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copied!<p>DISCLAIMER: I didn't really use ZeroMQ before</p> <p>At first note two things: </p> <ol> <li>Server prints "Receiving..." even if client.pl was not started yet. This means that io watcher callback is called even though nothing was sent to this socket yet</li> <li>In server if you substitute line setting up IO watcher (AE::io) with simple while(1) then server/client work correctly. This implies that IO callback is <em>not</em> fired correctly when we supposedly receive message from client (and this makes it "hang").</li> </ol> <p>I'm pretty sure you are running into issues described in <a href="http://funcptr.net/2012/09/10/zeromq---edge-triggered-notification/" rel="nofollow">http://funcptr.net/2012/09/10/zeromq---edge-triggered-notification/</a>. In short:</p> <ol> <li><p>Even if zmq filehandle is notified about being ready to read it does <em>not</em> mean there is message waiting to be received.</p> <p>Callback should check if this is not "false positive".</p> <pre><code>unless (ZMQ_POLLIN & zmq_getsockopt( $responder, ZMQ_EVENTS ) ) { say "Nothing to recv from socket, skipping"; return; } </code></pre> <p>at the beginning of callback seems to do the trick. But I don't think that is critical here. This problem just makes you call zmq_recv before data is there, so it makes your program more blocking</p></li> <li><p>IO watcher may be only called once even if multiple messages are received. This is described nicely in article linked above in "ZeroMQ uses edge triggered" paragraph. That is why you <em>should</em> loop inside callback until no more messages are there. You can do it in a non-blocking way by:</p> <pre><code>while (my $msg = zmq_recvmsg($responder,ZMQ_DONTWAIT)) { say "Received request: [".zmq_msg_data($msg)."]"; zmq_msg_send('World', $responder); } </code></pre> <p>This loop will process all messages there are pending and leave if there are no more.</p> <p><strong>UPDATE</strong> linked article suggests receiving messages in a loop while getsockopt for ZMQ_EVENTS reports that ZMQ_POLLIN is set. This seems more elegant to me. I didn't test it and don't know if there are practical differences.</p> <p>I have no idea <strong>how</strong> in your scenario multiple messages might be received by server on the same time, but that code change above alone seems to fix the problem. Possibly I don't know enough ZeroMQ. I'd be happy if someone can explain the "why" in this specific case to me.</p></li> </ol>

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