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From Jonathan Hodges <hodg...@gmail.com>
Subject Re: Arguments for Kafka over RabbitMQ ?
Date Fri, 07 Jun 2013 18:42:28 GMT
Sorry I forgot to add this RabbitMQ link as well as it also seems to
indicate the copying of messages to multiple queues with a topic exchange.


Maybe I am misunderstanding and topic exchange wouldn't be the approach to
take with Rabbit if you wanted to share the same stream of messages across
multiple consumers.

On Fri, Jun 7, 2013 at 12:03 PM, Jonathan Hodges <hodgesz@gmail.com> wrote:

> Hi Alexis,
> I appreciate your reply and clarifications to my misconception about
> Rabbit, particularly on the copying of the message payloads per consumer.
>  It sounds like it only copies metadata like the consumer state i.e.
> position in the topic messages.  I don’t have experience with Rabbit and
> was basing this assumption based on Google searches like the following -
> http://ilearnstack.com/2013/04/16/introduction-to-amqp-messaging-with-rabbitmq/.
>  It seems to indicate with topic exchanges that the messages get copied to
> a queue per consumer, but I am glad you confirmed it is just the metadata.
> While you are correct the payload is a much bigger concern, managing the
> metadata and acks centrally on the broker across multiple clients at scale
> is also a concern.  This would seem to be exasperated if you have consumers
> at different speeds i.e. Storm and Hadoop consuming the same topic.
> In that scenario, say storm consumes the topic messages in real-time and
> Hadoop consumes once a day.  Let’s assume the topic consists of 100k+
> messages/sec throughput so that in a given day you might have 100s GBs of
> data flowing through the topic.
> To allow Hadoop to consume once a day, Rabbit obviously can’t keep 100s
> GBs in memory and will need to persist this data to its internal DB to be
> retrieved later.  I believe when large amounts of data need to be persisted
> is the scenario described in the earlier posted Kafka paper (
> http://research.microsoft.com/en-us/um/people/srikanth/netdb11/netdb11papers/netdb11-final12.pdf)
> where Rabbit’s performance really starts to bog down as compared to Kafka.
> This Kafka paper is looks to be a few years old so has something changed
> within the Rabbit architecture to alleviate this issue when large amounts
> of data are persisted to the internal DB?  Do the producer and consumer
> numbers look correct?  If no, maybe you can share some Rabbit benchmarks
> under this scenario, because I believe it is the main area where Kafka
> appears to be the superior solution.
> Thanks for educating me on these matters.
> -Jonathan
> On Fri, Jun 7, 2013 at 6:54 AM, Alexis Richardson <alexis@rabbitmq.com>wrote:
>> Hi
>> Alexis from Rabbit here.  I hope I am not intruding!
>> It would be super helpful if people with questions, observations or
>> moans posted them to the rabbitmq list too :-)
>> A few comments:
>> * Along with ZeroMQ, I consider Kafka to be one of the interesting and
>> useful messaging projects out there.  In a world of cruft, Kafka is
>> cool!
>> * This is because both projects come at messaging from a specific
>> point of view that is *different* from Rabbit.  OTOH, many other
>> projects exist that replicate Rabbit features for fun, or NIH, or due
>> to misunderstanding the semantics (yes, our docs could be better)
>> * It is striking how few people describe those differences.  In a
>> nutshell they are as follows:
>> *** Kafka writes all incoming data to disk immediately, and then
>> figures out who sees what.  So it is much more like a database than
>> Rabbit, in that new consumers can appear well after the disk write and
>> still subscribe to past messages.  Instead, Rabbit which tries to
>> deliver to consumers and buffers otherwise.  Persistence is optional
>> but robust and a feature of the buffer ("queue") not the upstream
>> machinery.  Rabbit is able to cache-on-arrival via a plugin, but this
>> is a design overlay and not particularly optimal.
>> *** Kafka is a client server system with end to end semantics.  It
>> defines order to include processing order, and keeps state on the
>> client to do this.  Group management is via a 3rd party service
>> (Zookeeper? I forget which).  Rabbit is a server-only protocol based
>> system which maintains order on the server and through completely
>> language neutral protocol semantics.  This makes Rabbit perhaps more
>> natural as a 'messaging service' eg for integration and other
>> inter-app data transfer.
