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Subject: RE: [soa-rm] Opacity calumny
Agreed - so is there something about the loose coupling and the design of the interfaces that is peculiarly SOA (and thus would fit in the RM), or is a tightly coupled interface still ok? I tend to think that we need to capture something in the RM that focuses on loose coupling - which is similar to the OO concept of public vs. private (Frank's earlier assertions). -----Original Message----- From: Michael Stiefel [mailto:development@reliablesoftware.com] Sent: Wednesday, July 13, 2005 9:01 AM To: Bashioum,Christopher D; soa-rm@lists.oasis-open.org Subject: RE: [soa-rm] Opacity calumny I agree. Access through interface does not, however, automatically get you loose coupling. Poorly designed interfaces can produce tight coupling. Michael At 09:37 PM 7/12/2005, Bashioum,Christopher D wrote: > Ken, et. Al., > >I was reading your email thread and started thinking about opacity as it >relates to the concept of loose coupling. It seems to me that the real >issue is loose coupling as opposed to opacity per se. One could publish >all the specs related to a service, including the actual code, as part >of the service description. As long as access to the internals of the >service is strictly via the interface, then you still have loose >coupling (and, a service). If, on the other hand, you don't tell anyone >about the internals of a service, but you still allow access to the >internals via globals or back-door methods, you will not have loose >coupling. > >I think opacity is really a surrogate for loose coupling. In other >words, its not about what you know of the service via its metadata, its >what you can access of the internals of the service (thus the >private/public that Frank mentions). Ken - I think this is also what >you were trying to say also (thus, the ever more violent agreements >below). So, that being said, we should probably address this more >succinctly in the document. > >-----Original Message----- >From: Ken Laskey [mailto:klaskey@mitre.org] >Sent: Monday, July 11, 2005 5:24 PM >To: Michael Stiefel; Frank McCabe >Cc: soa-rm@lists.oasis-open.org >Subject: Re: [soa-rm] Opacity calumny > >Agree, agree, agree. This list is getting much too tame. > >I would suggest that the concept of opacity is relevant to the RM, and >the >fact that there are tradeoffs and examples of tradeoffs is relevant to >the >RM. I also think relating this to a/c/p is relevant to the RM. Being >an >engineer, it is difficult to list all the problems and not propose >solutions, but alas that is not part of the RM. So what of this is >appropriate for the RA? > >Ken > >At 05:09 PM 7/11/2005, Michael Stiefel wrote: > >I agree as well. > > > >Opacity is a design tradeoff and hence a concrete, not abstract issue. >All > >other things being equal more opacity is better than less, but in >reality > >you are trading off opacity for other things such as how much metadata >you > >publish and how much dependency you generate as a result. > > > >Michael > > > >At 04:31 PM 7/11/2005, Ken Laskey wrote: > >>Frank, > >> > >>To agree even more violently, this raises (and I allude to a little in > > >>the draft) the question of how to provide a means to publish metadata >as > >>it is found to be needed but in a way that supports a consistent use >of > >>the metadata in evaluating an entity against unambiguously stated >a/c/p, > >>which themselves can be augmented and will evolve to meet business >needs > >>of the users (where if I am a design engineer, my business need is to > >>find the technically correct solution engine). > >> > >>This represents a much ignored need as well as a terrible run-on > >>sentence. :-) > >> > >>Ken > >> > >>At 04:10 PM 7/11/2005, Frank McCabe wrote: > >>>Ken: > >>> > >>> I think we are in violent agreement. The essence of the Java > >>>execution engine example is that it is precisely that the > >>>implementation is important -- not whether or not its good style!! > >>> > >>> Incidentally, if it is important to know who wrote the > >>>implementation, then that too should be part of the public >description. > >>> > >>> Perhaps another aspect concerns the on-the-wire vs. resource model > >>>of service semantics. In the resource pov, issues of implementation > >>>(transparent or otherwise) figure prominently. In the on-the-wire > >>>pov, you only look at the message traffic; and do not even consider > >>>the implementation. > >>> > >>> They are related of course: a message is valid if it is well formed > >>>(in some communication language) and if the implied predicate is > >>>satisfied (there really is money in your bank account). > >>> > >>> But, a Service Oriented Semantics (SOS) would focus on the on-the- > >>> wire interpretation and require that the service participants > >>>faithfully reflect the messages in their systems. I.e., it is similar > >>>to the model/proof theory distinction: provided that the service > >>>implementation is a correct Interpretation of the communication, the > >>>participants