Excerpts from Mark McLoughlin's message of 2014-09-12 03:27:42 -0700:
> On Wed, 2014-09-10 at 14:51 +0200, Thierry Carrez wrote:
> > Flavio Percoco wrote:
> > > [...]
> > > Based on the feedback from the meeting[3], the current main concern is:
> > > 
> > > - Do we need a messaging service with a feature-set akin to SQS+SNS?
> > > [...]
> > 
> > I think we do need, as Samuel puts it, "some sort of durable
> > message-broker/queue-server thing". It's a basic application building
> > block. Some claim it's THE basic application building block, more useful
> > than database provisioning. It's definitely a layer above pure IaaS, so
> > if we end up splitting OpenStack into layers this clearly won't be in
> > the inner one. But I think "IaaS+" basic application building blocks
> > belong in OpenStack one way or another. That's the reason I supported
> > Designate ("everyone needs DNS") and Trove ("everyone needs DBs").
> > 
> > With that said, I think yesterday there was a concern that Zaqar might
> > not fill the "some sort of durable message-broker/queue-server thing"
> > role well. The argument goes something like: if it was a queue-server
> > then it should actually be built on top of Rabbit; if it was a
> > message-broker it should be built on top of postfix/dovecot; the current
> > architecture is only justified because it's something in between, so
> > it's broken.
> > 
> > I guess I don't mind that much zaqar being "something in between":
> > unless I misunderstood, exposing extra primitives doesn't prevent the
> > "queue-server" use case from being filled. Even considering the
> > message-broker case, I'm also not convinced building it on top of
> > postfix/dovecot would be a net win compared to building it on top of
> > Redis, to be honest.
> AFAICT, this part of the debate boils down to the following argument:
>   If Zaqar implemented messaging-as-a-service with only queuing 
>   semantics (and no random access semantics), it's design would 
>   naturally be dramatically different and "simply" implement a 
>   multi-tenant REST API in front of AMQP queues like this:
>     https://www.dropbox.com/s/yonloa9ytlf8fdh/ZaqarQueueOnly.png?dl=0
>   and that this architecture would allow for dramatically improved 
>   throughput for end-users while not making the cost of providing the 
>   service prohibitive to operators.
> You can't dismiss that argument out-of-hand, but I wonder (a) whether
> the claimed performance improvement is going to make a dramatic
> difference to the SQS-like use case and (b) whether backing this thing
> with an RDBMS and multiple highly available, durable AMQP broker
> clusters is going to be too much of a burden on operators for whatever
> performance improvements it does gain.

Having had experience taking queue-only data out of RDBMS's and even SMTP
solutions, and putting them into queues, I can say that it was generally
quite a bit more reliable and cheaper to maintain.

However, as I've been thinking about this more, I am concerned about the
complexity of trying to use a stateless protocol like HTTP for reliable
delivery, given that these queues all use a session model that relies
on connection persistence. That may very well invalidate my hypothesis.

> But the troubling part of this debate is where we repeatedly batter the
> Zaqar team with hypotheses like these and appear to only barely
> entertain their carefully considered justification for their design
> decisions like:
> https://wiki.openstack.org/wiki/Frequently_asked_questions_%28Zaqar%29#Is_Zaqar_a_provisioning_service_or_a_data_API.3F
> https://wiki.openstack.org/wiki/Frequently_asked_questions_%28Zaqar%29#What_messaging_patterns_does_Zaqar_support.3F
> I would like to see an SQS-like API provided by OpenStack, I accept the
> reasons for Zaqar's design decisions to date, I respect that those
> decisions were made carefully by highly competent members of our
> community and I expect Zaqar to evolve (like all projects) in the years
> ahead based on more real-world feedback, new hypotheses or ideas, and
> lessons learned from trying things out.

I have read those and I truly believe that the Zaqar team, who I believe
are already a valuable part of the OpenStack family, are doing good work.
Seriously, I believe it is valuable as is and I trust them to do what
they have stated they will do.

Let me explain my position again. Heat is in dire need of a way to
communicate with instances that is efficient. It has no need for a full
messaging stack.. just a way for users to have things pushed from Heat
to their instances efficiently.

So, to reiterate why I keep going on about this: If a messaging service
is to become an integrated part of OpenStack's release, we should think
carefully about the ramifications for operators _and_ users of not
having a light weight queue-only option, when that seems to fit _most_
of the use cases.

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