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From Fraser Adams <fraser.ad...@blueyonder.co.uk>
Subject Re: Message API - Real world usage issue
Date Thu, 13 Feb 2014 18:04:44 GMT
Hey again Clive,
Hope that you are well.

I think that I might have made a little progress against your use case.

All of the previous comments on this email thread still hold in terms of 
what has been said about queues, exchanges, bindings etc. Robbie's most 
recent comment "I believe its really only conveyed in the address string 
as a form of extension point offering some ability to leverage the AMQP 
0-10 bind commands" is exactly it.

However as Robbie and myself suggested earlier the move towards AMQP 1.0 
gives additional exciting options for message filtering in the form of 
JMS style Message Selectors.

In your original scenario you tried to do:

    Rxer 1 - "queue1; {create: receiver, node: 
{x-declare:{auto-delete:true}, x-bindings: [{exchange: 'amq.topic', 
queue: 'queue1', key: 'bill'}]}}"
    Rxer 2 - "queue1; {create: receiver, node: 
{x-declare:{auto-delete:true}, x-bindings: [{exchange: 'amq.topic', 
queue: 'queue1', key: 'ben'}]}}"
    Rxer 3 - "queue1; {create: receiver, node: 
{x-declare:{auto-delete:true}, x-bindings: [{exchange: 'amq.topic', 
queue: 'queue1', key: 'tim'}]}}"

And you were hoping that a message with the subject "tim" would only end 
up getting delivered to Rxer 3 and hopefully we've explained why that 
isn't the case.

But let's try using Message Selectors. I'm using drain and spout because 
I don't need to write any code but it should translate into what you are 
doing. I'm using qpid 0.27 compiled off trunk about three weeks ago.

So starting with qpidd --auth no for simplicity I fire up the following 
in three separate windows:

./drain --connection-options {protocol:amqp1.0} -b localhost -f \
"queue1; {create: receiver, link: {name: test-link, selector: 

./drain --connection-options {protocol:amqp1.0} -b localhost -f \
"queue1; {create: receiver, link: {name: test-link, selector: 

./drain --connection-options {protocol:amqp1.0} -b localhost -f \
"queue1; {create: receiver, link: {name: test-link, selector: 

What this is doing is to start up three separate AMQP 1.0 consumers that 
all consume from queue1 and have Message Selectors that inspect the 
property "test" for the values "bill", "ben" and "tim" respectively.

If you do qpid-config -r queues you'll see
Queue 'queue1'
     bind [queue1] => ''

I then do:

./spout --connection-options {protocol:amqp1.0} -b localhost --content 
"Hello World" -P test=tim "queue1"

Which sends a message with the text "Hello World" and the property 
"test" set to "tim" to the node addressed "queue1" e.g. to the queue 
queue1 in this case.

When I do this happily what I see is nothing in the bill and ben windows 

test:tim}, content='Hello World')

in the tim window.

I sent the message 20 times and each time it only arrived on the "tim" 

Although Message Selectors were added as part of the AMQP 1.0 work it 
seems that it also works in AMQP 0.10 because when I tried the following 
it worked too.

./drain -b localhost -f \
"queue1; {create: receiver, link: {name: test-link, selector: 

./drain -b localhost -f \
"queue1; {create: receiver, link: {name: test-link, selector: 

./drain -b localhost -f \
"queue1; {create: receiver, link: {name: test-link, selector: 

./spout -b localhost --content "Hello World" -P test=tim "queue1"

So it looks like it's *nearly* what you are looking for. One thing that 
I've *not* got working yet though is using the subject. I thought that

./drain -b localhost -f \
"queue1; {create: receiver, link: {name: test-link, selector: 

./spout -b localhost --content "Hello World" "queue1/tim"

would work, but it doesn't seem to. It's likely to be something quirky 
like the subject property needing some prefix or other, I don't know. If 
I get a moment I'll have a look - or perhaps someone else may know the 

Hope that helps. Do bear in mind that I've literally only spend 45 mins 
or so messing with this so there may be gotchas and I've not 
investigated the relative pros and cons of using a selector as a filter 
off a queue node versus the more traditional topic style subscriptions 
(you certainly run the risk of filling your queue up if "tim" stops 
consuming though that might be fine if you use a circular queue). I 
don't know the earliest broker version where this will work I *think* 
Message Selectors were first added to 0.20 but you'd be best to use 
something more recent if you want to try this approach.

