How do I send a dict with the PUB/SUB pattern using pyzmq?

Question

I'm trying to send a python dictionary, using something like:

import zmq
context = zmq.Context()
socket = context.socket(zmq.PUB)
socket.bind("tcp://*:%s" % port
socket.send_pyobj({'hello':'world'})

and receive it using:

socket.connect ("tcp://localhost:%s" % port)
topicfilter = "1"

socket.setsockopt_string(zmq.SUBSCRIBE, topicfilter)

while True:
    print(socket.recv_pyobj())

Questions:

1. I can't seem to figure out how to set the topic for send_pyobj().
2. Needless to say, I don't receive anything on the other end. I managed to get it going with strings and send_string(), so it's definitely connecting. What am I doing wrong?
3. Can I have many PUB servers broadcasting? Thereby creating a sort of many-to-many, where other apps can dip into the flow?

Answer 1

3. Can I have many PUB servers broadcasting?

Yes, you can

Easy.

---

2. I don't receive anything on the other end... What am I doing wrong?

Do not subscribe to "1" in cases, when it never comes

This single reason stands behind "not receiving anything" ( unless it started with "1", which it obviously never did, as the payload was assembled right by the socket.send_object( ... ) and there was never a "1" to match on the position of the string-alike byte[0].

---

1. ... how to set the topic for send_pyobj()?

Well, you never know

This ought be set using other means than guessing a few bytes, "matching" the initial section of the object-byte-string-representation.

Simplest ever is to "prepend" a controlled value on the sender side like this :

import dill as pickle

aConstantNbytesPUB_TOPIC_FILTER_PREFIX = "{UUID:0000-0000-0000-1111-2222}"

socket.send(aConstantNbytesPUB_TOPIC_FILTER_PREFIX + pickle.dumps(anObjectToSEND))

#...

aMSG = socket.recv()
print(pickle.loads(aMSG[len(aConstantNbytesPUB_TOPIC_FILTER_PREFIX) :]))

Using a multi-part message composition is also possible, yet a bit longer to demonstrate the concept here.

Both pickle and dill are limited in their SER/DES-strategies for object decomposition / traversal / reinstatement capabilities and security warnings are well known to be taken care off.