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How to make a server read the latest msg from a topic?

asked 2017-11-17 11:35:58 -0600

RobotWhisperer gravatar image

All I want to do is for my server to take a the latest msg from a topic and return it. The service server would take no inputs and return one output. Eg: Return robot position. Or Return a map.

I made a service server node. In the callback of the service I had the node subscribe to the topic I want. In the line were the service node subscribes to the topic, I expect the latest msg to be obtained right then and there from the topic because I made the topic latched by setting the latch argument to True. But it doesn't do as expected. As you can see from the code below I am forced to include the wait_for_message or else the server node never reads a msg.
The topic I am subscribed to doesn't publish frequently and waiting for a message is not practical, I just want the service to return the latest msg on the topic without waiting for the next one, I thought having the topic latched would take care of that. But it didn't.

Thank you

globvar = 0
def callback(msg_arg): # subscription callback 
    global globvar
    print "in sub cb ",globvar
    globvar = int(float( #because you cant int(float_string)

def service_callback(req): # service callback
    print "in server cb "
    sb = rospy.Subscriber('chatter', String, callback)
    rospy.wait_for_message("chatter", String)
    global globvar
    print (globvar)
    return globvar
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answered 2017-11-17 13:56:40 -0600

gvdhoorn gravatar image

updated 2017-11-17 13:59:45 -0600

The best way to implement something like this is rospy.wait_for_message(..). Creating Subscribers in callbacks is not a good idea.

If you're really only interested in a single message (which happens to be 'the last' only because of latching) then don't create the Subscriber, but use rospy.wait_for_message(..) only.

Note btw that what you show in your question text is really brittle: there is no way to guarantee that globvar will contain an actual message object at that point.

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It will also let you avoid all those ugly global vars.

gvdhoorn gravatar image gvdhoorn  ( 2017-11-17 13:56:58 -0600 )edit

Perfect. I didn't know I can use wait_for_message without subscribing. And I also didn't know that wait_for_message actually returns the message. My code is more robust now and doesn't have global variable. Thank you.

RobotWhisperer gravatar image RobotWhisperer  ( 2017-11-18 09:01:47 -0600 )edit

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Asked: 2017-11-17 11:35:58 -0600

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Last updated: Nov 17 '17