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

Yannick Hold-Geoffroy yannickhold at gmail.com
Thu Jul 25 22:05:53 CEST 2013

Hello Justin,

Is there a way to monitor the drop(s) from a router socket?

My first question was exactly about building something on top of a
router socket to know which connections are already made. But that
would involve adding an arbitrary sleep before sending data, which I
can't call reliable nor efficient.

Can an exception be raised when the socket drop a message?

Yannick Hold

>* 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.

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