ZeroMQ PUB/SUB - why won't multiple subscribers work?

Question

I'm using ZeroMQ to facilitate a publish/subscribe environment I'm needing. I'm running a publish server on machine A using Python (using EventLoop), and right now I have one subscriber running in C++ on machine B and a second subscriber running in Python (using EventLoop) on machine C.

If machine B subscribes to machine A before machine C does, then B gets subscribed messages and C does not. Furthermore, if I look at the established connections on machine A, there only exists a connection for machine B but not for C. If machine C subscribes to A before B does, then it's the other way around.

Here's my publisher code:

import zmq
from zmq.eventloop import ioloop, zmqstream

ioloop.install()

context   = zmq.Context(1)
socket    = context.socket(zmq.PUB)
publisher = zmqstream.ZMQStream(socket)
socket.bind("tcp://*:1337")

def publish():
  publisher.send_multipart(("heartbeat", "OHAI"))

ioloop.PeriodicCallback(publish, 5000).start()
ioloop.IOLoop.instance().start()

Here's my Python subscriber code:

import zmq
from zmq.eventloop import ioloop, zmqstream

ioloop.install()

context    = zmq.Context(1)
socket     = context.socket(zmq.SUB)
subscriber = zmqstream.ZMQStream(socket)
socket.setsockopt(zmq.SUBSCRIBE, "heartbeat")
socket.connect("tcp://pub.local:1337")

def subscription(message):
  print "Message Received: %s" % (message[1])

subscriber.on_recv(subscription)
ioloop.IOLoop.instance().start()

Why isn't my publisher accepting multiple incoming subscriber sockets? It's probably worth noting that multiple subscribers works fine when running them on machine A, but I don't think it's a firewall issue because I tested subscriber connections from B and C to A with the firewall disabled.

Answer 1

Thanks to everyone for all the helpful comments in the original posting. This behavior turned out to be due to a mismatch in ZeroMQ versions being used... an oversight on my part.