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Subject: RE: [soa-rm] Opacity calumny
We should also be careful not to handle coupling in a binary fashion - i.e. either it is tight or it is loose, with no in-between. Having an interface description (such as WSDL) may not in itself constitute loose coupling. It is how that interface is deployed within the bigger picture that further determines the degree of coupling. For example, consider the notion of whether or not a recipient is known to the sender of a message, in 2 cases in which WSDL is used. With a pub/sub architecture and design, the recipient of a message will (most often) not be known to the sender - it all depends on the subscriptions. We may consider this to be "very loose coupling". Now consider a case in which the recipient is always known to the sender - we may consider this to be "loose coupling" (or tighter coupling than the former case). In both cases, WSDL is used - yet it is the bigger picture (the overall architecture and design) that further determines the degree of coupling. Joe Joseph Chiusano Booz Allen Hamilton O: 703-902-6923 C: 202-251-0731 Visit us online@ http://www.boozallen.com > -----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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