On 09/09/2014 09:03 PM, Monty Taylor wrote:
> On 09/04/2014 01:30 AM, Clint Byrum wrote:
>> 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
>> graduate.
>>> # 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.
> It seems like you're EITHER describing something called XMPP that has at
> least one open source scalable backend called ejabberd. OR, you've
> actually hit the nail on the head with bringing up SMTP and IMAP but for
> some reason that feels strange.
> SMTP and IMAP already implement every feature you've described, as well
> as retries/failover/HA and a fully end to end secure transport (if
> installed properly) If you don't actually set them up to run as a public
> messaging interface but just as a cloud-local exchange, then you could
> get by with very low overhead for a massive throughput - it can very
> easily be run on a single machine for Sean's simplicity, and could just
> as easily be scaled out using well known techniques for public cloud
> sized deployments?
> So why not use existing daemons that do this? You could still use the
> REST API you've got, but instead of writing it to a mongo backend and
> trying to implement all of the things that already exist in SMTP/IMAP -
> you could just have them front to it. You could even bypass normal
> delivery mechanisms and do neat things with local injection.
> I don't care about the NoSQL question on its own. Mongo is fine. Redis
> is fine. I don't think either has any features for this use case that
> make a licks worth of difference compared to MySQL or Postgres, but I
> also don't think they are a PROBLEM in an of themselves.
> The main thing I care about here is every description I've heard of what
> zaqar wants to do (which does seem to be getting clearer through this
> thread) is still well implemented somewhere as an existing scalable
> service. Is zaqar actually Rabbit with a REST interface? Is it ejabberd
> with a rest interface? Or is it IMAP/SMTP with a REST interface. You'll
> note that probably nobody would think a single server that wanted to be
> both Rabbit AND IMAP/SMTP is a good idea ... at least this is one of the
> reasons why we all think Microsoft Exchange is a pile of garbage, no?
> I also worry about the fact that one description of zaqar was used to
> communicate a need for divergent requirements (it needs to be a
> high-volume fast message broker/queue - which, btw, sounds more like
> Rabbit/oslo.messaging and less like what Clint describes above) ... and
> that's why it wants to use falcon and not pecan and why it wants to use
> mongo and not SQL. And then what we're doing it reimplementing something
> like rabbit except in python (again, given as the justification for
> deviating from how other bits of OpenStack work)
> BUT - if that's not actually what zaqar is - if it isn't a rabbit
> replacement and doesn't need to do massive high volume sub-second
> queuing because what it's actually modeling is a message subscription
> service that's closer to email than to anything else, then there is
> nothing about the components that are happily used in the rest of
> OpenStack that should be precluded from being used. A REST api written
> in pecan should be fine ... as should an SQL backend, because 99% of all
> operations are going to be primary key lookups where even a moderately
> tuned database should be absolutely fine at keeping up.
> So which is it? Because it sounds like to me it's a thing that actually
> does NOT need to diverge in technology in any way, but that I've been
> told that it needs to diverge because it's delivering a different set of
> features - and I'm pretty sure if it _is_ the thing that needs to
> diverge in technology because of its feature set, then it's a thing I
> don't think we should be implementing in python in OpenStack because it
> already exists and it's called AMQP.
>>> 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.


(branching off from the old thread on the email that contains
information about the current concern related to Zaqar)

Last night, the TC held another meeting to evaluate Zaqar for
graduation. In the meeting, new concerns arose based on the email above.

To clarify the doubts of what Zaqar is or it's not, let me quote what's
written in the project's overview section[0]:

        "Zaqar is a multi-tenant cloud messaging service for web developers. It
combines the ideas pioneered by Amazon's SQS product with additional
semantics to support event broadcasting.

        The service features a fully RESTful API, which developers can use to
send messages between various components of their SaaS and mobile
applications, by using a variety of communication patterns. Underlying
this API is an efficient messaging engine designed with scalability and
security in mind.

        Other OpenStack components can integrate with Zaqar to surface events
to end users and to communicate with guest agents that run in the
"over-cloud" layer. Cloud operators can leverage Zaqar to provide
equivalents of SQS and SNS to their customers."

To be fair, we failed to update all the wiki pages[1] and even the
program's mission. These mentions of `queue` and the `Q` in `SQS` have
created lots of confusion about what Zaqar is aiming to do - we'll
obviously update the wiki and the program. However, the project's goals
have never changed and every feature that has been implemented aims to
serve for specific use-cases related to the project's goals.

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?

As I've mentioned in another email, I collected use-cases from folks
around our community and outside it. You can read the under-cloud
use-cases here[4] and the over-cloud use cases here[5].

I must admit that last night's meeting was *really* confusing so, if
there are other concerns besides the one I just mentioned, by all means,
add it to this thread.

I'd love to hear feedback from everyone. However, if your feedback is
not strictly related to the technical aspects of the project or the
relation between the project and OpenStack, please start a new thread
and avoid adding meta-discussions to this one.


[0] https://wiki.openstack.org/wiki/Zaqar#Overview
[1] https://wiki.openstack.org/wiki/Zaqar#Key_features
[4] https://etherpad.openstack.org/p/zaqar-integrated-projects-use-cases
[5] https://etherpad.openstack.org/p/zaqar-overcloud-use-cases

Flavio Percoco

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