[zeromq-dev] Router socket messages lost when too quick after connection and how to know if ready to send

[Justin Karneges]

On 07/25/2013 12:28 PM, Yannick Hold-Geoffroy wrote:
> Hello everyone,
>
> I've got two questions about router sockets.
>
> First, it seems I must wait for a while after connecting a router socket
> before sending data to its newly connected destination using pyzmq.
>
> This simple use case <http://pastebin.com/kPK2ZAc5> shows the problem on
> my system using pyzmq 13.0.0 with zeromq 3.2.2 (Arch Linux, kernel
> 3.9.9, GCC 4.8.1): Most of the time it runs correctly and terminates,
> but sometimes it prints up to around 900 and stall there. Note that
> adding a sleep before sending (I tried with ~50ms) always make the code
> works.
>
> The router socket always exhibits its POLLOUT flag even if the
> connection isn't ready to send to a newly connected peer. Is there a way
> to ask the zeromq socket when it's ready to accept messages to a given
> host without dropping them, aside from manually specifying a sleep?
>
>
> My second question is about sending. Still using a router socket, you
> could set as destination anything you'd like (even if the socket isn't
> currently connected to a peer with this identity). Is there a way to
> know if a connection is currently available for sending (ie. the message
> will be sent immediately, like POLLOUT for a given peer)?
>
> I've tried setting the socket's ZMQ_SNDTIMEO to zero and try sending
> using the ZMQ_NOBLOCK to see if it would raise (in pyzmq) an ZMQ_EAGAIN
> error, but it always seems to pass without problem even when there is
> nobody listening. A minimal code exhibiting my comprehension of how it
> should work can be found here <http://pastebin.com/zQE2Awt4>. Is there
> something I haven't understood about how it's supposed to work?

Hi Yannick,

The simple answer is that ROUTER has a drop policy, not a blocking one. 
And there is not a way to know with pure 0mq whether or not there is a 
recipient available. That is something you would need to build on top.

Justin