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From Fraser Adams <fraser.ad...@blueyonder.co.uk>
Subject Re: Use of subject for routing - moved thread to user list from earlier private discussion.
Date Fri, 29 Aug 2014 14:50:24 GMT

I guess that from my perspective one of my main concerns is consistency 
between the C++ and Java Brokers (and as much as possible with other 
vendors) without that things get very confusing.

Another thing I guess that I'd want to make sure gets thought about is 
how this stuff gets mapped to various APIs.

I think I've got some sympathy with the logic of Rob's comment "subject 
as "A common field for summary information about the message content and 
purpose" " and "The "to" field is defined as "identifies the node that 
is the intended destination of the message" " those sort of definitions 
seem to fit with traditional email interpretations of subject and to, 
however it's very definitely fairly "traditional" in Messaging to use 
the term "subject" when talking about publishing topics i.e. the 
traditional "UK.NEWS" subject, so it's not unreasonable that people 
might intuitively feel that subject and routing key have a relationship.

Back to my point about APIs though, one thing that I think is important 
is to be able to efficiently "multiplex" messages. In the current 
subject based world I could have a number of consumers to amq.direct 
called Q1, Q2, Q3 etc. and in JMS I could create a simple producer that 
had "amq.direct" as the JMS destination. From my client's perspective I 
can create that at startup time and send different messages to different 
places just by setting the subject. I wouldn't want to have to create 
new Destination objects every time that I wanted to send a message!! I 
know that "to" is an immutable AMQP 1.0 property, but what's the API 
mapping - and what should it contain??

what do I mean "what should it contain??" well different things might 
make different sense depending on whether one is using a 
"connection-oriented" API or not. For example in Messenger I "think" 
that the to would be the whole address - say 
"amqp://guest:guest@localhost:5673/qmf.default.direct/broker" what would 
it be in a connection-oriented API like JMS/qpid::messaging, would it be 
the same or would it just be "broker". We already have this sort of 
strange inconsistency around replyTo where one interpretation of a 
replyTo might be the fully qualified AMQP address of the originator, but 
the Broker's use the node name, both are reasonable interpretations, but 
they are clearly different and have different implications

Similarly in Messenger, as I say I think that the "to" gets set by 
setAddress() on Message, but I've no idea what caching (if any) takes 
place. Is it more efficient to do something like:

message.setAddress(address + '/' + subscriptionQueue);

where subscriptionQueue might be one of the Q1, Q2, Q3 things I 
mentioned above, or is it more efficient to do

message.setAddress(address);

and pass the subscriptionQueue information as the subject/routing-key 
whatever.


So I think it's not just about whether in the AMQP 1.0 spec. the subject 
and to now look more like the email interpretations (and I do have 
sympathy with that view) as I say though it's also about traditional 
interpretations of what "subject" means in messaging systems and also 
about how the various APIs allow multiplexing of messages.