can focus on the Formulae being communicated. > >>> > >>>That all being said, there are definite limits to descriptions. I.e., > >>>there will always be unstated (unstateable?) assumptions that are > >>>required to be shared for successful interactions. > >>> > >>>Frank > >>> > >>>On Jul 11, 2005, at 12:49 PM, Ken Laskey wrote: > >>> > >>>>Frank, > >>>> > >>>>Opacity/transparency is not a dimension on which to distinguish > >>>>services; it is more of a fallacy that we may need to explain our > >>>>way around. This ties in very much with what I've tried to say > >>>>about metadata and how metadata characterizing an instance is often > >>>>the basis upon which the instance is evaluated in the context of > >>>>assumptions, constraints, and policies (a/c/p) of providers and > >>>>consumers. In some circumstances, the amount of information that > >>>>needs to be published about an entity may maintain little in the > >>>>way of its opacity, but that is more likely related to a/c/p than > >>>>it is to WSDL. > >>>> > >>>>See more inline. > >>>> > >>>>At 03:23 PM 7/11/2005, Frank McCabe wrote: > >>>> > >>>>>I read the revised section on services. And an issue that made me > >>>>>uneasy before has resurfaced -- about opacity. > >>>>> > >>>>>I think that the opacity/transparency dimension is not a good basis > >>>>>for distinguishing services. I am aware of the intuition: that > >>>>>somehow when you use a service you should not have to worry about >the > >>>>>implementation. But what does it mean to say: "the implementation >is > >>>>>hidden from the service consumer"? If the service is to add lists >of > >>>>>numbers then its implementation is hidden provided numbers are >added > >>>>>up correctly. (I.e., we should not care about whether its in C++ or > >>>>>C#; multi-threaded or federated). > >>>> > >>>>So here the service adds numbers and while we can imagine what is > >>>>happening, we care no more of the specific than we do when we punch > >>>>numbers into a calculator. > >>>> > >>>> > >>>>>But, some services are explicitly about implementation issues: > >>>>>security services, workload distribution services, java remote > >>>>>execution engine services etc. etc. I.e., it is of the essence that > >>>>>to use a Java remote execution service, you have to send it a java > >>>>>class to execute. In that sense, the implementation is critical. > >>>> > >>>>But you call a java remote execution engine when you have an > >>>>appropriate java class to execute. If it is a general service that > >>>>is implemented as a java remote execution then the WS interface > >>>>should more likely be a collector of information that (inside the > >>>>service) is rewritten as the necessary java class. You could > >>>>require a java class as input but I'm not sure that would be > >>>>considered "good style". > >>>> > >>>> > >>>>>A clearer distinction is public/private. A service is characterized > >>>>>by a public description of its functionality. The public >description > >>>>>should be sufficient to be able to successfully interact with the > >>>>>service; and no additional information should be required. > >>>> > >>>>Not really because what you consider part of the private > >>>>information I might consider very pertinent to deciding whether > >>>>your service is suitable for my purposes, i.e. I need it to be > >>>>public. For an extreme example, I used to work for your company > >>>>and I know one of your developers is clearly incompetent and I'd > >>>>never use anything that idiot touched. > >>>> > >>>>The challenge is in defining "should be sufficient to be able to > >>>>successfully interact with the service". I think we need to > >>>>discuss opacity in these terms because it is strongly connected > >>>>with SOA and may be more useful as a vague concept than as an > >>>>absolute measure. > >>>> > >>>> > >>>>>Frank > >>>> > >>>>Ken > >>>> > >>>> > >>>> > >>>> > >>>> > >>>> > >>>>-- > >>>> > >>>>-------------------------------------------------------------------- >-- > >>>>----------- > >>>> / Ken > >>>>Laskey > >>>>\ > >>>> | MITRE Corporation, M/S H305 phone: 703-983-7934 | > >>>> | 7515 Colshire Drive fax: > >>>>703-983-1379 | > >>>> \ McLean VA > >>>>22102-7508 / > >>>> > >>>>-------------------------------------------------------------------- >-- > >>>>------------ > >> > >>-- > >>---------------------------------------------------------------------- >----------- > >> / Ken > >> Laskey >\ > >> | MITRE Corporation, M/S H305 phone: 703-983-7934 | > >> | 7515 Colshire Drive fax: 703-983-1379 >| > >> \ McLean VA 22102-7508 >/ > >>---------------------------------------------------------------------- >------------ > >> > >> > > > > > >-- > >----------------------------------------------------------------------- - >--------- > / Ken >Laskey \ > | MITRE Corporation, M/S H305 phone: 703-983-7934 | > | 7515 Colshire Drive fax: 703-983-1379 | > \ McLean VA 22102-7508 >/ > >----------------------------------------------------------------------- - >----------
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