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Subject: RE: [soa-rm] What is SOA (Really???)
The most simple level, the atomic level, for me is one service
and
one service consumer. That also defines a community and it allows
a
full description of both service and service consumer, which
together
form the architecture. It is possible to model a service
in
isolation, but I would say that that case is not the one that
interests
us. It is not why we are here doing this. We are here to
take the existing
community and abstract it's most basic fundamental
components in order to
refine a reference model for SOA, As for who
does the orienting, it is the
community, which is well beyond clearly
needing a model to guide future
development of SOA which already
exists in overabundance.
By building
the set of basic components that will allow the more
complete set of features
Ken described yesterday in response to what
makes SOA different from
Distributed AD, we are really just taking
what already exists and abstracting
from that, regardless of the fact
that we are attempting a top-down modeling
effort. To some extent, we
are also missing the argument about whether or not
a model-driven
architecture is the best direction for organizing this effort
going
forward. I think it is, but our socratic methodology insists
we
answer the question of why is a MDA better than, say Agile
Methodology
or Extreme Programming where everything is a special case
and ought to be
built around the specific existing situation.
Ciao,
Rex
At 4:30
PM +0200 5/27/05, Gregory A. Kohring wrote:
>You have not quite captured
the debate. It is not that I feel features
>needed to make multiple
services function are superfluous, it is just
>that no one has ever
clearly said what those features are.
>
>At the abstract level, what
concepts do you think are required?
>
>
>Take your community
example and suppose its communication model is
>the Internet. Would apply
such a model to a village of 3 houses,
>where people could just walk
across the grass to talk with each
>other? The concept of a community is
obviously more fundamental
>than such a communication model would allow,
so do you need to mention
>it all? While the communication model is
important, in my opinion it
>does not enter until you are ready to create
a reference architecture
>for a particular type of
community.
>
>As I see it, that is the problem we face. How to make
a reference
>model simple enough that it applies to simple
situations.
>
>
>-- Greg
>
>
>
>Don
Flinn wrote:
>> IMO the TC is spit into two camps and many times
the two contingents are
>> speaking past each other, enumerating
their own view. Rebekah's
>> variation of the house analogy
captured the difference:
>>
>> A- One side looks at a
Service Oriented Architecture from the viewpoint
>> of the
community whereas the architecture describes the houses
>>
(services) and their relationship to each other (coordination,
>>
choreography, etc.) and constructs their model from that
viewpoint.
>> B- The other side looks at a Service Oriented
Architecture from the
>> viewpoint of a single house (service)
and constructs their model from
>> that
viewpoint.
>>
>> Until and unless each viewpoint
addresses the concerns of the other
>> viewpoint we will never
reach consensus. One can not understand another
>> until you walk
a mile in their shoes.
>>
>> Following my suggestion,
being of the (A) viewpoint, let me attempt an
>> explanation of
the (B) viewpoint. B's contention is that the essence of
>>
what should be modeled is a service, where a service subsumes
the
>> service itself, Metadata and Discovery, Presence and
Availability
>> (Figure 1). Once we have fully modeled a
service, our customer, the
>> specification writer, can develop a
specification for any SOA
>> architecture, including the complex
scenario in Appendix B, by using the
>> concepts of a single
service multiple times, as needed. Thus, features,
>> which
are exogenous to the service, that are needed to make multiple
>
> services function as a unit are superfluous to the
model.
>>
>> Does this capture the (B) view of what our
RM should be?
>>
>> Could a (B) viewpointer summarize
the (A) viewpoints?
>>
>>
Don
>>
>>
>>
>> On Fri, 2005-05-27 at
11:41 +0200, Gregory A. Kohring
wrote:
>>
>>><quote>
>>>Make an example
of something that is not conformant to the SOA RM and
>>>explain
why.
>>></quote>
>>>
>>>
>>>One
of the problems we are having in this respect is
>>>generalizing
from the wrong basis model. Or more to the point,
>>>have we reached
agreement upon what basis model SOA is
generalizing
>>>from?
>>>
>>>In my opinion,
SOA RM generalizes Client-Server; whereby
>>>the "client" is
generalized to "consumer" and the "server" is
>>>generalized to
"service". (In this sense, SOA is a fundamental model
>>>and we
should try to keep it simple.)
>>>
>>>Seen from this
viewpoint, we should ask what is the difference
>>>between client
and consumer, server and service and the relationship
>>>between the
respective pairs.
>>>
>>>A "client" has the server's
description hard-wired. The policy,
>>>contract, data model and
processing model are all hard coded into both
>>>the client and the
server.
>>>
>>>A "consumer" on the other hand has some
goal to achieve and must
>>>first discover a service which can
achieve this goal, understand
>>>the service's policy and contract
to see if the service's policy is
>>>in alignment with its own
policy and constraints, examine the
>>>processing model to determine
whether a session needs to be
>>>established before the request can
be submitted and examine the
>>>data model to determine what format
is needed for the input data;
>>>only then can the consumer submit a
request to the service.
>>>
>>>If you accept this
scenario (which I know is a big "IF" ;-), then
>>>an example of
something which is Client-Server, but not SOA is
>>>FTP. With
FTP the policy (username-password authentication),
>>>contract (list
of allowed commands), data model (byte order of the
>>>ftp packet)
and processing model (control channel, data channel)
>>>are all
hard-coded in both the client and the server, there is no
room
>>>for dynamic inspection and
negotiation.
>>>
>>>In my opinion, it is this
inflexibility which forms the main
>>>demarcation between the
Client-Server model and the SOA
model.
>>>
>>>
>>>--
Greg
>>>
>>>
>>>
>>>
>
>
>--
>======================================================================
>G.A.
Kohring
>C&C Research Laboratories, NEC Europe
Ltd.
>======================================================================
--
Rex
Brooks
President, CEO
Starbourne Communications Design
GeoAddress:
1361-A Addison
Berkeley, CA 94702
Tel:
510-849-2309
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