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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