I hope this helps a bit,

On 11/02/14 21:57, CLIVE wrote:
> Robbie,
> Thanks for the response.
> You are confirming what Fraze has said, so I obviously need to take 
> this on board and rethink my understanding of the Receiver concept.
> In my mind I had a Receiver as an entity that received the messages 
> specified by the address string, but in fact I need to just look at it 
> as a conduit to a queue that has one or more bindings associated with it.
> Clive
> On 11/02/2014 21:35, Robbie Gemmell wrote:
>> On 11 February 2014 19:34, CLIVE <clive@ckjltd.co.uk> wrote:
>>> Ted,
>>> Thanks for the response and your comments.
>>> I have had to handle the case of multiple Receivers attaching to the 
>>> same
>>> queue on several occasions; primarily because the customer has felt 
>>> that it
>>> was easier to handle one queue with multiple bindings (up to 100), 
>>> rather
>>> than having a hundred queues with single bindings; message order was 
>>> also a
>>> contributing factor.
>>> The point of the post was just to raise it as a possible issue for 
>>> future
>>> improvement..
>>> I carried out a straw poll of 10 developers today at work. I gave 
>>> them the
>>> two examples previously described and asked what they would expect to
>>> happen for the case where multiple Receivers were created for the same
>>> queue. They all expected the correct Receiver to be returned from the
>>> nextReceiver method, not the undeterministic behaviour that they 
>>> would see.
>> As has been mentioned by others, the behaviour you are seeing is 
>> expected
>> because it is exactly what you are really asking the client and 
>> broker to
>> do currently: one queue which can receive messages via multiple binding
>> keys that have been added, and distribute them to any of the completely
>> equal multiple consumers receiving from it.
>> As Fraser has also beaten me to saying, if you really want to make
>> particular consumers only get particular messages from a shared 
>> queue, then
>> you will likely need to look at using selectors so that they can in fact
>> only receive those messages.
>>> I wouldn't have thought that it would take that much code/effort to add
>>> some additional functionality in the messaging API Implementation to
>>> support the behavior that, it would appear, most developers would 
>>> expect to
>>> see. If I find some time I will take a look and see how it could be 
>>> done.
>> As Fraser mentioned, I think there is some confusion as to what your
>> reciever creation calls are actually doing, but even if removing that
>> confusion from the equation the situation is not necessarily as 
>> simple as
>> it may seem. Suppose two receivers add the same binding key, which is 
>> the
>> 'correct' receiver to get the single message? Suppose wildcard 
>> matching is
>> in use on the bindings and multiple bindings then match a particular
>> message published, which receiver gets the single message? Imagine
>> selectors are also in use, but mutliple consumers selectors match the
>> message, which reciever gets the message? The list goes on...
>> You are effectively talking about turning the client into a sort of 
>> broker,
>> and since you already have one of those its probably easier to just 
>> ask it
>> to do what you actually want.
>> Robbie
>>> On 10/02/2014 22:17, Ted Ross wrote:
>>>> Clive,
>>>> What you are observing is what I expect:  In the second scenario where
>>>> you use the same queue for each of the three receivers, the 
>>>> receiver that
>>>> receives any particular message will be non-deterministic.
>>>> This is because the binding key is applied between the exchange and 
>>>> the
>>>> queue (i.e. it is used to determine which queue(s) the message 
>>>> should be
>>>> enqueued on).  Multiple receivers on a queue will receive messages 
>>>> from the
>>>> queue in an undetermined order, but no message shall be delivered 
