Re: [sca-bindings] ISSUE 2 - Callback support over the Web Servicebinding

[Anish Karmarkar <Anish.Karmarkar@oracle.com>]

Simon Nash wrote:
> Using the wsa:ReplyTo header for the callback endpoint does not seem to 
> exactly match SCA callback semantics, which allow callback messages to be 
> directed to a different endpoint from the endpoint that receives the reply 
> to the original request (by calling setCallbackObject() with a 
> ServiceReference).

I agree. It should be possible to say send the reply to the current 
request to my home address, but my callback address is my office 
address. Overloading ReplyTo for callback (as defined by SCA) would 
require introducing a new attribute/element of some sort in ReplyTo to 
distinguish between the two types of ReplyTos. If we are going to define 
a new element/attribute then I would prefer to define something like 
sca:Callback element of type EndpointReference.

>  Using wsa:ReplyTo also requires a message ID to be 
> added to the original request and the same message ID to be returned on 
> the reply and any callbacks in the wsa:RelatesTo header.

That is only true if you think of callback messages as "responses".

>  This is more of 
> an observation than a problem, though it does require extra state to be 
> maintained for the message IDs being exchanged.
> 
> In Tuscany, we did not use wsa:Reply To.  Instead we used the 
> WS-Addressing wsa:To endpoint reference with reference parameters to 
> represent the callback endpoint (as a wsa:EndpointReference), the callback 
> ID, and the conversation ID for stateful callbacks.
> 

Did you mean wsa:From and not wsa:To? wsa:To is a URI not an EPR.

Between ReplyTo and From, I would most certainly pick From. But there is 
still some overloading going on here. It is assumed that the sender of 
the message and the callback address is the same. It may not be.
The reason I prefer From over ReplyTo is because From is optional for 
all the WSDL MEPs and not many implementations rely on it. So the 
overloading problem is less of a problem than that in the case of ReplyTo.

> I did not propose a specific solution when opening this issue because I 
> wanted to open this up to as many suggestions and options as possible. The 
> discussion around wsa:ReplyTo has been interesting.  Perhaps someone will 
> have an idea on we can overcome the semantic mismatch that I mentioned 
> above.  I agree that defining a new header for SCA callbacks would be 
> undesirable.  Even the use of SCA-specific reference parameters seems less 
> than ideal, but without them I'm not sure how additional information like 
> the callback ID could be transmitted.  We can't use wsa:MessageID for 
> this, because of the statement in the WS-Addressing spec that "No two 
> messages with a distinct application intent may share a [message id] 
> property."  Perhaps the callback ID could be mapped into a wsa:MessageID 
> by adding a unique discriminator, so different callback requests could use 
> distinct message IDs from which the same callback ID could be extracted.
> 

I agree that inventing a new header (another WS-* spec anyone?) should 
be the last alternately. But I don't see how MessageId would do anything 
for callbacks (it is a URI not an EPR and it has completely different 
semantics). I also don't see how sca-specific refps would solve the problem.

Of the solutions that have been brought up only From and a new SOAP 
header block seem viable to me.

Comments?

-Anish
--

>     Simon
> 
> Simon C. Nash, IBM Distinguished Engineer
> Member of the IBM Academy of Technology
> Tel. +44-1962-815156  Fax +44-1962-818999
> 
> 
> 
> Khanderao Kand <khanderao.kand@oracle.com> 
> 01/10/2007 22:47
> 
> To
> Michael Rowley <mrowley@bea.com>
> cc
> sca-bindings@lists.oasis-open.org
> Subject
> Re: [sca-bindings] ISSUE 2 - Callback support over the Web Service binding
> 
> 
> 
> 
> 
> 
> Michael Rowley wrote:
>> One problem with using WS-Addressing wsa:ReplyTo is that it is usually
>> used to send the response message of a request/response pair.  I don't
>> think that WS-Address forbids its use for subsequent messages
>> (callbacks), but it would at least be unconventional.
>>
> [khanderao] IMHO WS-Addressing does not make any assumptions on the 
> number of returned messages. It is upto the integration scenario to have 
> one or many.
>> However, if we can't use wsa:ReplyTo, that would seem to imply that we
>> have to devise our own header to use, but that would be getting
>> dangerously close to inventing a wire-level protocol, which we don't
>> want to be doing.
>>
> [khanderao] Introducing SCA specific correlation / replyTo parameters 
> would not be essential. As far as possible we should be using the 
> available standards,  like WSA / WS-Coordination etc..
>> Michael
>>
>> -----Original Message-----
>> From: Eric Johnson [mailto:eric@tibco.com] 
>> Sent: Monday, October 01, 2007 12:16 PM
>> To: sca-bindings@lists.oasis-open.org
>> Subject: Re: [sca-bindings] NEW ISSUE: Callback support over the Web
>> Service binding
>>
>> Created as: http://www.osoa.org/jira/browse/BINDINGS-2
>>
>> -Eric.
>>
>> Simon Nash wrote:
>>
>>> TARGET:
>>>
>>> Web Service Binding specification, section TBD
>>>
>>> DESCRIPTION:
>>>
>>> The Web Service binding provides no example or suggestion for how SCA 
>>> callback semantics could be carried over Web services.  There is an 
>>> example in section 2.2.3 for how conversation semantics could be 
>>> supported.  It would be good to give some guidance (somewhere in the
>>>
>> range 
>>
>>> between example and normative) for what could be done for callbacks.
>>>
>> One 
>>
>>> possibility is to make use of the capabilities provided by
>>>
>> WS-Addressing.
>>
>>> PROPOSAL:
>>>
>>> None yet.
>>>
>>>
>>>
>>>
>>>
>>>
>>> Unless stated otherwise above:
>>> IBM United Kingdom Limited - Registered in England and Wales with
>>>
>> number 
>>
>>> 741598. 
>>> Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6
>>>
>> 3AU
>>
>>>
>>>
>>>
>>>
>>>
> 
> 
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> 
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> 
> Unless stated otherwise above:
> IBM United Kingdom Limited - Registered in England and Wales with number 
> 741598. 
> Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
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