ZeroMQ: have to sleep before send

Question

I'm write a zeromq demo with Forwarder device (with pyzmq)

Here are the codes(reference to https://learning-0mq-with-pyzmq.readthedocs.org/en/latest/pyzmq/devices/forwarder.html ):

forwarder.py

import zmq

context = zmq.Context()
frontend = context.socket(zmq.SUB)
frontend.bind('tcp://*:5559')
frontend.setsockopt(zmq.SUBSCRIBE, '')

backend = context.socket(zmq.PUB)
backend.bind('tcp://*:5560')

zmq.device(zmq.FORWARDER, frontend, backend)

sub.py

import zmq

context = zmq.Context()
socket = context.socket(zmq.SUB)
socket.connect('tcp://localhost:5560')
socket.setsockopt(zmq.SUBSCRIBE, '')

while True:
    print socket.recv()

pub.py

import zmq, time

context = zmq.Context()
socket = context.socket(zmq.PUB)
socket.connect('tcp://localhost:5559')
# time.sleep(0.01)
socket.send('9 hahah')

I run python forwarder.py, python sub.py in the terminal

then run python pub.py, the subscriber can't get the message. However, if I sleep a little time(for example 0.01s) before send, it works.

So my problem is, why have I sleep before send? thanks.

Answer 1

It's known as Slow Joiner syndrome. Read the guide, there are ways to avoid it using pub/sub syncing.