Open Stack

On 09/09/14 15:03, 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 was intrigued by the idea of an ejabberd backend to Zaqar, so I spent 
half a morning yesterday investigating it. (tl;dr - it won't work.)

XMPP does have a sort-of standard for queueing messages when a client is 
offline[1], and ejabberd does support it[2]. Amusingly, it does so by 
storing the queue in a RDBMS (the very thing that the TC has repeatedly 
called an 'anti-pattern'). Unfortunately, ejabberd does _not_ support[2] 
the extension that would allow the Zaqar API to request messages one at 
a time (in arbitrary order, though that's not important here) out of the 
queue[3], so if I understand your proposal correctly every time the API 
polled ejabberd it could potentially receive a flood of messages that it 
would then have to reliably buffer itself (i.e. duplicating all the work 
that ejabberd was supposed to eliminate). In fact, XMPP is not designed 
to be reliable at all. There is an XMPP extension that could potentially 
offer reliable delivery via acks[4], although it's not entirely clear to 
me if that requires the participation of the client (i.e. effectively 
becomes synchronous messaging).

So, in summary, not a good fit because it doesn't match the #1 
requirement, which is to never lose messages while remaining asynchronous.

I can't figure out if the suggestion to use dovecot was actually serious.

[1] http://www.xmpp.org/extensions/xep-0160.html
[2] http://www.ejabberd.im/protocols
[3] http://xmpp.org/extensions/xep-0013.html
[4] http://xmpp.org/extensions/xep-0079.html


In any event, I think it's probably unhelpful to come at this from the 
angle of "which orange is the best one to compare this to, and please 
don't even talk to me about other apples". The thing Zaqar is most 
directly comparable to is not email or XMPP, it's SQS.

SQS offers a guarantee of delivering each message *at least* once.[5] It 
is optimised for durability rather than latency. It also tries to 
minimise multiple deliveries in the case where multiple clients are 
polling the same queue (e.g. a work queue).

Zaqar offers somewhat more complicated semantics[6]. I think we should 
discuss those semantics and agree on which are essential and which 
dispensable, rather than trying to compare it to things like IMAP. Once 
we have agreement on what the semantics should be, then we can sensibly 
discuss which back ends are capable of satisfying them.

[5] 
https://en.wikipedia.org/wiki/Amazon_Simple_Queue_Service#Message_delivery
[6] 
https://wiki.openstack.org/wiki/Zaqar/Frequently_asked_questions#What_messaging_patterns_does_Zaqar_support.3F

Zaqar obviously supports point-to-point queues, with one producer and 
one consumer. I assume it also supports many:1 and anycast 1:many & 
many:many queues - it could hardly fail to do so, since it doesn't 
actually know who the producers and consumers are. Hopefully it takes 
steps to ensure that multiple workers rarely receive the same message 
before it is acknowledged.

However, Zaqar also supports the Pub-Sub model of messaging. I believe, 
but would like Flavio to confirm, that this is what is meant when the 
Zaqar team say that Zaqar is about messaging in general and not just 
queuing. That is to say, it is possible for multiple consumers to 
intentionally consume the same message, with each maintaining its own 
pointer in the queue. (Another way to think of this is that messages can 
be multicast to multiple virtual queues, with data de-duplication 
between them.) To a relative novice in the field like me, the difference 
between this and queuing sounds pretty academic :P. Call it what you 
will, it seems like a reasonable thing to implement to me.

What's not clear to me is whether Zaqar supports a model where multiple 
different publish queues are somehow multiplexed together into each 
subscription queue with the subscribers able to look and determine which 
messages to receive and which not to. I do *not* think Zaqar supports 
that (but again, would like Flavio to confirm). I definitely think it 
would be a mistake if it did. And I think that this is the kind of thing 
that Clint is referring to with the IMAP analogy.

The final question is the one of arbitrary access to messages in the 
queue (or "queue" if you prefer). Flavio indicated that this effectively 
came for free with their implementation of Pub-Sub. IMHO it is 
unnecessary and limits the choice of potential back ends in the future. 
I would personally be +1 on removing it from the v2 API, and also +1 on 
the v2 API shipping in Kilo so that as few new adopters as possible get 
stuck with the limited choices of back-end. I hope that would resolve 
Clint's concerns that we need a separate, light-weight queue system; I 
personally don't believe we need two projects, even though I agree that 
all of the use cases I personally care about could probably be satisfied 
without Pub-Sub.

As Rob pointed out, one of the more obvious choices of back end for an 
API like the one I just described would be Apache Kafka. Unfortunately 
it is a massive Java application with Zookeeper dependencies, and we all 
know how Monty feels about those ;) (FWIW, I agree with him.) Given 
that's a non-starter as the _default_ back-end, the current design of 
allowing multiple pluggable storage back ends, starting with MongoDB and 
Redis, seems like not a bad one to me.

> 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)

The idea of Zaqar is that it'll be the central place for polling stuff 
in OpenStack. So it's going to get hit a lot, and it makes sense to do 
as little work on each request as possible because work is expensive and 
there will be a lot of requests. It doesn't follow that the main aim is 
to optimise for latency and throughput (as it is with AMQP).

Last I checked, pretty much every OpenStack API was using a different 
web framework already and it hasn't been much more than a minor 
annoyance as far as I know.

> 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.

I don't think it's either of those things.

cheers,
Zane.

> 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.
>>
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