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From Robbie Gemmell <robbie.gemm...@gmail.com>
Subject Re: JMS Destination handling for AMQP 1.0
Date Thu, 18 Dec 2014 13:46:23 GMT
On 18 December 2014 at 12:29, Gordon Sim <gsim@redhat.com> wrote:
> On 12/17/2014 12:37 PM, Robbie Gemmell wrote:
>>
>> Hi everyone (please use reply-all to keep both lists on the trail),
>>
>> I would like to have a discussion around JMS destination handling in
>> the JMS Mapping for AMQP 1.0, in particular around how to handle JMS
>> Destination names via the AMQP "address" field of a link
>> (producer/consumer) source/target and the "to", and "reply-to" field
>> of messages.
>>
>> Apologies for the length of the mail, there is a fair bit to outline.
>> I moved some information for full context to the end to help a tiny
>> bit.
>>
>> JMS defines multiple Destination types that each have their own
>> inherent name space, so it is possible for example to have a Queue and
>> a Topic with the same name (e.g "foo"). AMQP defines an "address"
>> field on the source/target of links (producers/consumers), and a "to"
>> and "reply-to" field are available on messages, to indicate the
>> destination node (e.g queue/topic) address. These are typically string
>> values, and they form a single space since as there is no additional
>> node type information only the address name itself.
>>
>> This is is mostly an issue for non-temporary Queues and Topics since
>> TemporaryQueue and TemporaryTopic destinations will be given generated
>> addresses by the 'broker' peer through use of dynamic nodes, and so
>> can naturally be prevented from having the same addresses as each
>> other, and be made unlikely or unable to clash with non-temporary
>> nodes.
>>
>> To handle this mapping between JMS and AMQP it would seem we must either:
>> 1. Not support JMS Queues and Topics with the same name existing at all,
>> OR
>> 2. Allow multiple nodes to have the same address string but use type
>> metadata (via capabilities + annotations, see additional context) to
>> discriminate between them, OR
>> 3. Utilise address string naming conventions (e.g prefixes) for them
>> to separate the types into subspaces.
>>
>> The first option is an issue for implementations that already do, and
>> wish to continue to, allow Queues and Topics with the same name via
>> other protocols while also supporting AMQP, and would be a limitation
>> in terms of full JMS support. The second option would break reply-to
>> usage for any clients or intermediaries that don't understand the
>> message annotations and/or source+target capabilities carrying type
>> metadata (see additional context).
>
>
> Only in the case that the node name is ambiguous though, right? I.e. only if
> there exists both a queue and a topic with the same name. (Intermediaries
> shouldn't need to care, providing the annotations are defined such that they
> can be passed on without understanding them which they would need to be
> anyway for correct JMS behaviour at the receiving client).
>

Mostly in that case, but not always. There might be only one "foo"
that exists when they reply, but it might not be the correct type of
"foo" any more.

Warning, completely contrived example: An application sets JMSReplyTo
to a Queue "response" that currently exists and then sends it to the
"request" queue. Message sits on "request" Queue for a while.  Someone
decides to delete the "response" Queue because they think they are
done with it. Someone else creates a "response" Topic. A client which
doesnt understand the annotations/capabilities consumes the message
and proceeds to created a producer and reply to "response".  That
message is going to end up in the wrong place if we rely on the
address, since it will match but the missing annotations/capabilities
would have told us it isnt the correct destiantion.


> I don't really like dividing the namespace by type. Many applications
> shouldn't need to care about the type. Having to change the node name to get
> a different behaviour, rather than just changing the configuration of that
> node seems undesirable to me, at least for some cases.
>

I dont like it either, but as the spec only gives one namespace and
JMS requires multiple spaces I dont see another way which works. It
would have been nice if the core 1.0 spec handled this 'type' in some
way. It does for the message transfer formats for example, just not
for the things they are being sent to.

JMS does have the different types, and expects them to behave in
certain ways. For a JMS applications behaviour to be affected such as
in that contrived example above is also not very nice.

> While I wouldn't necessarily object to use of address prefixes in cases
> where disambiguation is needed, I would be disappointed if that were to
> become the accepted or even 'blessed' pattern.
>
> If a broker receives an attach to/from a node 'foo' and there is either a
> queue or a topic of that name (but not both), I think they should resolve
> the name to that existing entity. They should not *require* you to specify a
> prefix when there is no ambiguity.

In the JMS case I think there will always be ambiguity without a
naming scheme and/or metadata. As I have said before in other
contexts, if the application wants to use a JMS Topic "foo", it would
preferable it didnt get attached to a Queue "foo" because that
happened to be there.

