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Re: multiple descriptions [was: [soa-rm-ra] Questions on action m odel]
Michael,
I think the more important point is that the descriptions are not isolated entities but leverage common resources and each other, so a collection of descriptions should tell you more (OK, at least as much) about the subject than you get from any single description.
Ken
On Oct 16, 2007, at 11:44 AM, Poulin, Michael wrote:
Thank you, Ken. I think we have agreed with Danny that multiple descriptors make sense in multiple business domains/contexts and/or multiple registries/repositories. This, probably, may be reflected in the RA.
- Michael
From: Ken Laskey [mailto:]
Sent: 16 October 2007 16:27
To: Michael Poulin
Cc: Duane Nickull; Scott Came;
Subject: multiple descriptions [was: [soa-rm-ra] Questions on action model]
Michael,
I see your discomfort and if all we had were multiple, independently generated descriptions, this would certainly lead to ambiguity and contradiction.
I have often talked about a service description as being a table of contents, so less than the source of many description elements, it is the collection points of things created for other, related purposes. For example, I do not see writing policies into each service description but I see those with expertise in rules systems developing policies in parallel and the service description author would link to the existing policies. In the same way, I do not see duplicating my address and phone number for every service for which I am a responsible party, but I do see a link to the resource where my contact (and possibly other personal information) is maintained.
In this way, a different subset of possible description could be emphasized for different contexts, but the subsets would leverage common resources for the description elements.
At one time, I advocated the idea of a "complete description" where the description would explicitly collect the description elements needed for the context but would provide other links where additional description sets or description elements could be found. The idea was a given description would highlight what was needed for the context but there would be tendrils to the rest of the world where more information resides.
Ken
On Oct 15, 2007, at 4:43 AM, Poulin, Michael wrote:
Duane wrote: "In an ecosystem of services, there could potentially be many service descriptions for a service. The service description a consumer uses may be dependent on the entity the consumer interacts with when they use the service, or it could be dependent on the particular context the service description is provided. Because of unforeseen possibilities for a service to have multiple service descriptions, I do not think the OASIS SOA RA can qualify one service to one service description."
I have a lot of concerns about underlined statements.
1) Service description gets used as the service definition now. I agree that in an ecosystem not all information about the service is in the Service Descriptor. However, in this case, I prefer to recognise that I have different 'domains' in the ecosystem of services (voice, signs, UDDI, etc.) where the Service Description is the single unique one.
2) To me, the "service description a consumer uses may be dependent on the entity the consumer interacts with when they use the service" looks much more like an execution context and related Service Contract than a service description. Again, "be dependent on the particular context the service description is provided" is either services ecosystem domain specific Description or the EC and Service Contract.
3) as I mentioned before, SOA RM states that EC is not a context of service interaction only ( though in many places it says so ) but also the context of service execution per se. If we allow "a service to have multiple service descriptions" (w/o defining concrete conditions where it is possible), we have to clearly distinguish it from a service to have multiple service descriptions in multiple service execution contexts - interaction and execution ones.
4) I, personally, dislike the idea of "a service to have multiple service descriptions", at least, in the same Service Description Repository. Though this is a lower level technical implementation detail, it is VERY important for practical use of SOA and quoted statement above can easily screw it, which I hate.
- Michael
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From: Duane Nickull [mailto:]
Sent: 12 October 2007 16:49
To: Ken Laskey; Scott Came
Cc:
Subject: Re: [soa-rm-ra] Questions on action model
Concur. Look at the cardinality of WSDL. Not everything is mandatory either. On a purely logical level, consumers can use only that which they require. It is unlikley that any one service description artifact would in fact contain *all* the information required.
At least some info is known via alternative mechanisms (voice, signs, UDDI, etc.)
Duane
On 10/11/07 6:22 PM, "Ken Laskey" <> wrote:
The RM says
The service description represents the information needed in order to use a service. In most cases, there is no one “right” description but rather the elements of description required depend on the context and the needs of the parties using the associated entity.
Thus, the implication is there may be overlapping descriptions relevant to different contexts.
