ZeroMQ XPUB/XSUB Serious Flaw?

Question

It seems as though the XPUB/XSUB socket types have a serious flaw that is difficult to work around:

This is my implementation of that center node:

#include <zmq.hpp>

int main()
{
    zmq::context_t context(1);

    //Incoming publications come here
    zmq::socket_t sub(context, ZMQ_XSUB);
    sub.bind("ipc://subscriber.ipc");

    //Outgoing publications go out through here.
    zmq::socket_t pub(context, ZMQ_XPUB);
    pub.bind("ipc://publisher.ipc");

    zmq::proxy(sub, pub, nullptr);

    return 0;
}

The problem is, of course, slow joiner syndrome. If I connect a new publisher to XSUB and publish some messages, they disappear into the void:

#include "zhelpers.hpp"

int main () {
    //  Prepare our context and publisher
    zmq::context_t context(1);
    zmq::socket_t publisher(context, ZMQ_PUB);
    publisher.connect("ipc://subscriber.ipc");

    s_sendmore (publisher, "B");
    s_send (publisher, "Disappears into the void!!");

    return 0;
}

However, if I sleep(1) after connecting to XSUB, it magically works:

#include "zhelpers.hpp"

int main () {
    //  Prepare our context and publisher
    zmq::context_t context(1);
    zmq::socket_t publisher(context, ZMQ_PUB);
    publisher.connect("ipc://subscriber.ipc");

    sleep(1);
    s_sendmore (publisher, "B");
    s_send (publisher, "Magically works!!");

    return 0;
}

The Guide claims there is a simple solution to this "slow joiners" syndrome, but then never delivers a working synchronized XSUB/XPUB implementation. After much searching it looks like most people are just sleeping, which is really bad.

Why hasn't this ever been fixed? Are there any known workarounds? All my google queries just point me back to the guide...

Answer 1

I found one workaround here, and that is to use PUSH/PULL on the publisher side, and PUB/SUB on the subscriber side. The new topology looks like this:

This is the code you need for the center node:

#include <zmq.hpp>

int main()
{
    zmq::context_t context(1);

    //Incoming publications come here
    zmq::socket_t sub(context, ZMQ_PULL);
    sub.bind("ipc://subscriber.ipc");

    //Outgoing publications go out through here.
    zmq::socket_t pub(context, ZMQ_PUB);
    pub.bind("ipc://publisher.ipc");

    zmq::proxy(sub, pub, nullptr);

    return 0;
}

And then for publishers:

#include "zhelpers.hpp"

int main () {
    //  Prepare our context and publisher
    zmq::context_t context(1);
    zmq::socket_t publisher(context, ZMQ_PUSH);
    publisher.connect("ipc://subscriber.ipc");

    s_sendmore (publisher, "B");
    s_send (publisher, "No sleep!");

    return 0;
}

This solution seems to work fairly well, and I hope people chime in if they see any downsides to it. If I come across a better answer, I'll post here.

Answer 2

The downside is your publishers are always publishing. In the XSUB/XPUB case, subscriptions are forwarded to your publishers so that they can restrict what they are sending to the proxy. That results in less network traffic and less work for the proxy. In the PULL/PUB case, the publishers never see the subscription information. A worst case scenario would be a subscriber's subscription means they only want data from one publisher. All publishers would still be sending their data to the proxy and the proxy would filter out everything but the one publisher's messages.