Excerpts from Flavio Percoco's message of 2014-09-04 00:08:47 -0700:
> Greetings,
> Last Tuesday the TC held the first graduation review for Zaqar. During
> the meeting some concerns arose. I've listed those concerns below with
> some comments hoping that it will help starting a discussion before the
> next meeting. In addition, I've added some comments about the project
> stability at the bottom and an etherpad link pointing to a list of use
> cases for Zaqar.

Hi Flavio. This was an interesting read. As somebody whose attention has
recently been drawn to Zaqar, I am quite interested in seeing it

> # Concerns
> - Concern on operational burden of requiring NoSQL deploy expertise to
> the mix of openstack operational skills
> For those of you not familiar with Zaqar, it currently supports 2 nosql
> drivers - MongoDB and Redis - and those are the only 2 drivers it
> supports for now. This will require operators willing to use Zaqar to
> maintain a new (?) NoSQL technology in their system. Before expressing
> our thoughts on this matter, let me say that:
>     1. By removing the SQLAlchemy driver, we basically removed the chance
> for operators to use an already deployed "OpenStack-technology"
>     2. Zaqar won't be backed by any AMQP based messaging technology for
> now. Here's[0] a summary of the research the team (mostly done by
> Victoria) did during Juno
>     3. We (OpenStack) used to require Redis for the zmq matchmaker
>     4. We (OpenStack) also use memcached for caching and as the oslo
> caching lib becomes available - or a wrapper on top of dogpile.cache -
> Redis may be used in place of memcached in more and more deployments.
>     5. Ceilometer's recommended storage driver is still MongoDB, although
> Ceilometer has now support for sqlalchemy. (Please correct me if I'm wrong).
> That being said, it's obvious we already, to some extent, promote some
> NoSQL technologies. However, for the sake of the discussion, lets assume
> we don't.
> I truly believe, with my OpenStack (not Zaqar's) hat on, that we can't
> keep avoiding these technologies. NoSQL technologies have been around
> for years and we should be prepared - including OpenStack operators - to
> support these technologies. Not every tool is good for all tasks - one
> of the reasons we removed the sqlalchemy driver in the first place -
> therefore it's impossible to keep an homogeneous environment for all
> services.

I whole heartedly agree that non traditional storage technologies that
are becoming mainstream are good candidates for use cases where SQL
based storage gets in the way. I wish there wasn't so much FUD
(warranted or not) about MongoDB, but that is the reality we live in.

> With this, I'm not suggesting to ignore the risks and the extra burden
> this adds but, instead of attempting to avoid it completely by not
> evolving the stack of services we provide, we should probably work on
> defining a reasonable subset of NoSQL services we are OK with
> supporting. This will help making the burden smaller and it'll give
> operators the option to choose.
> [0] http://blog.flaper87.com/post/marconi-amqp-see-you-later/
> - Concern on should we really reinvent a queue system rather than
> piggyback on one
> As mentioned in the meeting on Tuesday, Zaqar is not reinventing message
> brokers. Zaqar provides a service akin to SQS from AWS with an OpenStack
> flavor on top. [0]

I think Zaqar is more like SMTP and IMAP than AMQP. You're not really
trying to connect two processes in real time. You're trying to do fully
asynchronous messaging with fully randomized access to any message.

Perhaps somebody should explore whether the approaches taken by large
scale IMAP providers could be applied to Zaqar.

Anyway, I can't imagine writing a system to intentionally use the
semantics of IMAP and SMTP. I'd be very interested in seeing actual use
cases for it, apologies if those have been posted before.

> Some things that differentiate Zaqar from SQS is it's capability for
> supporting different protocols without sacrificing multi-tenantcy and
> other intrinsic features it provides. Some protocols you may consider
> for Zaqar are: STOMP, MQTT.
> As far as the backend goes, Zaqar is not re-inventing it either. It sits
> on top of existing storage technologies that have proven to be fast and
> reliable for this task. The choice of using NoSQL technologies has a lot
> to do with this particular thing and the fact that Zaqar needs a storage
> capable of scaling, replicating and good support for failover.

What's odd to me is that other systems like Cassandra and Riak are not
being discussed. There are well documented large scale message storage
systems on both, and neither is encumbered by the same licensing FUD
as MongoDB.

Anyway, again if we look at this as a place to storage and retrieve
messages, and not as a queue, then talking about databases, instead of
message brokers, makes a lot more sense.

> - concern on the maturity of the NoQSL not AGPL backend (Redis)
> Redis backend just landed and I've been working on a gate job for it
> today. Although it hasn't been tested in production, if Zaqar graduates,
> it still has a full development cycle to be tested and improved before
> the first integrated release happens.

I'd be quite interested to see how it is expected to scale. From my very
quick reading of the driver, it only supports a single redis server. No
consistent hash ring or anything like that.

> # Use Cases
> In addition to the aforementioned concerns and comments, I also would
> like to share an etherpad that contains some use cases that other
> integrated projects have for Zaqar[0]. The list is not exhaustive and
> it'll contain more information before the next meeting.
> [0] https://etherpad.openstack.org/p/zaqar-integrated-projects-use-cases

Just taking a look, there are two basic applications needed:

1) An inbox. Horizon wants to know when snapshots are done. Heat wants
to know what happened during a stack action. Etc.

2) A user-focused message queue. Heat wants to push data to agents.
Swift wants to synchronize processes when things happen.

To me, #1 is Zaqar as it is today. #2 is the one that I worry may not
be served best by bending #1 onto it.

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