>> *** Rabbit is a general purpose messaging system with extras like
>> federation.  It speaks many protocols, and has core features like HA,
>> transactions, management, etc.  Everything can be switched on or off.
>> Getting all this to work while keeping the install light and fast, is
>> quite fiddly.  Kafka by contrast comes from a specific set of use
>> cases, which are interesting certainly.  I am not sure if Kafka wants
>> to be a general purpose messaging system, but it will become a bit
>> more like Rabbit if that is the goal.
>> *** Both approaches have costs.  In the case of Rabbit the cost is
>> that more metadata is stored on the broker.  Kafka can get performance
>> gains by storing less such data.  But we are talking about some N
>> thousands of MPS versus some M thousands.  At those speeds the clients
>> are usually the bottleneck anyway.
>> * Let me also clarify some things:
>> *** Rabbit does NOT store multiple copies of the same message across
>> queues, unless they are very small (<60b, iirc).  A message delivered
>> to >1 queue on 1 machine is stored once.  Metadata about that message
>> may be stored more than once, but, at scale, the big cost is the
>> payload.
>> *** Rabbit's vanilla install does store some index data in memory when
>> messages flow to disk.  You can change this by using a plugin, but
>> this is a secret-menu undocumented feature.  Very very few people need
>> any such thing.
>> *** A Rabbit queue is lightweight.  It's just an ordered consumption
>> buffer that can persist and ack.  Don't assume things about Rabbit
>> queues based on what you know about IBM MQ, JMS, and so forth.  Queues
>> in Rabbit and Kafka are not the same.
>> *** Rabbit does not use mnesia for message storage.  It has its own
>> DB, optimised for messaging.  You can use other DBs but this is
>> Complicated.
>> *** Rabbit does all kinds of batching and bulk processing, and can
>> batch end to end.  If you see claims about batching, buffering, etc.,
>> find out ALL the details before drawing conclusions.
>> I hope this is helpful.
>> Keen to get feedback / questions / corrections.
>> alexis
>> On Fri, Jun 7, 2013 at 2:09 AM, Marc Labbe <mrlabbe@gmail.com> wrote:
>> > We also went through the same decision making and our arguments for
>> Kafka
>> > where in the same lines as those Jonathan mentioned. The fact that we
>> have
>> > heterogeneous consumers is really a deciding factor. Our requirements
>> were
>> > to avoid loosing messages at all cost while having multiple consumers
>> > reading the same data at a different pace. On one side, we have a few
>> > consumers being fed with data coming in from most, if not all, topics.
>> On
>> > the other side, we have a good bunch of consumers reading only from a
>> > single topic. The big guys can take their time to read while the smaller
>> > ones are mostly for near real-time events so they need to keep up the
>> pace
>> > of incoming messages.
>> >
>> > RabbitMQ stores data on disk only if you tell it to while Kafka
>> persists by
>> > design. From the beginning, we decided we would try to use the queues
>> the
>> > same way, pub/sub with a routing key (an exchange in RabbitMQ) or topic,
>> > persisted to disk and replicated.
>> >
>> > One of our scenario was to see how the system would cope with the
>> largest
>> > consumer down for a while, therefore forcing the brokers to keep the
>> data
>> > for a long period. In the case of RabbitMQ, this consumer has it owns
>> queue
>> > and data grows on disk, which is not really a problem if you plan
>> > consequently. But, since it has to keep track of all messages read, the
>> > Mnesia database used by RabbitMQ as the messages index also grows pretty
>> > big. At that point, the amount of RAM necessary becomes very large to
>> keep
>> > the level of performance we need. In our tests, we found that this an
>> > adverse effect on ALL the brokers, thus affecting all consumers. You can
>> > always say that you'll monitor the consumers to make sure it won't
>> happen.
>> > That's a good thing if you can. I wasn't ready to make that bet.
