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.
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.
Clive
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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