Frase

On 29/08/14 15:12, Rob Godfrey wrote:
> So, I think the issue has come about on the Java Broker because I have been
> trying to move some of the other behaviours to be in line with the upcoming
> work on management and addressing.  In these cases the "to" field of an
> AMQP 1.0 message is used for routing purposes.  The inadvertent side effect
> of the changes (that to was promoted "to" be the primary routing attribute
> of the message) was that the exchanges which previously used the "subject"
> field of a 1.0 message where they would use "routing-key" of an 0.x message
> currently use "to".  Given the way that the legacy filters were constructed
> this is an error and should probably be changed so that the Java Broker
> reverts to routing on subject for the amq.direct and amq.topic exchanges
> (and any other address which in AMQP 1.0 offers the ability to use the
> legacy filters).
>
> In general, however, I think the subject is not the best fit for a mapping
> of the semantics of the 0.x "routing-key".  The AMQP 1.0 spec defines
> subject as "A common field for summary information about the message
> content and purpose", rather than giving it and explicit or implicit
> routing semantics.  The "to" field is defined as "identifies the node that
> is the intended destination of the message" which definitely implies some
> routing semantics but does not make them explicit.
>
> If an application is designed around using AMQP 1.0, I would not normally
> expect them to be putting routing information in the subject field.
>
> -- Rob
>
>
> On 29 August 2014 15:55, Fraser Adams <fraser.adams@blueyonder.co.uk> wrote:
>
>> All,
>> I've moved this thread to the user list. It started out on the back of an
>> email conversation between Rob Godfrey. Gordon Sim and and myself when I
>> couldn't get my QMF messages routed on the Java Broker.
>>
>> The original context was that I was sending messages to qmf.default.direct
>> and setting the message subject to "broker", this is pretty much exactly
>> what the python qpid tools such as qpid-config do.
>>
>> in AMQP 0.10 this works fine because the subject (which in AMQP 0.10 is a
>> user property called qpid.subject) gets treated as the routing key when
>> sent to a direct exchange.
>>
> When I did my AMQP 1.0 JavaScript qpid-config port I set the AMQP 1.0
>> subject (which is now an immutable message property and not an application
>> property) to "broker" and sent to amqp://guest:guest@localhost:
>> 5673/qmf.default.direct
>>
>>
>> This worked fine with the C++ broker, but with the Java Broker the message
>> wasn't routed and in subsequent conversations with Rob and Gordon I
>> discovered that the Java Broker doesn't route on direct exchanges based on
>> Subject rather it first tries to use to "to" - in other words if I sent to
>> amqp://guest:guest@localhost:5673/qmf.default.direct/broker it'd work or
>> if the to isn't set it uses the application property "routing-key".
>>
>> So basically this thread is around an inconsistency between the C++ and
>> Java Brokers where the C++ Broker continues to route on Subject for AMQP
>> 1.0 but the Java Broker does not.
>>
>> Rob and Gordon can fill in if I've missed anything, but it's probably best
>> to share this discussion on the user list.
>>
>> Frase
>>
>>
>>
>>
>> On 29/08/14 14:03, Rob Godfrey wrote:
>>
>>> I think in the context of where AMQP 1.0 is now, routing by subject is
>>> somewhat counter-intuitive, though when we started we did see the subject
>>> field in a routing-key sort of role.
>>>
>>  From my perspective I'd say exactly the opposite, that is routing by
>> subject seems entirely intuitive and is also consistent with what happened
>> with AMQP 0.10. Given the use of subject as the defacto routing key on all
>> of the python tools I don't think I'm alone in feeling that's intuitive.
>> I'm pretty sure that the subject is used for routing on topic exchanges so
>> I'm not sure why you think its counter-intuitive on derect exchanges?
>>
>>
>>> In terms of current behaviour - the legacy filters in the Java Broker
>>> just set up the bindings between the queue and the exchange and thus use
>>> the same routing as previously described (i.e. they're not currently using
>>> subject). Inside the Java Broker an exchange routes based on an abstract
>>> notion of the "routingAddress" of a message instance.  In AMQP 0.x that's
>>> taken to be the routing key.  In 1.0 it is as I described in my previous
>>> mail.
>>>
>>> If we collectively (and this discussion should really be on the users
>>> list) think that 0.x exchanges should route on subject when routing a 1.0
>>> message then I'm happy to change the default behaviour of the Java Broker -
>>> except for the "no name" exchange which will route on "to" because that's
>>> what we're defining on the addressing spec.  I'll then probably add options
>>> to the exchange implementation so that a user can configure on a per
>>> exchange basis to route on something else (except for the non default amq.*
>>> exchanges).
>>>
>>> -- Rob
>>>
>>>
>>> On 29 August 2014 13:45, Gordon Sim <gsim@redhat.com <mailto:
>>> gsim@redhat.com>> wrote:
>>>
>>>      On 08/29/2014 11:49 AM, Rob Godfrey wrote:
>>>
>>>          For the moment I guess we'll just have to live with the C++
>>>          and Java
>>>          Brokers having different ideas about how to route 1.0 messages
>>>          at exchanges.
>>>
>>>
>>>      The legacy-amqp filters are defined to work on the subject when
>>>      using the exchange as the source. I thought the java broker
>>>      supported those?
>>>
>>>      If I create a receiving link from amq.direct with a
>>>      apache.org:legacy-amqp-direct-binding:string of foo, then send a
>>>      message to amq.direct whose subject is foo, my expectation would
>>>      be that the receiver will receive that.
>>>
>>>
>>>


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