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.

Sweeping

Answer 1

It's known as Slow Joiner syndrome. Read the guide, there are ways to avoid it using Node Coordination.

Deus

specifically, look at the pub-sub synchronization example to see it working. As the figure shows, you'll use an additional REP/REQ pair of sockets to sync your pub and sub sockets. – Pentothal

It also says, that the example it shows has an unrobust 1 second delay in the subscriber, and then suggests you basically need to send bogus data from the publisher to the subscriber until everything is flushed through. Look at the "more robust model" blurb at the very end. – Gamopetalous

Everything is 404. – Wiser

looks like it moved here. – Arnaud