[zeromq-dev] Pipeline Reliability

Mark Botner mbotner at gmail.com
Tue Jul 3 17:35:31 CEST 2018

I wonder if the default setting for ZMQ_LINGER is causing the zmq_close()
to block since there are unsent messages?  From the ref. guide:

The default setting of *ZMQ_LINGER* does not discard unsent messages; this
behaviour may cause the application to block when calling *zmq_ctx_term()*.


On Tue, Jul 3, 2018 at 9:08 AM, Charles Bouillaguet <
charles.bouillaguet at gmail.com> wrote:

> Dear zeromq'ers,
> I'm facing a reliability problem that I couldn't solve by myself so far.
> I have two machines with two asymetric programs running. Machine A creates
> a
> PULL socket and binds it. Machine B creates a PUSH socket and connects it
> (to
> the PULL socket of machine A), using the TCP transport. Machine B then
> sends
> messages like crazy (about 500/s). Basically, B is an low-cost device
> equiped
> with sensors and A is a server that just stores the data.
> This works like a charm... until the inevitable happens: some network event
> occurs, and the messages cannot be transmitted from machine B to machine A.
> With a blocking send, the process on machine B then gets stuck in
> zmq_send(),
> once the high water mark is reached, and the whole pipeline grinds to a
> halt.
> To avoid this, I tried the "Lazy Pirate Pattern". I use something like:
>    if (-1 == zmq_send(socket, msg, size, ZMQ_DONTWAIT)) {
>       if (errno == EAGAIN) {
>          zmq_close(socket);
>          socket = zmq_socket(context, ZMQ_PUSH);
>          zmq_connect(socket, address);
>       }
>    }
> I don't care if I lose some messages. What I don't want is the pipeline to
> stop
> forever.
> At first, this seems to work as intended. When the network is down, the
> program
> actually closes and re-creates the socket; the call to zmq_connect()
> succeeds... but the messages are still not sent, and the process in
> machine B
> ends up in a loop where it fills the ZMQ buffers, destroy the socket,
> re-create
> it, re-connect, rinse, repeat. I observed the loop for several hours.
> Just stopping the UNIX process and re-starting it solved the problem
> (i.e. messages get transmitted normally, instantaneously).
> Is there something I am doing wrong? What are my options to avoid this
> problem?
> [I can consider moving away from ZMQ to nanomsg or nng].
> Thanks,
> --
> Université de Lille - Sciences et Technologies
> charles.bouillaguet at univ-lille1.fr | www.univ-lille1.fr
> Laboratoire CRIStAL - Bât M3 - Bureau 332 - 59655 Villeneuve d'Ascq
> Tél. +33 (0)3 28 77 85 84
> homepage:  http://cristal.univ-lille.fr/~bouillag/
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