>> >
>> > Another point is the fact that, since we wanted to use pub/sub with a
>> > exchange in RabbitMQ, we would have ended up with a lot data duplication
>> > because if a message is read by multiple consumers, it will get
>> duplicated
>> > in the queue of each of those consumer. Kafka wins on that side too
>> since
>> > every consumer reads from the same source.
>> >
>> > The downsides of Kafka were the language issues (we are using mostly
>> Python
>> > and C#). 0.8 is very new and few drivers are available at this point.
>> Also,
>> > we will have to try getting as close as possible to once-and-only-once
>> > guarantee. There are two things where RabbitMQ would have given us less
>> > work out of the box as opposed to Kafka. RabbitMQ also provides a bunch
>> of
>> > tools that makes it rather attractive too.
>> >
>> > In the end, looking at throughput is a pretty nifty thing but being sure
>> > that I'll be able to manage the beast as it grows will allow me to get
>> to
>> > sleep way more easily.
>> >
>> >
>> > On Thu, Jun 6, 2013 at 3:28 PM, Jonathan Hodges <hodgesz@gmail.com>
>> wrote:
>> >
>> >> We just went through a similar exercise with RabbitMQ at our company
>> with
>> >> streaming activity data from our various web properties.  Our use case
>> >> requires consumption of this stream by many heterogeneous consumers
>> >> including batch (Hadoop) and real-time (Storm).  We pointed out that
>> Kafka
>> >> acts as a configurable rolling window of time on the activity stream.
>>  The
>> >> window default is 7 days which allows for supporting clients of
>> different
>> >> latencies like Hadoop and Storm to read from the same stream.
>> >>
>> >> We pointed out that the Kafka brokers don't need to maintain consumer
>> state
>> >> in the stream and only have to maintain one copy of the stream to
>> support N
>> >> number of consumers.  Rabbit brokers on the other hand have to
>> maintain the
>> >> state of each consumer as well as create a copy of the stream for each
>> >> consumer.  In our scenario we have 10-20 consumers and with the scale
>> and
>> >> throughput of the activity stream we were able to show Rabbit quickly
>> >> becomes the bottleneck under load.
>> >>
>> >>
>> >>
>> >> On Thu, Jun 6, 2013 at 12:40 PM, Dragos Manolescu <
>> >> Dragos.Manolescu@servicenow.com> wrote:
>> >>
>> >> > Hi --
>> >> >
>> >> > I am preparing to make a case for using Kafka instead of Rabbit MQ
>> as a
>> >> > broker-based messaging provider. The context is similar to that of
>> the
>> >> > Kafka papers and user stories: the producers publish monitoring data
>> and
>> >> > logs, and a suite of subscribers consume this data (some store it,
>> others
>> >> > perform computations on the event stream). The requirements are
>> typical
>> >> of
>> >> > this context: low-latency, high-throughput, ability to deal with
>> bursts
>> >> and
>> >> > operate in/across multiple data centers, etc.
>> >> >
>> >> > I am familiar with the performance comparison between Kafka, Rabbit
>> MQ
>> >> and
>> >> > Active MQ from the NetDB 2011 paper<
>> >> >
>> >>
>> http://research.microsoft.com/en-us/um/people/srikanth/netdb11/netdb11papers/netdb11-final12.pdf
>> >> >.
>> >> > However in the two years that passed since then the number of
>> production
>> >> > Kafka installations increased, and people are using it in different
>> ways
>> >> > than those imagined by Kafka's designers. In light of these
>> experiences
>> >> one
>> >> > can use more data points and color when contrasting to Rabbit MQ
>> (which
>> >> by
>> >> > the way also evolved since 2011). (And FWIW I know I am not the
>> first one
>> >> > to walk this path; see for example last year's OSCON session on the
>> State
>> >> > of MQ<http://lanyrd.com/2012/oscon/swrcz/>.)
>> >> >
>> >> > I would appreciate it if you could share measurements, results, or
>> even
>> >> > anecdotal evidence along these lines. How have you avoided the
>> "let's use
>> >> > Rabbit MQ because everybody else does it" route when solving
>> problems for
>> >> > which Kafka is a better fit?
>> >> >
>> >> > Thanks,
>> >> >
>> >> > -Dragos
>> >> >
>> >>

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