Why a code, trying to read messages from two ZeroMQ sockets, fails? - python-3.7

Question

I have issues with reading messages from two zmq servers (one set to REQ|REP and one PUB|SUB)
The two servers are running on another computer. When I read just the REQ|REP connection everything works perfectly but as soon as I also try to read the PUB|SUB connection the program freezes (I guess it waits forever for a message)
from PyQt5 import QtCore, QtGui, QtWidgets
import zmq
import ui_mainwindow
class MainWindow(QtWidgets.QMainWindow, ui_mainwindow.Ui_MainWindow):
def __init__(self, parent = None):
super(MainWindow, self).__init__(parent)
self.context = zmq.Context()
try:
self.stateSocket = self.context.socket(zmq.REQ)
self.stateSocket.connect("tcp://134.105.89.197:5555")
except zmq.ZMQError as e:
print('States setup failed: ', e)
try:
self.context = zmq.Context()
self.anglesSocket = self.context.socket(zmq.SUB)
self.anglesSocket.connect("tcp://134.105.89.197:5556")
except zmq.ZMQError as e:
print('angles setup failed: ', e)
self.timer = QtCore.QTimer()
self.timer.timeout.connect(self.publishState)
self.timer.setInterval(500)
self.timer.start()
self.timer2 = QtCore.QTimer()
self.timer2.timeout.connect(self.publishAngles)
self.timer2.setInterval(500)
self.timer2.start()
# +more variables unrelated to problem
def publishState(self):
request= "a string"
try:
self.stateSocket.send_string(request)
self.reset = 0
message = self.stateSocket.recv()#flags=zmq.NOBLOCK)
values = [float(i) for i in message.decode("UTF-8").split(',')]
print("Status: ", message)
except zmq.ZMQError as e:
print('State communication: ', e)
values = [0] * 100
def publishAngles(self):
try:
message = anglesSocket.recv_string() # flags=zmq.NOBLOCK)
#values = [float(i) for i in message.decode("UTF-8").split(',')]
print("Angles: ", message)
except zmq.ZMQError as e:
print('Angles communication: ', e)
values = [0] * 100
edit: added the full relevant code.
What I observe is the deadlock does not come from the REQ|REP, This part alone works perfectly fine. But it seems that the PUB|SUBpart does not work in the timer function. When I make a minimal example with a while loop inside publishAngels() it works.
So is there an elegant way to use a PUB|SUB socket in a Qt Timer connected function?

Answer 1

In case one has never worked with ZeroMQ,one may here enjoy to first look at "ZeroMQ Principles in less than Five Seconds"before diving into further details
Q: "Is there any stupid mistake I am overlooking?"
Yes, there are a few and all easy to refine.
1) The, so far incomplete, visible ZeroMQ part exhibits principal uncertainty of what type of the subscription and other safe-guarding settings were, if ever, when and where, applied to the SUB-socket-Archetype AccessPoint. The same applied to the REQ-socket-Archetype AccessPoint, except the subscription management related kind(s) of setting(s) for obvious reasons.
2) The code ignores the documented principles of the known rules for the distributed-Finite-State-Automaton's (dFSA) logic, hardwired into the REQ/REP Scalable Formal Communication Archetype. Avoid this using a correct logic, not violating the, here mandatory, dFSA-stepper of REQ-REP-REQ-REP-REQ-REP, plus make either of the REQ and SUB handling become mutually independent and you have it. In other words, a naive, dFSA-rules ignoring use of the zmq.NOBLOCK flag does not solve the deadlock either.
If you feel to be serious into becoming a distributed-computing professional, a must read is the fabulous Pieter Hintjen's book "Code Connected, Volume 1"