On Mon, Sep 22, 2014 at 7:04 PM, Joe Gordon <joe.gord...@gmail.com> wrote:
>
>
> On Mon, Sep 22, 2014 at 5:47 PM, Zane Bitter <zbit...@redhat.com> wrote:
>
>> On 22/09/14 17:06, Joe Gordon wrote:
>>
>>> On Mon, Sep 22, 2014 at 9:58 AM, Zane Bitter <zbit...@redhat.com> wrote:
>>>
>>> On 22/09/14 10:11, Gordon Sim wrote:
>>>>
>>>> On 09/19/2014 09:13 PM, Zane Bitter wrote:
>>>>>
>>>>> SQS offers very, very limited guarantees, and it's clear that the
>>>>>> reason
>>>>>> for that is to make it massively, massively scalable in the way that
>>>>>> e.g. S3 is scalable while also remaining comparably durable (S3 is
>>>>>> supposedly designed for 11 nines, BTW).
>>>>>>
>>>>>> Zaqar, meanwhile, seems to be promising the world in terms of
>>>>>> guarantees. (And then taking it away in the fine print, where it says
>>>>>> that the operator can disregard many of them, potentially without the
>>>>>> user's knowledge.)
>>>>>>
>>>>>> On the other hand, IIUC Zaqar does in fact have a sharding feature
>>>>>> ("Pools") which is its answer to the massive scaling question.
>>>>>>
>>>>>>
>>>>> There are different dimensions to the scaling problem.
>>>>>
>>>>>
>>>> Many thanks for this analysis, Gordon. This is really helpful stuff.
>>>>
>>>> As I understand it, pools don't help scaling a given queue since all
>>>> the
>>>>
>>>>> messages for that queue must be in the same pool. At present traffic
>>>>> through different Zaqar queues are essentially entirely orthogonal
>>>>> streams. Pooling can help scale the number of such orthogonal streams,
>>>>> but to be honest, that's the easier part of the problem.
>>>>>
>>>>>
>>>> But I think it's also the important part of the problem. When I talk
>>>> about
>>>> scaling, I mean 1 million clients sending 10 messages per second each,
>>>> not
>>>> 10 clients sending 1 million messages per second each.
>>>>
>>>> When a user gets to the point that individual queues have massive
>>>> throughput, it's unlikely that a one-size-fits-all cloud offering like
>>>> Zaqar or SQS is _ever_ going to meet their needs. Those users will want
>>>> to
>>>> spin up and configure their own messaging systems on Nova servers, and
>>>> at
>>>> that kind of size they'll be able to afford to. (In fact, they may not
>>>> be
>>>> able to afford _not_ to, assuming per-message-based pricing.)
>>>>
>>>>
>>> Running a message queue that has a high guarantee of not loosing a
>>> message
>>> is hard and SQS promises exactly that, it *will* deliver your message.
>>> If a
>>> use case can handle occasionally dropping messages then running your own
>>> MQ
>>> makes more sense.
>>>
>>> SQS is designed to handle massive queues as well, while I haven't found
>>> any
>>> examples of queues that have 1 million messages/second being sent or
>>> received 30k to 100k messages/second is not unheard of [0][1][2].
>>>
>>> [0] https://www.youtube.com/watch?v=zwLC5xmCZUs#t=22m53s
>>> [1] http://java.dzone.com/articles/benchmarking-sqs
>>> [2]
>>> http://www.slideshare.net/AmazonWebServices/massive-
>>> message-processing-with-amazon-sqs-and-amazon-
>>> dynamodb-arc301-aws-reinvent-2013-28431182
>>>
>>
>> Thanks for digging those up, that's really helpful input. I think number
>> [1] kind of summed up part of what I'm arguing here though:
>>
>> "But once your requirements get above 35k messages per second, chances
>> are you need custom solutions anyway; not to mention that while SQS is
>> cheap, it may become expensive with such loads."
>
>
> If you don't require the reliability guarantees that SQS provides then
> perhaps. But I would be surprised to hear that a user can set up something
> with this level of uptime for less:
>
> "Amazon SQS runs within Amazon’s high-availability data centers, so queues
> will be available whenever applications need them. To prevent messages from
> being lost or becoming unavailable, all messages are stored redundantly
> across multiple servers and data centers." [1]
>
>
>>
>>
>> There is also the possibility of using the sharding capabilities of the
>>>>
>>>>> underlying storage. But the pattern of use will determine how effective
>>>>> that can be.
>>>>>
>>>>> So for example, on the ordering question, if order is defined by a
>>>>> single sequence number held in the database and atomically incremented
>>>>> for every message published, that is not likely to be something where
>>>>> the databases sharding is going to help in scaling the number of
>>>>> concurrent publications.
>>>>>
>>>>> Though sharding would allow scaling the total number messages on the
>>>>> queue (by distributing them over multiple shards), the total ordering
>>>>> of
>>>>> those messages reduces it's effectiveness in scaling the number of
>>>>> concurrent getters (e.g. the concurrent subscribers in pub-sub) since
>>>>> they will all be getting the messages in exactly the same order.
>>>>>
>>>>> Strict ordering impacts the competing consumers case also (and is in my
>>>>> opinion of limited value as a guarantee anyway). At any given time, the
>>>>> head of the queue is in one shard, and all concurrent claim requests
>>>>> will contend for messages in that same shard. Though the unsuccessful
>>>>> claimants may then move to another shard as the head moves, they will
>>>>> all again try to access the messages in the same order.
