I tried sending the following over email, but I guess it didn't post 
correctly.  Apologies for spam if this is your second time seeing my email.

> What you are likely running into is a failure to propagate subscriptions 
on the PUB/SUB channel.

Thanks, this was spot on.  I added a millisecond wait time and that fixed 
the problem (as fixed as I cared about for a toy project).

Now I'm curious, though.  I see that there's a ZMQ Socket 
option ZMQ_INVERT_MATCHING which would cause the PUB socket to push all 
messages to all new SUB connections, iiuc.  This behavior also seems to fit 
the Jupyter messaging model as, per the docs, the IOPub topics are 
irrelevant and completely ignored because frontends just subscribe to all 

If I wanted to hack the Jupyter kernel code to set ZMQ_INVERT_MATCHING on 
the IOPub PUB side, where would I go do that?

On Thursday, February 8, 2018 at 2:49:47 PM UTC-8, Trevor Murphy wrote:
> I'm playing around with a hand-rolled jupyter client, mainly to learn ZMQ 
> and the jupyter messaging protocol.
> My "client" starts a jupyter console and then starts sending messages to 
> the shell channel / receiving from the shell and iopub channels.
> I notice that, often, I'm unable to read from the shell and iopub channels 
> if I send a message as soon as possible after the console program starts.  
> Like, my code does:
>   1. shell out a jupyter console, wait for return
>   2. poll the shell socket's ZMQ_EVENTS option until ZMQ_POLLOUT is true 
> (incidentally, this always succeeds on the first poll)
>   3. send a message to the shell socket
>   4. poll the shell and iopub sockets' ZMQ_EVENTS options until ZMQ_POLLIN 
> is true (both polls spin forever).
> If I make the code wait for a few seconds, the send and receive roundtrip 
> completes just fine, a/k/a the polls at (4) succeed and my code continues.
> Can anybody point me to debug this?  I tried starting the console with 
> --log-level=DEBUG but none of the messages talk about what's going on at 
> this level.
> Or, am I just doing the poll wrong?  I would've though it would be fine to 
> send when ZMQ_POLLOUT is true, but it looks like the kernel isn't receiving 
> my message in this case.

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