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Rosbag: play on demand service

asked 2013-02-27 16:23:52 -0500

TRS gravatar image

updated 2013-02-27 20:17:31 -0500

Is there a way to get rosbag to publish the next message by calling a service. A use-case is that a node request the rosbag player to publish the next message without a fixed hz, but when the node is ready to receive new info. This will allow for a lot of flexibility during the development of complex systems.

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answered 2013-02-28 07:17:22 -0500

pgrice gravatar image

updated 2013-02-28 07:19:12 -0500

I don't believe this exists out-of-the-box.

Rosbag command line tools do allow you to play back topics at a modified (possibly very slow) constant rate with --rate. If you want very strict control over the publishing, this code snippet should serve:

import roslib, rospy, rosbag
from std_srvs.srv import Empty

class BagByService(object):
    def __init__(self, bagfile):
        self.bag = rosbag.Bag(bagfile, 'r')
        self.message_generator = self.bag.read_messages()
        self.next_msgs_srv = rospy.Service('/next_msg', Empty, self.pub_next_msg)

        #Create a publisher for each topic
        self.publishers = {}
        for con in self.bag._get_connections():
            msg_class = roslib.message.get_message_class(con.datatype)
            self.publishers[con.topic] = rospy.Publisher(con.topic, msg_class)

    def pub_next_msg(self, req):
        topic, msg, time =

if __name__=='__main__':
    import sys
    bbs = BagByService(sys.argv[1])

Run this with the .bag file as the only commandline argument. Calling the Empty '/next_msg' service will publish the next message in the bag file. Clearly, more interesting things could be done from here (filtering by time, topic, etc.)

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pub_next_msg() needs to return a response, otherwise a "service handler returned None" error occurs. I add: return EmptyResponse() at the end of pub_next_msg() which requires a from std_srvs.srv import EmptyResponse and that solves the problem

martinakos gravatar image martinakos  ( 2019-06-20 05:54:04 -0500 )edit

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Asked: 2013-02-27 16:23:52 -0500

Seen: 789 times

Last updated: Feb 28 '13