Retrieving subscriber count using zeromq PUB/SUB sockets

Question

Is it possible to get the total count of subscribers from a PUB socket in zeromq?

Thanks!

Answer 1

There doesn't seem to be any direct way. Below is Python code to monitor socket events which can be used to maintain count:

import zmq
from zmq.eventloop import ioloop, zmqstream
import zmq.utils.monitor

class Publication:
    def start(self, port, host):
        context = zmq.Context()
        self._socket = context.socket(zmq.PUB)
        self._socket.bind("tcp://%s:%d" % (host, port))
        self._mon_socket = self._socket.get_monitor_socket(zmq.EVENT_CONNECTED | zmq.EVENT_DISCONNECTED)
        self._mon_stream = zmqstream.ZMQStream(self._mon_socket)
        self._mon_stream.on_recv(self._on_mon)

    def _on_mon(self, msg):
        ev = zmq.utils.monitor.parse_monitor_message(msg)
        event = ev['event']
        endpoint = ev['endpoint']
        if event == zmq.EVENT_CONNECTED:
            pass
            # print(endpoint)
        elif event == zmq.EVENT_DISCONNECTED:
            pass
            #print(endpoint)

One issue is that for some reason CONNECTED event is not firing. Another issue is that even when event fires, you only get endpoint ID which is like tcp://ip:port string. So for multiple clients on same node you get same endpoint ID.