>>>> to more
>>>> than one receiver.  In the second case, all of the messages are 
>>>> placed on
>>>> the same queue in the order in which they arrive.  The queue acts as a
>>>> buffer between the routing rule that matched the message and the 
>>>> receiver
>>>> that provided the routing rule.
>>>> It would be simpler to do the following:
>>>>    Rxer 1 - "amq.topic/bill; {link: {x-declare: {auto-delete:true}}}"
>>>>    Rxer 2 - "amq.topic/ben; {link: {x-declare: {auto-delete:true}}}"
>>>>    Rxer 3 - "amq.topic/tim; {link: {x-declare: {auto-delete:true}}}"
>>>> This will give you the determinism you want.  This will cause the
>>>> creation of a temporary queue for each receiver that will receive the
>>>> messages that match the topic key (following the slash in the 
>>>> address).
>>>> -Ted
>>>> On 02/10/2014 04:39 PM, CLIVE wrote:
>>>>> Fraser,
>>>>> Thanks for the response. The real problem is that the behavior of a
>>>>> Receiver is different depending on the multiplicity of the binding 
>>>>> strategy
>>>>> used. If you use a single queue with a single binding then 
>>>>> messages will
>>>>> get delivered to the required receiver. If you use multiple 
>>>>> Receivers bound
>>>>> to the same queue, the Receiver called by the messaging API, when
>>>>> delivering a message to your application, may not be the one that you
>>>>> think!!
>>>>> So if I create three Receivers in the same application, with the
>>>>> following bindings (note unique queue names)
>>>>>     Rxer 1 - "queue1; {create: receiver, node:
>>>>> {x-declare:{auto-delete:true}, x-bindings: [{exchange: 
>>>>> 'amq.topic', queue:
>>>>> 'queue1', key: 'bill'}]}}"
>>>>>     Rxer 2 - "queue2; {create: receiver, node:
>>>>> {x-declare:{auto-delete:true}, x-bindings: [{exchange: 
>>>>> 'amq.topic', queue:
>>>>> 'queue1', key: 'ben'}]}}"
>>>>>     Rxer 3 - "queue3; {create: receiver, node:
>>>>> {x-declare:{auto-delete:true}, x-bindings: [{exchange: 
>>>>> 'amq.topic', queue:
>>>>> 'queue1', key: 'tim'}]}}"
>>>>> And then send a message on the amq.topic exchange with a subject of
>>>>> 'tim'. Then Rxer3 will get returned by the 'nextReceiver' method 
>>>>> on the
>>>>> associated Session object.
>>>>> But if I change the bindings so they related to the same queue
>>>>>     Rxer 1 - "queue1; {create: receiver, node:
>>>>> {x-declare:{auto-delete:true}, x-bindings: [{exchange: 
>>>>> 'amq.topic', queue:
>>>>> 'queue1', key: 'bill'}]}}"
>>>>>     Rxer 2 - "queue1; {create: receiver, node:
>>>>> {x-declare:{auto-delete:true}, x-bindings: [{exchange: 
>>>>> 'amq.topic', queue:
>>>>> 'queue1', key: 'ben'}]}}"
>>>>>     Rxer 3 - "queue1; {create: receiver, node:
>>>>> {x-declare:{auto-delete:true}, x-bindings: [{exchange: 
>>>>> 'amq.topic', queue:
>>>>> 'queue1', key: 'tim'}]}}"
>>>>> And send the same message again, Which Receiver would you expect 
>>>>> to get
>>>>> returned from the sessions nextReceiver method?
>>>>> I would expect the same result as in the first example, Rxer 3. 
>>>>> But this
>>>>> does not happen, anyone of the three receivers might get called.
>>>>> This doesn't seem right to me and as a result you have to produce 
>>>>> quite
>>>>> a bit of application level logic to handle this scenario; 
>>>>> especially when
>>>>> your bindings are being passed down to you dynamically by several 
>>>>> client
>>>>> applications.
>>>>> Hope this explains it a bit better than my last attempt.
>>>>> Clive
>>>>> On 07/02/2014 10:03, Fraser Adams wrote:
>>>>>> On 06/02/14 19:07, CLIVE wrote:
>>>>>>> Hi,
>>>>>>> [snip]
>>>>>>> The first use case requires the dynamic creation of Receivers,
>>>>>>> before creating a new receiver, I would like to know if I 
>>>>>>> already have a
>>>>>>> receiver that would match the required binding. This is not 
>>>>>>> possible at the
>>>>>>> moment because the binding matching algorithms are hidden from

