[zeromq-dev] HWM behaviour & blocking

Steffen Mueller zeromq at steffen-mueller.net
Wed May 9 18:29:59 CEST 2012

Hi Chuck,

On 05/09/2012 03:57 PM, Chuck Remes wrote:
> On May 9, 2012, at 12:45 AM, Steffen Mueller wrote:
>> I have an application of 0MQ where I have one or multiple server
>> instances that use a PUSH socket to send messages that must be
>> processed by any one of potentially many workers. The servers are
>> single-threaded apart from 0MQ's IO thread and this is hard to
>> change.
>> The documentation for the PUSH socket type (and others like XREQ)
>> explain that sending messages on such a socket will be a blocking
>> operation IF
>> - the HWM is hit - OR there are no peers
>> In my scenarios, I want to be resilient against intermittent
>> client failure (due to whatever -- coordinated restart, failure,
>> ...). But for this time, the server processes will block on the
>> write and that is not acceptable. Is it reasonable to hope for a
>> way to achieve the following behaviour?
>> Sending a messages down a socket of this time will block if:
>> - the HWM is hit for all peers (as per docs, presumably, this
>> means all and each separately since the buffers are probably
>> per-client) - OR a single global HWM is hit if there are no peers
>> IOW, I'd like to be able to shove up to $HWM messages down a pipe
>> no matter what, and have the PUSH/whatever socket use a single
>> $HWM-depth buffer if there is no peer. As soon as a peer connects,
>> it could do one of two things:
>> - simply swap that buffer in to become the first connecting peer's
>> send queue (which might be undesirable in some cases since it
>> doesn't load balance but it's likely much easier to implement and
>> more efficient) - use that buffer as another queue stage to load
>> balance from
>> Any chance I could have such a functionality? Of course, being able
>> to determine whether there are any peers connected would be great,
>> too.
> I recommend that you do non-blocking writes and check the return
> code. If zmq_errno is equal to EAGAIN, then you know that you have
> either hit HWM or that there are no peers.

thanks for your advice.

Alas, I can't see how that would help me with the particular issue I 
have. In a nutshell, I want to have the HWM apply also while there's no 
listeners. Right now, I'd have to do non-blocking writes and implement 
my own buffering. In particular when you include multi-frame messages 
into the picture, that's just a lot of silly effort. Thus, I am asking 
whether having a mode of operation or socket type in 0MQ where 0MQ does 
buffering and applies HWM even without currently connected peers.

Furthermore, "there are no peers or HWM hit" doesn't help me AT ALL in 
this scenario since I am attempting to *distinguish* between the two 
scenarios. So even if I was to implement my own message queuing in the 
application (yikes, really?), I would essentially do double buffering if 
the HWM is hit since it's not distinguishable from having no peers. IOW, 
what I want is currently NOT POSSIBLE AT ALL with 0MQ unless I'm missing 


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