>>>>>
>>>>> So if Zaqar's goal is to scale the number of orthogonal queues, and the
>>>>> number of messages held at any time within these, the pooling facility
>>>>> and any sharding capability in the underlying store for a pool would
>>>>> likely be effective even with the strict ordering guarantee.
>>>>>
>>>>>
>>>> IMHO this is (or should be) the goal - support enormous numbers of
>>>> small-to-moderate sized queues.
>>>>
>>>
>>>
>>> If 50,000 messages per second doesn't count as small-to-moderate then
>>> Zaqar
>>> does not fulfill a major SQS use case.
>>>
>>
>> It's not a drop-in replacement, but as I mentioned you can recreate the
>> SQS semantics exactly *and* get the scalability benefits of that approach
>> by sharding at the application level and then round-robin polling.
>
>
>> As I also mentioned, this is pretty easy to implement, and is only
>> required for really big applications that are more likely to be written by
>> developers who already Know What They're Doing(TM). While the reverse
>> (emulating Zaqar semantics, i.e. FIFO, in SQS) is tricky, error-prone, and
>> conceivably required by or at least desirable for all kinds of
>> beginner-level applications. (It's also pretty useful for a lot of use
>> cases in OpenStack itself, where OpenStack services are sending messages to
>> the user.)
>
>
>
> I'm not convinced that is as simple to implement well as you make it out
> to be, now every receiver has to poll N endpoints instead of 1. How would
> this work with Long Polling? What is the impact on expanding the number of
> connections? How do you make this auto scale? etc.
>
> I don't know enough about the target audience to know if adding the FIFO
> guarantee is a good trade off or not. But I don't follow your use case for
> OpenStack service sending messages to the user; when do these need to be in
> order?
>
>
>>
>>
>> If scaling the number of communicants on a given communication channel
>>>>
>>>>> is a goal however, then strict ordering may hamper that. If it does, it
>>>>> seems to me that this is not just a policy tweak on the underlying
>>>>> datastore to choose the desired balance between ordering and scale, but
>>>>> a more fundamental question on the internal structure of the queue
>>>>> implementation built on top of the datastore.
>>>>>
>>>>>
>>>> I agree with your analysis, but I don't think this should be a goal.
>>>>
>>>> Note that the user can still implement this themselves using
>>>> application-level sharding - if you know that in-order delivery is not
>>>> important to you, then randomly assign clients to a queue and then poll
>>>> all
>>>> of the queues in the round-robin. This yields _exactly_ the same
>>>> semantics
>>>> as SQS.
>>>>
>>>
>>>
>>> The reverse is true of SQS - if you want FIFO then you have to implement
>>>> re-ordering by sequence number in your application. (I'm not certain,
>>>> but
>>>> it also sounds very much like this situation is ripe for losing messages
>>>> when your client dies.)
>>>>
>>>> So the question is: in which use case do we want to push additional
>>>> complexity into the application? The case where there are truly massive
>>>> volumes of messages flowing to a single point? Or the case where the
>>>> application wants the messages in order?
>>>>
>>>> I'd suggest both that the former applications are better able to handle
>>>> that extra complexity and that the latter applications are probably more
>>>> common. So it seems that the Zaqar team made a good decision.
>>>>
>>>>
>>> If Zaqar is supposed to be comparable to amazon SQS, then it has picked
>>> the
>>> wrong choice.
>>>
>>
>> It has certainly picked a different choice. It seems like a choice that
>> is friendlier to beginners and simple applications, while shifting some
>> complexity to larger applications without excluding them as a use case.
>> That's certainly not an invalid choice.
>
>
>> It isn't OpenStack's job to be cloning AWS services after all... we can
>> definitely address the same problems better when we see the opportunity. We
>> should, of course, think very carefully about all the consequences,
>> intended and unintended, of changing a model that is already proven in the
>> field and the market, so I'm very glad this discussion is happening. But
>> after digging into it, the choice doesn't seem "wrong" to me.
>>
>
> To me this is less about valid or invalid choices. The Zaqar team is
> comparing Zaqar to SQS, but after digging into the two of them, zaqar
> barely looks like SQS. Zaqar doesn't guarantee what IMHO is the most
> important parts of SQS: the message will be delivered and will never be
> lost by SQS. Zaqar doesn't have the same scaling properties as SQS. Zaqar
> is aiming for low latency per message, SQS doesn't appear to be. So if
> Zaqar isn't SQS what is Zaqar and why should I use it?
>
>
One more: SQS uses Distributed Queues, Zaqar does not.
>
> [0] ACK is sent before message is stored on disk by default.
>
>
>> cheers,
>> Zane.
>>
>>
>> (Aside: it follows that Zaqar probably should have a maximum throughput
>>>> quota for each queue; or that it should report usage information in
>>>> such a
>>>> way that the operator could sometimes bill more for a single queue than
>>>> they would for the same amount of usage spread across multiple queues;
>>>> or
>>>> both.)
>>>>
>>>> I also get the impression, perhaps wrongly, that providing the strict
>>>>
>>>>> ordering guarantee wasn't necessarily an explicit requirement, but was
>>>>> simply a property of the underlying implementation(?).
>>>>>
>>>>>
>>>> I wasn't involved, but I expect it was a bit of both (i.e. it is a
>>>> chicken/egg question).
>>>>
>>>> cheers,
>>>> Zane.
>>>>
>>>>
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>>>
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