>>>>>>> public view;
>>>>>>> they are buried deep inside the Brokers Exchange Implementation

>>>>>>> code.
>>>>>> You know that you can get the binding information from QMF don't
>>>>>> Clive? I guess I'm missing what you're looking for if it's something
>>>>>> different than that. And I guess to be fair to get the 
>>>>>> information via QMF
>>>>>> you'd need a bit of code, but I'd have thought that this would be

>>>>>> the most
>>>>>> appropriate way to get the information.
>>>>>> Out of curiosity why do you need to know if you already have a 
>>>>>> receiver
>>>>>> that would match the binding?
>>>>>> One thing that's worth mentioning, I'm suspecting that (like me)

>>>>>> you've
>>>>>> mainly been using AMQP 0.10 - If I'm reading you correctly you 
>>>>>> sound like
>>>>>> you are dynamically creating queue nodes and passing x-bindings.
>>>>>> I've been doing that for a few years, but a few weeks back I started
>>>>>> looking at AMQP 1.0 and that primarily takes a perspective of 
>>>>>> addressing
>>>>>> the topic like exchanges and the queues end up being subscription

>>>>>> queues
>>>>>> and all of the stuff that relates to binding and the like ends up

>>>>>> in the
>>>>>> link (not node) config.
>>>>>> For me at any rate that was quite a different perspective on 
>>>>>> things (I
>>>>>> wrote up what I was up to in the "A write up of some AMQP 1.0 
>>>>>> Experiments"
>>>>>> post) previously I've been focussing on the queues, so I was 
>>>>>> dynamically
>>>>>> creating queue nodes and passing x-bindings in AMQP 0.10, but in

>>>>>> AMQP 1.0
>>>>>> I've been addressing the exchanges (topic type nodes) and using 
>>>>>> the link to
>>>>>> specify what I need. For me it took a bit of getting used to 
>>>>>> because I was
>>>>>> so ingrained doing it the other way, but I think I'm getting it now.
>>>>>>> The second use case in question requires a client application
>>>>>>> dynamically create multiple receivers for the same queue, but

>>>>>>> with slightly
>>>>>>> different binding keys bound to an exchange. When a message from
>>>>>>> exchange gets put in the queue and delivered to the client (via

>>>>>>> a receiver)
>>>>>> I'm not sure if I'm correctly interpreting what you are saying 
>>>>>> here, so
>>>>>> you want a client that has a single queue, but each receiver adds

>>>>>> different
>>>>>> binding keys right? You do know that this will result in what 
>>>>>> amounts to an
>>>>>> OR condition - both keys will be bound and a message will be put

>>>>>> on the
>>>>>> queue if either match so consumer A of the queue would receive 
>>>>>> messages due
>>>>>> to consumer B's key - is that what you mean.
>>>>>> The following AMQP 1.0 consumers will do what you seem to be saying,
>>>>>> there's a single shared subscription queue called queue1, the first
>>>>>> consumer binds *.news the second *.weather
>>>>>> ./drain --connection-options {protocol:amqp1.0} -b localhost -f \
>>>>>> "amq.topic/*.news; {node: {capabilities: [shared]}, link: {name:
>>>>>> queue1}}"
>>>>>> ./drain --connection-options {protocol:amqp1.0} -b localhost -f \
>>>>>> "amq.topic/*.weather; {node: {capabilities: [shared]}, link: {name:
>>>>>> queue1}}"
>>>>>> qpid-config -r queues gives
>>>>>> Queue 'queue1'
>>>>>>      bind [queue1] => ''
>>>>>>      bind [*.news] => amq.topic
>>>>>>      bind [*.weather] => amq.topic
>>>>>> For AMQP 0.10 the following would create a similar effect (not 
>>>>>> sure if
>>>>>> you want auto delete or not, if not remove the x-declare below 
>>>>>> and for the
>>>>>> AMQP 1.0 example above add reliability: at-least-once to the link

