the order of 7th and 8th lines, i.e.
>>    rospy.init_node('talker', anonymous=True)
>>    pub = rospy.Publisher('chatter', String)
>> ...it does NOT publish the message.
>>
>> However, when I add a sleep(1) command after Publisher line, then it
>> works fine.
>>
>> Is this...
>> A) A bug
>> B) A situation where I should just sleep
>> C) A situation where I should publish the message repeatedly until I'm
>> sure it's received.
>>
>> Thanks,
>> -David!!
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>>
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