Open Stack

On Wed, 2014-03-19 at 12:37 -0700, Devananda van der Veen wrote:
> Let me start by saying that I want there to be a constructive discussion
> around all this. I've done my best to keep my tone as non-snarky as I could
> while still clearly stating my concerns. I've also spent a few hours
> reviewing the current code and docs. Hopefully this contribution will be
> beneficial in helping the discussion along.

Thanks, I think it does.

> For what it's worth, I don't have a clear understanding of why the Marconi
> developer community chose to create a new queue rather than an abstraction
> layer on top of existing queues. While my lack of understanding there isn't
> a technical objection to the project, I hope they can address this in the
> aforementioned FAQ.
> 
> The reference storage implementation is MongoDB. AFAIK, no integrated
> projects require an AGPL package to be installed, and from the discussions
> I've been part of, that would be a show-stopper if Marconi required
> MongoDB. As I understand it, this is why sqlalchemy support was required
> when Marconi was incubated. Saying "Marconi also supports SQLA" is
> disingenuous because it is a second-class citizen, with incomplete API
> support, is clearly not the recommended storage driver, and is going to be
> unusuable at scale (I'll come back to this point in a bit).
> 
> Let me ask this. Which back-end is tested in Marconi's CI? That is the
> back-end that matters right now. If that's Mongo, I think there's a
> problem. If it's SQLA, then I think Marconi should declare any features
> which SQLA doesn't support to be optional extensions, make SQLA the
> default, and clearly document how to deploy Marconi at scale with a SQLA
> back-end.
> 
> 
> Then there's the db-as-a-queue antipattern, and the problems that I have
> seen result from this in the past... I'm not the only one in the OpenStack
> community with some experience scaling MySQL databases. Surely others have
> their own experiences and opinions on whether a database (whether MySQL or
> Mongo or Postgres or ...) can be used in such a way _at_scale_ and not fall
> over from resource contention. I would hope that those members of the
> community would chime into this discussion at some point. Perhaps they'll
> even disagree with me!
> 
> A quick look at the code around claim (which, it seems, will be the most
> commonly requested action) shows why this is an antipattern.
> 
> The MongoDB storage driver for claims requires _four_ queries just to get a
> message, with a serious race condition (but at least it's documented in the
> code) if multiple clients are claiming messages in the same queue at the
> same time. For reference:
> 
> https://github.com/openstack/marconi/blob/master/marconi/queues/storage/mongodb/claims.py#L119
> 
> The SQLAlchemy storage driver is no better. It's issuing _five_ queries
> just to claim a message (including a query to purge all expired claims
> every time a new claim is created). The performance of this transaction
> under high load is probably going to be bad...
> 
> https://github.com/openstack/marconi/blob/master/marconi/queues/storage/sqlalchemy/claims.py#L83
> 
> Lastly, it looks like the Marconi storage drivers assume the storage
> back-end to be infinitely scalable. AFAICT, the mongo storage driver
> supports mongo's native sharding -- which I'm happy to see -- but the SQLA
> driver does not appear to support anything equivalent for other back-ends,
> eg. MySQL. This relegates any deployment using the SQLA backend to the
> scale of "only what one database instance can handle". It's unsuitable for
> any large-scale deployment. Folks who don't want to use Mongo are likely to
> use MySQL and will be promptly bitten by Marconi's lack of scalability with
> this back end.
> 
> While there is a lot of room to improve the messaging around what/how/why,
> and I think a FAQ will be very helpful, I don't think that Marconi should
> graduate this cycle because:
> (1) support for a non-AGPL-backend is a legal requirement [*] for Marconi's
> graduation;
> (2) deploying Marconi with sqla+mysql will result in an incomplete and
> unscalable service.
> 
> It's possible that I'm wrong about the scalability of Marconi with sqla +
> mysql. If anyone feels that this is going to perform blazingly fast on a
> single mysql db backend, please publish a benchmark and I'll be very happy
> to be proved wrong. To be meaningful, it must have a high concurrency of
> clients creating and claiming messages with (num queues) << (num clients)
> << (num messages), and all clients polling on a reasonably short interval,
> based on what ever the recommended client-rate-limit is. I'd like the test
> to be repeated with both Mongo and SQLA back-ends on the same hardware for
> comparison.

My guess (and it's just a guess) is that the Marconi developers almost
wish their SQLA driver didn't exist after reading your email because of
the confusion it's causing. My understanding is that the SQLA driver is
not intended for production usage.

If Marconi just had a MongoDB driver, I think your points would boil
down to:

  "I don't have a clear understanding of why the Marconi developer
   community chose to create a new queue rather than an abstraction
   layer on top of existing queues"

  "no integrated projects require an AGPL package to be installed, and
   from the discussions I've been part of, that would be a show-stopper
   if Marconi required MongoDB"

  "whether a database like Mongo can be used in such a way _at_scale_
   and not fall over from resource contention. The MongoDB storage
   driver for claims requires _four_ queries just to get a message, with
   a serious race condition if multiple clients are claiming messages in
   the same queue at the same time."

I think the first two concerns have been expressed already, but the
third one is new. I've put my attempt at drafting a FAQ here:

  https://etherpad.openstack.org/p/draft-marconi-faq

and added the scalability concern to it.

Thanks,
Mark.