>>>>>> Map)
>>>>>> ./drain -b localhost -f \
>>>>>> "queue1; {create: receiver, node: {x-declare:{auto-delete:True},
>>>>>> x-bindings: [{exchange: 'amq.topic', queue: 'queue1', key: 
>>>>>> '*.news'}]}}"
>>>>>> ./drain -b localhost -f \
>>>>>> "queue1; {create: receiver, node: {x-declare:{auto-delete:True},
>>>>>> x-bindings: [{exchange: 'amq.topic', queue: 'queue1', key: 
>>>>>> '*.weather'}]}}"
>>>>>> The following also works for AMQP 0.10
>>>>>> ./drain -b localhost -f \
>>>>>> "queue1; {create: receiver, node: {x-declare:{auto-delete:True}},
>>>>>> link: {x-bindings: [{exchange: 'amq.topic', queue: 'queue1', key:
>>>>>> '*.news'}]}}"
>>>>>> ./drain -b localhost -f \
>>>>>> "queue1; {create: receiver, node: {x-declare:{auto-delete:True}},
>>>>>> link: {x-bindings: [{exchange: 'amq.topic', queue: 'queue1', key:
>>>>>> '*.weather'}]}}"
>>>>>> Don't know if this is what you are looking for.
>>>>>> Note that in none of the cases above have I worked out how to 
>>>>>> remove a
>>>>>> binding other than by removing the queue so if you add the first

>>>>>> then the
>>>>>> second then delete the second both bindings remain in place - I 
>>>>>> did wonder
>>>>>> about putting the x-declare/auto delete stuff on the link in the

>>>>>> second
>>>>>> AMQP 0.10 example, but that doesn't seem to remove the binding, 
>>>>>> so I'm not
>>>>>> sure if that's possible.
>>>>>>   I need to route the message to the correct application level
>>>>>>> destination(s). To do this I need to undertake a matching 
>>>>>>> operation between
>>>>>>> the routing key of the message and the binding key(s) of the

>>>>>>> created
>>>>>>> receivers; qpid does not deliver the message to the receiver

>>>>>>> with the most
>>>>>>> exact binding key match.
>>>>>> I guess than I'm not understanding you here. As far as I'm aware
>>>>>> you've got multiple bindings between an exchange and a queue then

>>>>>> the
>>>>>> message will be delivered on to the queue if either binding 
>>>>>> matches, so it
>>>>>> behaves like a logical OR. In your scenario if the first receiver

>>>>>> adds
>>>>>> *.news then the second adds *.weather then from that point on 
>>>>>> they will
>>>>>> *both* start to receive (*.news OR *.weather)
>>>>>>   So basically the receivers, and their bindings, enable the 
>>>>>> required
>>>>>>> messages to get delivered to the required client, but I then

>>>>>>> need to
>>>>>>> undertake application level routing to route the message to one

>>>>>>> or more
>>>>>>> application level classes, based on message routing key/ 
>>>>>>> receiver binding
>>>>>>> key matches.
>>>>>> So I'm still totally baffled why you want to send them to the same
>>>>>> queue if you are then demultiplexing at the application level. 
>>>>>> Surely (for
>>>>>> example) you'd be better having a news queue for the *.news 
>>>>>> messages and a
>>>>>> weather queue for the *.weather messages. If you force them down

