[rabbitmq-discuss] questions about distributed queue

Alexis Richardson alexis.richardson at gmail.com
Tue Aug 18 09:20:34 BST 2009


On Mon, Aug 17, 2009 at 8:36 PM, Paul Dix<paul at pauldix.net> wrote:
> Yeah, that's what I'm talking about. There will probably be upwards of
> a few hundred producers and a few hundred consumers.


So one question you need to answer is: do you want all the consumers
to receive the same messages?  I.e.:

* are you aggregating all the producers into one 'firehose', and then
sending the whole firehose on to all connected consumers?


* are you planning to in some way share messages out amongst connected
consumers, eg on a round robin basis

See more below re flow1, flow2...

> The total ingress
> is definitely what I'm most worried about right now.


Be aware that in high ingress rate cases you may be limited by the
client egress rate, which is strongly implementation and platform
dependent.  Also, see Matthias' notes on testing performance, which
are googleable from the rabbitmq archives, if you want to run some
test cases at any point.

> Later, memory may
> be a concern, but hopefully the consumers are pulling so quickly that
> the queue never gets extremely large.


> Can you give me more specific details (or a pointer) to how the flow1,
> flow2 thing work (both producer and consumer side)?


First you need to read up on what 'direct exchanges' are and how they
work in AMQP.  I recommend Jason's intro to get you started:


More background info can be found here: www.rabbitmq.com/how

In a nutshell, RabbitMQ will route any message it receives on to one
or more queues.

Each queue lives on a node, and nodes are members of a cluster.  You
can have one or more nodes per machine - a good guide is to have one
per core.  You can send messages to any node in the cluster and they
will get routed to the right places (adding more nodes to a cluster is
how you scale ingress and availability).

The routing model is based on message routing keys: queues receive
messages whose routing keys match routing patterns ("bindings").  Note
that multiple queues can request messages matching the same key,
giving you 1-many pubsub.  This is explained in Jason's article.  I
suggest you use the 'direct exchange' routing model, in which each
message has one routing key, e.g.: "flow1", "flow2".

Take a look at the article and let us know if it all makes sense.


> Thanks,
> Paul
> On Mon, Aug 17, 2009 at 2:32 PM, Alexis
> Richardson<alexis.richardson at gmail.com> wrote:
>> On Mon, Aug 17, 2009 at 5:22 PM, Paul Dix<paul at pauldix.net> wrote:
>>> So what exactly does option 1 look like?
>>> It sounds like it's possible to have a queue with the same id on two
>>> different nodes bound to the same exchange.
>> Not quite.  Same routing - two queues, two ids.  Actually now that I
>> think about it that won't give you exactly what you need.  More below.
>>> Will the exchange will
>>> then round robin the messages to the two different queues? If so,
>>> that's exactly what I'm looking for. I don't really care about order
>>> on this queue.
>> No it won't and that's why my suggestion was wrong.
>> Round robin does occur when you have two consumers (clients) connected
>> to one queue.  This WILL help you by draining the queue faster, if
>> memory is a limitation.
>> If total ingress is the limitation you can increase that by splitting
>> the flow.  Suppose you start with one queue bound once to one exchange
>> with key "flow1".  Then all messages with routing key flow1 will go to
>> that queue.  When load is heavy, add a queue with key "flow2", on a
>> second node.  Then, alternate (if you prefer, randomly) between
>> routing keys flow1 and flow2.  This will spread the load as you
>> require.  And so on, for more queues.
>> You can make this part of a load balancing layer on the server side,
>> so that clients don't have to be coded too much.
>> Is this along the lines of what you need?  Let me know, and I can elaborate.
>> alexis
>>> Thanks,
>>> Paul
>>> On Mon, Aug 17, 2009 at 10:55 AM, Alexis
>>> Richardson<alexis.richardson at gmail.com> wrote:
>>>> Paul
>>>> On Mon, Aug 17, 2009 at 3:34 PM, Paul Dix<paul at pauldix.net> wrote:
>>>>> Sorry for the confusion. I mean scalability on a single queue. Say I
>>>>> want to push 20k messages per second through a single queue. If a
>>>>> single node can't handle that it seems I'm out of luck. That is, if
>>>>> I'm understanding how things work.
>>>> You can in principle just add more nodes to the cluster.  More details below.
>>>>> So I guess I'm not worried about total queue size, but queue
>>>>> throughput (although size may become an issue, I'm not sure). It seems
>>>>> the solution is to split out across multiple queues, but I was hoping
>>>>> to avoid that since it will add a layer of complexity to my producers
>>>>> and consumers.
>>>> 1. To maximise throughput, don't use persistence.  To make it bigger,
>>>> forget about ordering.  So for example, you can easily have two
>>>> queues, one per node, subscribed to the same direct exchange with the
>>>> same key, and you ought to double throughput (assuming all other
>>>> things being equal and fair).
>>>> 2. If you want to be both fast and 'reliable' (no loss of acked
>>>> messages), then add more queues and make them durable, and set
>>>> messages to be persistent.
>>>> 3. If you want to preserve ordering, label each message with an ID and
>>>> dedup at the endpoints.  This does as you say, add some small noise to
>>>> your producers and consumers, but the above two options 1 and 2, do
>>>> not.
>>>>> I don't think I understand how using Linux-HA with clustering would
>>>>> lead to a splitting a single queue across multiple nodes. I'm not
>>>>> familiar with HA, but it looked like it was a solution to provide a
>>>>> replicated failover.
>>>> You are right that HA techniques, indeed any kind of queue replication
>>>> or replicated failover, will not help you here.
>>>> What you want is 'flow over' ie. "when load is high, make a new node
>>>> with the same routing info".  This is certainly doable.
>>>> alexis
>>>>> Thanks again,
>>>>> Paul
>>>>> On Mon, Aug 17, 2009 at 10:24 AM, Tony Garnock-Jones<tonyg at lshift.net> wrote:
>>>>>> Paul Dix wrote:
>>>>>>> Do you have a roadmap for when a scalable queue
>>>>>>> will be available?
>>>>>> If by "scalable" you mean "replicated", then that's available now, by
>>>>>> configuration along the lines I hinted at in my previous message. Adding
>>>>>> clustering into the mix can help increase capacity, on top of that (at a
>>>>>> certain cost in configuration complexity).
>>>>>> If instead you mean "exceeding RAM+swap size", we're hoping to have that
>>>>>> for the 1.7 release -- which ought to be out within a month or so.
>>>>>>> Just to give you a little more information on what I'm doing, I'm
>>>>>>> building a live search/aggregation system. I'm hoping to push updates
>>>>>>> of a constant internet crawl through the messaging system so workers
>>>>>>> can analyze the content and build indexes as everything comes in.
>>>>>> Sounds pretty cool!
>>>>>> Tony
>>>>>> --
>>>>>>  [][][] Tony Garnock-Jones     | Mob: +44 (0)7905 974 211
>>>>>>   [][] LShift Ltd             | Tel: +44 (0)20 7729 7060
>>>>>>  []  [] http://www.lshift.net/ | Email: tonyg at lshift.net
>>>>> _______________________________________________
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