[zeromq-dev] HWM behaviour & blocking

[Steffen Mueller]

Dear all,

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.

--Steffen