Re: Message API - Real world usage issue

[Fraser Adams]

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: 
\"test='bill'\"}}"

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

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


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 
and

Message(properties={spout-id:aa92e8c4-3d12-40f6-bc89-8cabe91500e6:0, 
test:tim}, content='Hello World')

in the tim window.

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


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: 
\"test='bill'\"}}"

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

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

./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: 
\"subject='tim'\"}}"

./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 
answer.


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,
Frase


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 <cl...@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, but
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 you
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 to
dynamically create multiple receivers for the same queue, but 
with slightly
different binding keys bound to an exchange. When a message from an
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 if
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 to
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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