Node to NOde synchronous communication using Publisher Subscriber Architecture

edit

I want to design three nodes such that node 1 is publishing a message to node 2 and node 3, node 2 and 3 is processing the message and sending back to node 1, and once node 1 receives both the messages from node 2 and node 3, it sends the message again and cycle continues. The entire thing must be carried out synchronously. Please suggest me the ways to do it. Thank you,

Node 1(code)---------------------------------------------------------------------------

#!/usr/bin/env python

import rospy
from beginner_tutorials.msg import Num

msg=Num()
msg.message="Message sent to listener node"
msg.age1=0
msg.age2=0
msg.flag=1

def callback(data):
    rospy.loginfo("Hello! I'm node 1. I received the message %s.My age is: %d" % (data.message, data.age1))
    pub=rospy.publisher('node_chat',Num,queue_size=1)
    msg=Num()
    msg.message="Message sent to listener node"
    msg.age1=data.age1
    msg.age2=data.age2+1
    msg.flag=1
    rospy.loginfo(msg)
    pub.publish(msg)
    rospy.spin()


if __name__ == '__main__':
    try:

        pub = rospy.Publisher('node_chat', Num, queue_size=1)
        rospy.init_node('node1', anonymous=True)
        while not rospy.is_shutdown():
                print ("Hello! I'm node 1")
                msg.age2+=1
                rospy.loginfo(msg)
                pub.publish(msg)
                rospy.sleep(10)
                rospy.Subscriber('node_chat',Num, callback)

    except rospy.ROSInterruptException:
        pass

node2(code)-----------------------------------------------------------------------------------------------------------------------------------

#!/usr/bin/env python
import rospy
from beginner_tutorials.msg import Num

def callback(data):
    rospy.loginfo("Hello! I'm node 2. I received the message %s.My age is: %d" % (data.message, data.age2))
    pub=rospy.publisher('node_chat',Num,queue_size=1)
    msg=Num()
    msg.message="Message sent back to talker node"
    msg.age1=data.age1+1
    msg.age2=data.age2
    msg.flag=2
    rospy.loginfo(msg)
    pub.publish(msg)
   # rospy.spin()

def listener():
    rospy.init_node('node2', anonymous=True)
    rospy.Subscriber('node_chat',Num, callback)

    # spin() simply keeps python from exiting until this node is stopped
    rospy.spin()

if __name__ == '__main__':
    listener()

I am getting this error also, which shows----------------

[ERROR] [WallTime: 1481180187.955999] bad callback: <function callback at 0x7f22                                                                                                                                        22ddca28>
Traceback (most recent call last):
  File "/opt/ros/indigo/lib/python2.7/dist-packages/rospy/topics.py", line 720,                                                                                                                                         in _invoke_callback
    cb(msg)
  File "/home/collab/catkin_ws/src/beginner_tutorials/scripts/customlistener.py"                                                                                                                                        , line 7, in callback
    pub=rospy.publisher('node_chat',Num,queue_size=1)
AttributeError: 'module' object has no attribute 'publisher'