[openstack-dev] [Marconi] Why is marconi a queue implementation vs a provisioning API?
joe.gordon0 at gmail.com
Thu Mar 20 00:58:09 UTC 2014
On Wed, Mar 19, 2014 at 12:37 PM, Devananda van der Veen <
devananda.vdv at gmail.com> 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.
> 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
storage = mongodb
uri = mongodb://localhost:27017/marconi
On an related note I see that marconi has no gating integration tests.
But then again that is documented in
We have a devstack-gate job running and will be making it voting this week.
Of the non-gating integration test job, I only see one marconi test being
> 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:
> 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...
> 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
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