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From Rob Godfrey <rob.j.godf...@gmail.com>
Subject Re: AMQP 1.0 Message Encoding - using AMQP typed data vs. Mime content-type / content-encoding (Was (unintentionally I assume): P)
Date Wed, 10 Sep 2014 19:02:58 GMT
On 10 September 2014 20:54, Fraser Adams <fraser.adams@blueyonder.co.uk>

> Sorry I missed this one,
> Comments inline.
> On 10/09/14 18:08, Rob Godfrey wrote:
>> I think it's inconsistent to use a mechanism (content type) intended for
>> opaque data on data which is not opaque... only in a couple of special
>> cases (an amqp-value section containing a string or binary value) does it
>> make sense.
> Hmm I could interpret opacity in a number of ways. For example the data
> that we are talking about is JSON, now that happens to be using a String as
> a container format but the fact that it is JSON is *entirely opaque* to the
> AMQP type system hence why IMHO it entirely makes sense to pass additional
> metadata to detail how it should be interpreted.
> Now described might be OK, but I assert that following common design
> patterns is a very good idea and the pattern that I suspect 99 per cent of
> the population would instinctively choose would be the pattern employed by

OK, to reveal my personal preference - in this case I would avoid using the
AMQP type system, and I would be using the content-type and sticking the
json string in a data section.  I think this is much more sensible than
using a described type and trying to invent yet another way of encoding

>    In general the question is who is the field aimed at - the
>> AMQP library or the client application? I think the content-type should
>> actually be interpreted by the AMQP library - in the same way that I would
>> expect the AMQP types to be.
> I'd probably have assumed it would be more the client application, but no
> matter I think that if you expect the AMQP library you actually strengthen
> my argument. If I want to do what I've suggested I can have my JavaScript
> library take a JavaScript object, set the content-type and use that to tell
> the recipient how to deserialise it transparently passing a JavaScript
> Object to the consumer client.
> That's fine and grand except that you seem to be telling me that
> content-type can't actually be used to do *anything useful*!!!

No - I'm telling you you should use the data section type and not the
amqp-value section type :-)

>> I think it "appears" to be more convenient to set the content-type on the
>> properties field for a string value because we perceive that the client
>> library would be able to understand AmqpValue(String("{ }")) but not
>> AmqpValue(Described(Symbol("org.json"),String("{ }"))) and that the
>> content-type would be made available to the end user of the API for them
>> to
>> then be able to understand that this is Json, and not intercepted by the
>> client library.
> No what I'm saying is that it "appears" to be more convenient because
> there's about half a billion AJAX based web-pages that set
> content-type=application/json and send the JSON string in the HTTP Message
> Body.
> As it happens my JavaScript Library can do this fairly trivially with
> message.body = new proton.Data.Described({"some": "json"},
> 'application/json');
> And it's roughly as easy to add some code to the serialisation of
> Described to call toJSON() when it encounters a descriptor of
> "application/json" as it would be to add something around content-type
> (though other APIs might have more fun :-)) but although in implementation
> terms it's roughly as easy it seems an awful lot weirder and less intuitive
> than
> message.setContentType('application/json');
> message.body = {"some": "json"};
And I'm saying I agree - but that this should go in a data section not an
amqp-value section containing a amqp string object.  I've no idea how
Messenger / Proton API works around these things, but in general I would
say the 90% use case should be the data section rather than using the amqp
type system to encode your data.  I agree on using the more widely used
HTTP / MIME style for describing content.

>     However that's not necessarily the case (the current JMS
>> client doesn't expose the content type IIRC, and may use it for switching
>> behaviours on opaque data - e.g. for object messages).
> That's a reasonable point, though in that case I'd suggest that the
> content type should be an extension type "application/x-amqp-map" and
> shouldn't be interfering with any IANA registered content types.
>> More generally I think we collectively (in Qpid and in the wider AMQP
>> world) need to decide when/if we use the AMQP type system to encode data,
>> versus using MIME types.
> For the most part I'm fairly happy with the AMQP type system, though I can
> recall Gordon and Rafi had quite an exciting debate about the relative
> merits of JSON versus the AMQP type system.
> I wasn't specifically looking to rekindle that in this discussion, this
> was more about using content-type in a manner that I would assert the *vast
> majority* of the broader technical community would intuitively expect it to
> be used.

Basically I'm saying we should use things consistently.  If you want to use
HTTP / MIME style (which I think is better for pretty much everything
except for maybe a few cases) then just stick your message body data in a
data section.  Only where there is some specific advantage to be gained
over the HTTP / MIME encoding style would I consider using the AMQP type
system for your message data.

>> If I want to start compressing data as I generally send large messages,
>> should I send my data with the content-encoding as gzip, or should I
>> define
>> a new described type for compressed data.  If the former - what if the
>> large message I want to send is a map?
> well I think that I'd probably follow an Internet style pattern along the
> lines of:
> message.setContentType('application/x-amqp-map');
> message.setContentEncoding('gzip');

Yep - which would all go in a data section... which is in compliance with
the spec.

>> My general advice is to choose one mechanism or the other and not to
>> confuse the two.
>> Of course you have an inconsistency to resolve if somebody sets an AMQP
>>> string body with content-type='application/json; charset="utf-16"'
>>> Could we say: You MAY use content-type with an AMQP string body but it
>>> MUST NOT not have a charset and MUST be ignored by the sender. It MAY be
>>> used as a decoding hint on the receiver.
>>>  The spec is the spec, and now ratified at ISO, so the wording there is
>> not
>> going to change.  We could interpret the spec more loosely, but - as above
>> - i think that ultimately that will lead to more confusion.
>> -- Rob
> Yes the spec is the spec, but in my opening paragraph I asserted that
> opacity is in the eye of the beer-holder and the fact that this stuff is
> JSON is very definitely opaque from the perspective of AMQP in the
> scenarion that I've been describing.
Yep - so the appropriate section type is "data"...  :-)

-- Rob

> Frase
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