|Hi Mike, |
Sorry for the delay but I've been traveling again....Comments inline.
On Oct 13, 2009, at 3:52 PM, Mike Edwards wrote:
Thanks to Jim for these comments - they help the debate here.
I've put 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
|From: ||Jim Marino <firstname.lastname@example.org> |
|To: ||OASIS Bindings <email@example.com> |
|Date: ||10/10/2009 22:24 |
|Subject: ||Re: [sca-bindings] Re: [sca-bindings-comment] Response to: "Microsoft technical comment: Develop interoperable approach notspecific to SCA for callbacks"|
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 interoperability (http://www.osoa.org/jira/browse/BINDINGS-87).
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 here?
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 callback protocol?
Separately, do you want to make the SCA Web services callback protocol the same as the WCF callback protocol?
To answer the first question, I'd like mandatory interoperability for the Web Services binding based on basic profile and for any other capabilities specified by the binding (e.g. policies) to always support interoperability (otherwise, call it something other than "Web Services"). For example, if the binding specifies how callback interactions are manifested, I'd like that to use an interoperable rather than specific SCA mechanism. While an SCA specific callback mechanism may add value to the use case where an SCA component needs to talk to a bidirectional SCA service hosted on a different runtime implementation, I think it will be extremely limited. This is because the target service will not be interoperable and will only be callable by another SCA client. In contrast, if the Web Services binding used an interoperable mechanism for callbacks, the target service would be callable by other SCA components hosted in different runtime implementations as well as clients such as .NET, non-SCA BPEL, JAX-WS, etc. Another advantage to this approach is it would make the development experience much easier as it is more likely for tooling such as wsimport (JDK) and SrvUtil (.NET) to support generating proxies for these types of services.
In terms of the second question, my preferred approach would be to form a separate TC that took as input ideas from the Web Services binding spec as well as WCF. I have some ideas on how this might look we could explore.
A separate question: is the WCF callback protocol standardized in any way?
Not to my knowledge although it uses standardized "protocols" (a combination of WS-Addressing and WS-RM).
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 this functionality.
As far as SCA interoperation is concerned, I forsee 2 cases of interest:
a) WCF client talking to an SCA service which has a callback
b) SCA client talking to a WCF service with a callback
Yes although other non-SCA clients and services could be involved.
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 WS-RM etc.
This does not solve the statefulness problem of b) above
I'm approaching this from the view that a user wants to create a bidirectional service which is interoperable and does not place platform-specific demands on clients. We could define a mapping from SCA to .NET and vice versa but that wouldn't really be interoperable since it would require clients to be either SCA or .NET.
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 stateful.
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 protocols.
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?
Yes, with the idea that a separate interoperable mechanism will replace it.
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.
Yes, I think it would definitely take more time.
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?
To be clear, I think it is important to note that the current mechanism does not define an interoperable mechanism for handling callbacks/ bidirectional communication for SCA services. What it defines is a way for an SCA client component hosted in one runtime implementation to invoke a bidirectional service offered by an SCA component hosted in another vendor runtime. IMO, this is of very limited value given that if the current mechanism is used, the target service will not be callable by anything other than an SCA client or a client that directly implements the SCA-specific protocols.
That said, I think the alternative of defining this in a non-interoperable way is much worse. It solves the limited use case mentioned above but will lead to serious long-term migration issues for end-users. I think it is safe to say the proposed mechanism will need to be changed if it is to be interoperable. Even in the hypothetical case where the SCA approach is adopted wholesale by other platforms, the namespaces and way of representing callbacks would need to be changed to fit into a more general approach. If the current approach is adopted, there is the real potential that an interoperable approach will be needed later that is incompatible with the former. Existing deployments that used the current approach and switched over to the interoperable approach would either require mediation or duplicate services to avoid breaking existing clients. From a vendor perspective, this would require runtimes to support both approaches for a very long time.
If the current callback mechanism was removed, this would mean there would be no standardized way for a bidirectional service to be called by an SCA client hosted in a different runtime implementation over a Web Service protocol. Given the above scenario, I think this is a good thing for end users. Basically, it requires them to use interoperable mechanisms for communicating outside a domain over Web Services protocols, which would mean services need have either request-response or simple one-way operations. While this inhibits the very specific use case mentioned above, it avoids a migration issue down the road and allows time for a proper mechanism to be defined based on input from other stakeholders (i.e. technology platforms).