ZMQ connect and push - how to determine if it fails

Question

I have a simple ZMQ program that establishes a zmq.PUSH socket, connects, and then tries to send messages.

import zmq

zcontext = zmq.Context()
zsock = zcontext.socket(zmq.PUSH)
zsock.connect("tcp://localhost:12345")

with open(sys.argv[1]) as f:
   for line in f:
       zsock.send(line)

This works fine when the other side is there. But if there's any problems with the listener on the other side (e.g. forgot to start listener, or I connected to wrong port), it just hangs after it tries to send about 1000 messages (depends on size of the default queue).

What's the right thing to do? If there's any issue with other side, I'd rather just print an error message and exit gracefully.

Did you try with the `flags=NOBLOCK`? – Benyamin Jafari Oct 29 '18 at 23:42 — Benyamin Jafari, Oct 29 '18 at 23:42

Benyamin Jafari · Accepted Answer · 2018-10-29T23:41:29.773

1

You can get zmq.error with the following snippet code:

import zmq
import time

zcontext = zmq.Context()
zsock = zcontext.socket(zmq.PUSH)
zsock.connect("tcp://localhost:12345")

try:
    with open(sys.argv[1]) as f:
        for line in f:
            zsock.send(line, flags=NOBLOCK)
            time.sleep(.1)

except zmq.ZMQError as exc:
    print(exc)

[NOTE]:

You can change the .connect() method with the .bind() in above code, and also change the other side bind to connect for a test case.
In .bind() method should be used 127.0.0.1 instead of localhost.

[UPDATE]:

Check the Limiting_ZeroMQ_Queue

With flags=NOBLOCK in .send() method, this raises ZMQError if the queue is full; otherwise, this waits until space is available. Thus, in this case, will be as the following:

zsock.send(line, flags=NOBLOCK)

edited Oct 29 '18 at 23:41

answered Oct 22 '18 at 21:51

Benyamin Jafari

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I don't think that works. Did you try it? It just tries to send it down the socket without throwing an exception – user3240688 Oct 22 '18 at 21:56
why not works?! if you have an error on this loop you will know it. – Benyamin Jafari Oct 22 '18 at 22:08
i tried it. picked a random port with nobody listening. it doesn't throw an exception – user3240688 Oct 23 '18 at 02:37
still hanged after 1000 itteration loop? – Benyamin Jafari Oct 23 '18 at 04:01
yes. i think 1000 might be the default queue size. it goes up to 1000, and then hangs. – user3240688 Oct 23 '18 at 12:04
@user3240688 check this [link](https://stackoverflow.com/a/52353685/3702377) and change the `SNDHWM` and `RCVHWM` from `1` to `2000` and remove the `sock.setsockopt(zmq.CONFLATE, 1)` – Benyamin Jafari Oct 23 '18 at 17:25
@user3240688 I updated my answer with `flags=NOBLOCK` argument. – Benyamin Jafari Oct 28 '18 at 10:42

ZMQ connect and push - how to determine if it fails

1 Answers1