XSUB not receiving in ZeroMQ XSUB/PUB setup

Question

I would like to use XSUB/XPUB to enable multiple ZMQ publishers and subscribers. Everything works when I use zmq.proxy(xpub_socket, xsub_socket), but I need something custom because I need to write code between XSUB and XPUB that examines the messages.

Here's where I'm at:

import time
import zmq

context = zmq.Context()

address = '127.0.0.1'
pub_port = '3000'
sub_port = '3001'

# XSUB socket
xsub_socket = context.socket(zmq.XSUB)
xsub_socket.bind(f'tcp://{address}:{pub_port}')

# XPUB socket
xpub_socket = context.socket(zmq.XPUB)
xpub_socket.bind(f'tcp://{address}:{sub_port}')

time.sleep(1)

# PUB socket
pub_socket = context.socket(zmq.PUB)
pub_socket.connect(f'tcp://{address}:{pub_port}')

# SUB socket
sub_socket = context.socket(zmq.SUB)
sub_socket.subscribe('')
sub_socket.connect(f'tcp://{address}:{sub_port}')

time.sleep(1)

pub_socket.send_string('test')

time.sleep(1)

print(poller.poll(0))

The values sent from the PUB socket do not reach the XSUB socket.

I read here that the first byte needs to be 1. Both of these also don't work:

pub_socket.send(b'\x01')
pub_socket.send_multipart([b'\x01', 'test'.encode('utf-8')])

What am I doing wrong here?

Answer 1

A PUB socket won't send any messages to an XSUB socket unless it has received a subscription request, which you get by calling subscribe on a SUB socket.

The only way those subscription messages get passed through is if you set up your XSUB/XPUB proxy.

Here's a simple proxy that connects an XPUB and XSUB socket, printing out messages it receives in either direction:

import zmq

ctx = zmq.Context()

xpub_sock = ctx.socket(zmq.XPUB)
xpub_sock.bind("tcp://127.0.0.1:3000")

xsub_sock = ctx.socket(zmq.XSUB)
xsub_sock.bind("tcp://127.0.0.1:3001")

poller = zmq.Poller()
poller.register(xpub_sock, zmq.POLLIN)
poller.register(xsub_sock, zmq.POLLIN)

while True:
    socks = dict(poller.poll())
    if xpub_sock in socks and socks[xpub_sock] == zmq.POLLIN:
        msg = xpub_sock.recv_multipart()
        print("(sub)", msg)
        xsub_sock.send_multipart(msg)
    elif xsub_sock in socks and socks[xsub_sock] == zmq.POLLIN:
        msg = xsub_sock.recv_multipart()
        print("(pub)", msg)
        xpub_sock.send_multipart(msg)

If I connect to this with an PUB socket, like this...

import zmq
import time

ctx = zmq.Context()
pub_sock = ctx.socket(zmq.PUB)
pub_sock.connect("tcp://localhost:3001")

while True:
    pub_sock.send_string("test")
    time.sleep(1)

...I won't see any messages arriving at the XSUB socket, because there are no active subscriptions. However, if I connect a SUB socket to the XPUB socket and set a subscription...

import zmq

ctx = zmq.Context()
sub_sock = ctx.socket(zmq.SUB)
sub_sock.connect("tcp://localhost:3000")
sub_sock.subscribe("")

while True:
    msg = sub_sock.recv()
    print(msg)

...then I will start to see messages passing from the PUB socket to the XSUB socket, and then from the XPUB socket to the SUB socket.