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On 09/16/2014 08:55 AM, Flavio Percoco wrote:
> pub/sub doesn't necessarily guarantees messages delivery, it really
> depends on the implementation.

As I understand it, the model for pub-sub in Zaqar is to have multiple 
subscribers polling the queue with gets, and have the messages removed 
from the queue only when they expire. Is that right?

If the ttl of the messages is long enough, a subscriber can start 
getting the queue from where they left off (if they have or can recover 
their last used marker) or from the head of the queue.

So although not acknowledged, subscribers can retry on failover 
providing they do so before the message expires.

> That said, there are ways to guarantee
> that depending on the method used. For example, if the subscriber is a
> webhook, we can use the response status code to ack the message. if it
> has a persistent connection like websocket or even (long|short)-poll an
> ack may be needed.

In the pub-sub case, to remove a message based on acks you need to wait 
until all known subscribers have acked it. With the current model there 
is no explicit concept of subscriber (nor of ack in the non-competing 
consumer case). Without changing that I don't think you can use the 
response of a webhook anyway (unless of course there are not get style 
subscribers on the queue).