python - Pyzmq high-water mark not working on pub socket

Question

According to the ZeroMQ documentation a pub socket is supposed to drop messages once the number of queued messages reaches the high-water mark.

This doesn't seem to work in the following example (and yes I do set the hwm before bind/connect):

import time
import pickle
from threading import Thread
import zmq

ctx = zmq.Context()

def pub_thread():
    pub = ctx.socket(zmq.PUB)
    pub.set_hwm(2)
    pub.bind('tcp://*:5555')

    i = 0
    while True:
        # Send message every 100ms
        time.sleep(0.1)
        pub.send_string("test", zmq.SNDMORE)
        pub.send_pyobj(i)
        i += 1

def sub_thread():
    sub = ctx.socket(zmq.SUB)
    sub.subscribe("test")
    sub.connect('tcp://localhost:5555')
    while True:
        # Receive messages only every second
        time.sleep(1)
        msg = sub.recv_multipart()
        print("Sub: %d" % pickle.loads(msg[1]))

t_pub = Thread(target=pub_thread)
t_sub = Thread(target=sub_thread)
t_pub.start()
t_sub.start()

while True:
    pass

I'm sending messages on pub 10 times faster than reading them on the sub socket, hwm is set to 2. I would expect to only receive about every 10th message. Instead, I see the following output:

Sub: 0
Sub: 1
Sub: 2
Sub: 3
Sub: 4
Sub: 5
Sub: 6
Sub: 7
Sub: 8
Sub: 9
Sub: 10
Sub: 11
Sub: 12
Sub: 13
Sub: 14
...

so I see all messages arriving, thus they are held in some queue until I read them. Same holds true when adding a hwm=2 on the sub socket as well before connect.

What am I doing wrong or am I misunderstanding hwm?

I use pyzmq version 17.1.2

Answer 1

With borrowing an answer to the issue which I opened in Github, I've updated my answer as follows:

---

Messages are held in operating system's network buffers. I have found HWMs to be not that useful because of that. Here is modified code where subscriber misses messages:

import time
import pickle
import zmq
from threading import Thread
import os

ctx = zmq.Context()

def pub_thread():
    pub = ctx.socket(zmq.PUB)
    pub.setsockopt(zmq.SNDHWM, 2)
    pub.setsockopt(zmq.SNDBUF, 2*1024)  # See: http://api.zeromq.org/4-2:zmq-setsockopt
    pub.bind('tcp://*:5555')
    i = 0
    while True:
        time.sleep(0.001)
        pub.send_string(str(i), zmq.SNDMORE)
        pub.send(os.urandom(1024))
        i += 1

def sub_thread():
    sub = ctx.socket(zmq.SUB)
    sub.setsockopt(zmq.SUBSCRIBE, b'')
    sub.setsockopt(zmq.RCVHWM, 2)
    sub.setsockopt(zmq.RCVBUF, 2*1024)
    sub.connect('tcp://localhost:5555')
    while True:
        time.sleep(0.1)
        msg, _ = sub.recv_multipart()
        print("Received:", msg.decode())

t_pub = Thread(target=pub_thread)
t_pub.start()
sub_thread()

---

Output looks something like this:

Received: 0
Received: 1
Received: 2
Received: 3
Received: 4
Received: 5
Received: 6
Received: 47
Received: 48
Received: 64
Received: 65
Received: 84
Received: 85
Received: 159
Received: 160
Received: 270

---

Messages are missed because all queues/buffers are full and publisher starts to drop messages (see documentation for ZMQ_PUB: http://api.zeromq.org/4-2:zmq-socket).

---

[NOTE]:

• You should use the high-water mark option in listener/subscriber and advertiser/publisher.
• These posts are also relevant (Post1 - Post2)
• sock.setsockopt(zmq.CONFLATE, 1) is another option to get the last message only which defined in subscriber side.