>>>>>> the same
>>>>>> queue then you are going to have to do application level 
>>>>>> demultiplexing,
>>>>>> which it sounds like you don't want to do, but why use a single 
>>>>>> queue.
>>>>>> What's actually driving the single queue requirement? That sounds

>>>>>> like
>>>>>> the root of your problems, without knowing the nuance of your 
>>>>>> scenario it
>>>>>> feels like your approaching the problem from the wrong angle and

>>>>>> fighting
>>>>>> the middleware rather than letting it work for you. I'm sure I've

>>>>>> missed
>>>>>> something subtle in your use case.
>>>>>>> Unfortunately in both cases the messaging API does not provide
>>>>>>> visibility of the bind matching algorithms and so I have to 
>>>>>>> create several
>>>>>>> utility classes to support this functionality.
>>>>>>> Would it be possible to create a Binding.h class in the 
>>>>>>> messaging API
>>>>>>> to support matching of bindings from all the supported exchange

>>>>>>> types?
>>>>>> I'm not actually sure what you are asking for here. Are you 
>>>>>> asking for
>>>>>> a client side filtering API?
>>>>>> As I say I'm having trouble getting under the skin of your use 
>>>>>> case. If
>>>>>> I'm reading it correctly it sounds like you are wanting to have a

>>>>>> single
>>>>>> queue but have multiple bindings between an exchange and that 
>>>>>> queue, which
>>>>>> will result in messages for both bindings making their way on to

>>>>>> the queue
>>>>>> and then, to get around that, to apply a client side filter to 
>>>>>> deliver the
>>>>>> right message to the right receiver - is that correct?
>>>>>> I'm afraid that I'm still not clear why you want to do that on the
>>>>>> client rather than on the broker??
>>>>>> Other's might have a better view, but I'm not sure that client side
>>>>>> filtering fits into the qpid::messaging API per se (and binding 
>>>>>> probably
>>>>>> wouldn't be a good idea anyway as it's a legacy AMQP 0.10 concept).
>>>>>> One thought moving forward (and I'm far from an expert) might be
>>>>>> think in terms of AMQP 1.0, so the Qpid Broker may be viewed as 
>>>>>> essentially
>>>>>> an AMQP 1.0 container and it has a whole bunch of capabilities, 
>>>>>> including
>>>>>> the ability to filter (the traditional bindings plus - really cool
>>>>>> message selectors). The qpid::messaging API is about interacting

>>>>>> with nodes
>>>>>> on a container and attaching links with specified properties.
>>>>>> As it happens though an AMQP 1.0 client application can also be 
>>>>>> thought
>>>>>> of as a container, so an interesting thought might be a client 
>>>>>> application
>>>>>> containing its own addressable node. In this scenario you'd 
>>>>>> establish all
>>>>>> the stuff previously discussed with the broker and the consumer 
>>>>>> client
>>>>>> would have all messages delivered to the node on the client, you

>>>>>> could then
>>>>>> (in theory 'cause none of this exists) create AMQP links (on the

>>>>>> client) to
>>>>>> the node (on the client) passing filter properties on attachment

>>>>>> (such as a
>>>>>> selector).
>>>>>> As I say none of this exists at the moment (except on the broker)

>>>>>> but
>>>>>> it might be interesting to consider if it would be possible to 
>>>>>> modularise
>>>>>> things such that some of these fairly general purpose AMQP 1.0 
>>>>>> "services"
>>>>>> could be extracted from the broker and made available as a 
>>>>>> toolkit for
>>>>>> creating general purpose AMQP 1.0 containers.
>>>>>> As I say I'm no expert and tentatively finding my feet with AMQP

>>>>>> 1.0,
>>>>>> Gordon Sim would be far better placed than I to say whether that

>>>>>> a) makes
>>>>>> sense from an AMQP 1.0 perspective b) how feasible it is and c) 
>>>>>> how likely
>>>>>> it is to happen :-)
>>>>>> Hope I've managed to be at least some help Clive,
>>>>>> Cheers,
>>>>>> Frase
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