[zeromq-dev] Some clarifications about HWM for pub/sub

Emmanuel TAUREL taurel at esrf.fr
Mon Nov 28 17:55:41 CET 2011

Hi Chuck,

On 28/11/2011 17:26, Chuck Remes wrote:
> On Nov 28, 2011, at 10:19 AM, Emmanuel TAUREL wrote:
>> Hello all,
>> I am using ZMQ 3.0.x on linux boxes with  the PUB/SUB pattern.
>> I have one publisher and two subscribers. I have a HWM set on the publisher to 50 and no HWM on the subscribers.
>> When the HWM limit is reached on the publisher, following messages are dropped on the publisher side
>> and both subscribers do not receive them.
>> "man zmq_setsockopt" tell me this for the ZMQ_SNDHWM option:
>> -------------------
>> The high water mark is a hard limit
>> on the maximum number of outstanding messages 0MQ shall queue in memory
>> for any single peer that the specified socket is communicating with.
> The part that says "for any single peer" means that each peer has its own SNDHWM enforcement.
>> -------------------
>> Do I have to understand that all peers are affected when the pub HWM is reached?
> No. Only the peer that has reached the limit is affected. Other peers that have not reached the limit should not see any message drops.

But this is not what I see on wireshark and on my terminal windows!
When the HWM is reached, messages are dropped for all peers (at least 
for the 2 I have in my tests).
I don't see them on the wire on both connections (connection1 with port1 
for sub1 and connection2 with port2 for sub2) and my two
applications using the subscribers complain.

Therefore, I am a bit confused and I don't really know what should be 
the wanted behavior.


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