[zeromq-dev] will receive fail if TCP goes down in REQ socket

Justin Karneges justin at affinix.com
Sat Dec 14 02:46:25 CET 2013

ZeroMQ does not resend messages, so while the reconnect/queuing logic 
will protect you to some degree, you still need to account for message loss.

If you're using REQ then you'll need to timeout the request, otherwise 
if a request or response message is lost then you'll never be able to 
make a request on the socket ever again. So don't just indefinitely 
block on a send or receive. Further, ZeroMQ historically hasn't had a 
way to get a REQ socket out of the "waiting for response" state in the 
event of a timeout other than by closing the socket and starting over. 
This means the REQ type is not really usable in production. Better to 
use DEALER and format a REQ-compatible message yourself. REP does not 
suffer from these problems, so you can keep on using that and have 
DEALER talk to REP.

Note: it is possible that very recent versions of ZeroMQ allow REQ 
sockets to revert state on error but I haven't been following this closely.

On 12/13/2013 04:17 PM, Sean Robertson wrote:
> I believe the REQ will simply wait for the REP to come back up,
> re-bind and send something.
> On Fri, Dec 13, 2013 at 2:53 PM, Mohit Jaggi <mohitjaggi at gmail.com> wrote:
>> Hi,
>> In most ZMQ sockets, the underlying TCP socket can change transparently.
>> Does that apply to REQ-REP sockets as well? Or will the receive call to ZMQ
>> socket fail?
>> sock = new REQ socket;
>> connect(sock);
>> while(1) {
>> request = receive(...);
>> ...
>> send(response);
>> }
>> For example in the above code, let us say that the server with REP socket on
>> the other side crashed and restarted. What will happen?
>> Thanks,
>> Mohit.
>> _______________________________________________
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>> zeromq-dev at lists.zeromq.org
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