>
> A further complication is where nodes are created on-demand as attach
> requests are made. However in this case the broker can provide the means to
> define patterns allowing particular node names to be matched to desired
> policy.
>

In addition to allowing the client to optionally assert it is getting
the behaviour it wants, creating nodes on demand is another reason we
defined capabilities for the different JMS destintion types (needed
for temporary queue vs topic, but useful for queue vs topic if the
broker supports auto-creating those with arbitrary addresses).

>
>> The third option either requires
>> clients to always utilise the full address strings in
>> session.createQueue("<queue-prefix>foo") etc calls, or providing a
>> means to configure the prefixes within the client so that they are
>> added/removed behind the scenes and the application just uses
>> session.createQueue("foo"), but the resulting AMQP address string
>> would be "<queue-prefix>foo". The main issue with requiring clients
>> always use the full address as the session.createQueue(..) value would
>> be for bridging between different systems using different conventions,
>> though the values for those methods are noted as being
>> provider-specific.
>>
>> Both the old Qpid AMQP 1.0 JMS client, and the new JMS client we are
>> creating that implements the JMS Mapping for AMQP being worked on,
>> currently do some form of the third option, providing a way to
>> configure a 'queue prefix' and 'topic prefix' that are used to prefix
>> the application provided strings in session.createQueue(..) etc for
>> outgoing addresses used for links and messages and be stripped from
>> incoming addresses on messages to give the names used for the
>> JMSDestination and JMSReplyTo objects. Temporary destinations are
>> named by the 'broker' peer and their addresses are used as provided.
>>
>> The main issue with this approach is that such configuration makes it
>> more difficult to use the client against a number of different
>> brokers, which is a goal, since this configuration is likely to differ
>> between them meaning even the simplest HelloWorld type example may be
>> unable to work against them without additional configuration.
>
>
> Even the simplest example may require some configuration of the broker in
> the form of creating the required queue/topic (unless they are created
> on-demand).
>

This is true. I was speaking about the client, I didnt mean to suggest
additional steps wouldnt still be needed for the broker.

> If brokers can handle the non-ambiguous case without prefixing, which I
> think is the ideal, then simply configuring the broker for the name used by
> the example should be sufficient. (In fact having the name the example uses
> be a command line option would also make sense, making adaptation even
> easier).
>

As above, I dont think there is a non-ambugious case for JMS. I also
dont mean to suggest the example cant have configuration so a user
could e.g use it with a different queue they already had rather than
create a new one, just that it would be nice if they didnt almost
always have to actually configure it.

>> Between
>> ActiveMQ and Qpid we currently appear to have 3 different options for
>> our brokers (two different prefixes being required, or it being
>> optional [at the cost of being unable to support Queues and Topics
>> with the same name]), and considering others would likely expand this.
>>
>> An idea to handle this was to have the brokers use connection
>> properties to inform the client of the prefixes (if any) they require
>> it to use, allowing different brokers to supply their own specific
>> value (if any) to meet their requirements, and allowing clients/simple
>> applications to work against many of them without further
>> configuration change.
>>
>> An alternative suggestion was to have the JMS Mapping define a set of
>> standard name prefixes the client would use by default, such that the
>> issue of Topics and Queues with the same name is addressed by the
>> mapping, while also allowing brokers to specify their own values via
>> connection properties so that their specific needs can still be met if
>> different (e.g they have existing naming conventions they wish/need to
>> retain).
>>
>> There was also a suggestion that something beyond a simple prefix may
>> be needed, I will let the person behind those thoughts expand further
>> to stop this getting any longer for now.
>>
>> Thoughts?
>
>
> I think 'recommended' practice should be to avoid ambiguous node names.
>
> I accept of course that we have to be able to cope with the situations where
> they exist (this is the case for brokers built around the pre 1.0 AMQP model
> of exchanges and queues also). These should be seen as workarounds corner
> cases however, not standardised as 'normal' behaviour.
>

I would say this is a normal behaviour for JMS, not really a corner case.

> Personally I think the behaviour you describe already in the JMS clients -
> the ability to configure the client to use prefixing - is sufficient. I
> don't think it needs to be something blessed by the AMQP mapping
> specification.
>

I think it solves the problem if you use it (or will do if we
incorporate the white/blacklisting idea in some way to prevent messing
with certain addresses), it would just be nice if it was slightly
slicker.

The old client wont work against 2 of our 4 brokers without
configuring that. Due to its heritage, the new client currently has
its prefixes defaulted to work with one of those brokers, and so it
will behave oddly or not work against the 3 others unless it is
reconfigured.

Adding a mechanism to convey the prefix automatically was simply a way
to avoid such hassle in some cases, and in others it wont be used and
would thus have no effect (potentially at the expense of needing to
ensure applications dont use JMS queues and Topics with the same
name).

> I would rather see the end user experience switching between brokers
> improved by growing consensus on (configurable) node resolution than have an
> additional standard that tries to paper over differences.
>

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