[zeromq-dev] Identities & PUB/SUB

[Pieter Hintjens]

On Fri, Apr 8, 2011 at 8:11 AM, Martin Sustrik <sustrik at 250bpm.com> wrote:

> If that's you use case, it should be solved inside 0mq by providing acks in
> push/pull model. That would allow us to limit number of messages on the
> flight (HWM) to particular worker to an exact number (even though the
> network buffers are not yet filled).

Doing acks in sockets is a great idea. It raises many questions, I'm
sure you've thought of already:

* When does the socket ack, when it receives the message, or when it
delivers it to the application, or when the application has finished
processing it? Only the last actually solves the use case.
* Do you ack every single message (slow) or do you allow acking of
multiple messages?
* If you allow ack of multiple messages, how do you do that? Do you
number messages? Who numbers them? Where?
* If it's the application that acks messages, what is the API for
this? Another zmq method?
* Do you allow nack? If so, do you allow nack-drop vs. nack-requeue?
* If you allow nack, do you detect poison payloads? I.e. messages that
kill consumers.
* If you allow redelivery, do you have any indicator that a message is
'redelivered'? If so, where?
* Do you allow consumers to control the flow, i.e. stop and restart
getting messages? Or can they only do this by closing their sockets?
What if they are connected to multiple producers?
* If you don't implement real-world ack/nacking like this, do you ban
0MQ for applications that need it?

-Pieter