From: Mike Edwards
Sent: Tuesday, October 13, 2009 3:52 PM
To: OASIS Bindings
Subject: Re: [sca-bindings] Response to: "Microsoft technical
comment: Develop interoperable approach notspecific to SCA for callbacks"
Thanks to Jim
for these comments - they help the debate here.
comments inline as:
Strategist - Emerging Technologies, SCA & SDO.
Co Chair OASIS SCA Assembly TC.
IBM Hursley Park, Mail Point 146, Winchester, SO21 2JN, Great Britain.
Phone & FAX: +44-1962-818014 Mobile: +44-7802-467431
[sca-bindings] Re: [sca-bindings-comment] Response to: "Microsoft
technical comment: Develop interoperable approach notspecific to SCA for
On Oct 7, 2009, at 7:42 PM, Eric Johnson wrote:
Here are my immediate thoughts,
in response, for the TC's consumption:
Michael Champion wrote:
Thank you for considering
Microsoft's suggestion for improving the SCA Web Services Binding spec's
We suggested that Web Services
callbacks in the SCA Web Services Binging spec should interoperate with
comparable frameworks such as JAX-WS and WCF, and not be limited to various
implementations of SCA. This would promote the original goals of the Web
Services standards to achieve wire-level interoperability among diverse
run-times and platforms.
I'm unaware of anything in
JAX-WS that addresses callbacks, so I don't understand that point, or how it is
comparable. Nor am I aware of anything that we've specified that prevents
a vendor from using JAX-WS to implement callbacks support within the SCA
environment. Presumably implementation experience will reveal details
I have run into a number of difficulties trying to integrate SCA callbacks with
WCF services (specifically, duplex services, see next comment) for a client.
Basically, the way SCA represents callbacks at the protocol level (in WSDL and
on the wire) is not interoperable with WCF. The only way to make these types of
interactions work is to introduce proprietary behavior in an SCA runtime to
support WCF's protocols.
So, Jim, do you want to make it an objective that
the SCA Web service binding should be capable of interoperating with the WCF
Separately, do you want to make the SCA Web services
callback protocol the same as the WCF callback protocol?
A separate question: is the WCF callback protocol
standardized in any way?
With respect to WCF, I'm not
aware of its capabilities, and whether or not it provides anything equivalent
to the callback functionality of SCA. Does anyone else on the TC have
insight into this?
The analog of SCA callbacks in WCF is duplex services, which provide
bidirectional communication. One difference between WCF duplex services and SCA
callbacks are the former are stateful. That is, the WCF client instance
originating the forward call will receive callbacks. Prior to the deferral of
conversational services, SCA conversational callbacks could be used to model
As far as SCA interoperation is concerned, I forsee
2 cases of interest:
a) WCF client talking to an SCA service which has a
b) SCA client talking to a WCF service with a
a) could be remodelled in WCF as a stateless client
that offers a service which is has the callback interface of the SCA service
- this clearly is not the same as current WCF
callbacks, but it is a potential practical approach
b) gives the problem that the WCF service may have a
business interface that makes the assumption that there is out-of-band
data that ties the callback messages to a given
forward message. This is indeed tricky to deal with.
At the protocol level, duplex services (which use the WCF duplex binding) rely
on WS-RM and sequence ids for correlation. In WSDL, WCF represents duplex
services as a a form of solicit-response with an output/input sequence.
For SCA at the binding level, in principle this could
be handled by a particular policy specification which would mandate the use of
This does not solve the statefulness problem of b)
At the wire level, what we've
describe remains interoperable and compatible. Seems to me like the
actual issue is at the protocol level.
At the wire level, things as they currently stand are not strictly
interoperable because SCA callbacks are stateless and WCF duplex services are
The SCA Binding TC responded
by saying that the SCA Web Services binding protocol "defines an *SCA* Web
service callback protocol standard” and that it "is not meant to
satisfy general purpose callback requirements with a broadest scope
possible". In other works, the TC believes that the SCA Web Services
callbacks will NOT be interoperable with non-SCA implementations
That sounds incorrect to me. Seems like it is completely out of scope for
an SCA TC to dictate how or if any standard outside of SCA reaches
compatibility with something inside of SCA. Nothing prevents other
implementations (Mike Champion's word choice) from achieving interoperability.
The TC's response goes on to
say that "This TC does believe that it should define an interoperable Web
services protocol that implements SCA callback and it has done that. It does
not believe that it is in the scope or interest of this TC to define a callback
protocol for all architectures and programming models." We respectfully
find this statement contradictory, unless the TC defines the term "interoperability"
in its narrowest form: SCA implementations will only be interoperable amongst
themselves, and not with other frameworks and runtimes. We would find this
unfortunate, as OASIS is committed to broad interoperability, especially when
it comes to use of Web Service wire protocols. It would be better to
standardize a Web Services callback in a separate spec, with the participation
of all vendors who build platforms and products that support Web Services wire
Yes, we define interoperability amongst SCA runtimes (including from different
vendors), since that's within the scope of our charter. The
"narrowest" sense would be only interoperability among a single
vendor (see JMS itself, the "binding.sca" binding, and various
Microsoft networking protocols, at some point in time.). So I disagree
with the characterization of this as the "narrowest" possible form of
interoperability. Further, we don't deny the utility of a broad
specification, and yes, we believe interoperability at a larger scale would be
useful. It just isn't in our charter.
Is Microsoft explicitly requesting a change in charter? If so, they
should explicitly state that.
It is clear from 
that the TC is aware that interoperability with non-SCA runtimes is an issue.
The TC discussed the idea of moving the callback portion of the protocol into
its own document in order to address "the use case of non-SCA clients does
walk into the more general territory alluded to by MS." We highly
recommend that the Binding and Assembly TCs work together to design a Web
Services Binding spec that is interoperable with non-SCA technologies.
Without interoperability, software developers and users will find it
difficult to use SCA in the heterogeneous, multi-vendor environments that all
our customers live in.
Absolutely nothing here prevents vendors from building compatible systems with
both SCA and non-SCA parts that interact. In fact, we expect it.
Further, when bridging from a non-SCA environment to an SCA environment,
a vendor such as Microsoft should only have to implement compatibility with the
SCA WS Callbacks mechanism once, and it should work with all vendors providing
SCA environments. If they believe this last point is incorrect, it would
be enormously useful for Microsoft to identify the specific oversights of the
currently specified approach.
These last statements seem a bit odd. I thought one of the purposes of the web
services binding was to provide interoperability into and out of a domain.
For example, I think the requirement to integrate with WCF and non-SCA
software will be much more common than integration between different SCA vendor
runtimes. Of course, other systems could integrate with SCA by implementing
support for proprietary SCA protocols but that kind of defeats the purpose of
interoperability (a general integration mechanism).
It seems that Microsoft is proposing the SCA TCs not define a callback
mechanism and instead work jointly in some other TC to define an interoperable
protocol for bidirectional communication. If this is the case, such an approach
would provide a lot more value to end-users than a proprietary SCA mechanism
since it would allow SCA, WCF, JAX-WS, etc. to interoperate at a much deeper
level. Maybe this is something we should ask Microsoft if they would be
interested in pursuing?
1) Are you proposing that we should remove the
current SCA callback description from the specification?
2) I suspect that if we went down the road of
standardizing a generalized callback mechanism in some new TC, that it would
take quite some time to complete.
Are you happy for there to be no defined way of
providing SCA callbacks over Web services for a significant amount of time -
with the probable consequence
of vendors building SCA runtimes simply each
implementing their own way of doing things in the interim?
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