Ken
On Oct 11, 2007, at 8:08 PM, Scott Came wrote:
Danny:
I don't personally have a strong feeling one way or the other on this
issue. I did sense the "one description" position as the subcommittee
consensus, however...mostly from reading the RA 0.2 draft.
Thanks.
--Scott
-----Original Message-----
From: Danny Thornton [mailto:]
Sent: Thursday, October 11, 2007 3:55 PM
To: Scott Came;
Subject: RE: [soa-rm-ra] Questions on action model
Scott,
Thank you for the summarization. The only statement I
have an exception with is 3, A service has one
description.
In an ecosystem of services, there could potentially
be many service descriptions for a service. The
service description a consumer uses may be dependent
on the entity the consumer interacts with when they
use the service, or it could be dependent on the
particular context the service description is
provided. Because of unforeseen possibilities for a
service to have multiple service descriptions, I do
not think the OASIS SOA RA can qualify one service to
one service description.
Danny
--- Scott Came <> wrote:
Subcommittee:
I'd like to take attempt a summarization of this
thread. Note the word
"attempt"...please let me know if I've misconstrued
anything said so
far.
1. The action model of a service may contain
multiple actions
2. The actions in a given service's action model may
produce
distinct real-world effects, meaning that a consumer
may choose to
interact with (invoke?) some subset of actions and
not others
3. A service has one description, but that one
description may make
specific reference to particular actions in order to
describe their
individual effects, and it also may specify specific
policies for
individual actions, though those specific individual
policies can be
viewed as part of one whole policy for the service.
4. The process model of a service shows how a
consumer may interact
with (invoke?) specific actions in sequence to
accomplish some business
function aligned with the service's RWE
5. There may be multiple processes in the process
model, involving
different sequences and different functions
6. A consumer may interact with (invoke?) a single
action to
achieve a particular effect (whether that is a
process with one step, or
no process, is an issue I suppose)
7. The mechanism by which a consumer interacts with
a service is by
invoking an action through message exchange; that
is, the consumer and
service exchange information (in the form of a
message) to achieve some
effect
8. Interaction with different actions can be by
different MEPs
9. The decision about service boundaries-which of
all possible
actions should be in a given service's action
model-can be productively
guided by a set of design principles, which the RA
may include at some
future time. Principles should be carefully and
precisely defined.
The example again is an Employee Time Management
service. The action
model of this service might consist of four actions:
add time records,
update existing time records, delete time records,
and view time
records. A process model for this service may, for
example, indicate
that it makes sense to view time records prior to
updating them. But,
it is also possible to add time records in
isolation...without any of
the other actions being involved. The overall RWE
of this service is
management of employee time, but each action has a
separate effect of
independent value to a consumer. There is no
absolute rule regarding
whether it makes sense to include all four of these
actions in one
service-without more analysis, it is not clear
whether this is a "good"
service model or not. However, there are some
design principles that an
architect (creator of a concrete architecture) can
apply in making this
determination: loose coupling, statelessness,
autonomy, abstraction,
etc.
Thanks.
--Scott
________________________________
From: Ken Laskey [mailto:]
Sent: Thursday, October 11, 2007 11:09 AM
To: Scott Came
Cc:
Subject: Re: [soa-rm-ra] Questions on action model
Scott,
Indeed I do not expect we (and anyone else) will
provide a definitive
cookbook for defining services. However, it is
exactly those guiding
principles that I think will be valuable and with
which the RA should be
consistent. Thomas Erl's book may provide a good
starting point. If
anyone can summarize between now and when I
eventually make it to a book
store (or log into Amazon), we can begin considering
those.
Note when we mention principles, we have to do much
more than use terms
which are familiar but overused and never really
defined. The RM
specifically took shots at loose-coupling and
coarse-grained. I also
put agility into that category. When we state the
principles (assuming
we can), those SHOULD be phrases that convey enough
meaning that they
cannot be automatically misconstrued (by accident or
on purpose) and
then crisply elaborated.
Again, I'm energized to revise the service
description model but have a
long list of household chores to catch up on first.
Keep sending ideas.
Alternately, you can come over and fix the lawn
mower.
Ken
On Oct 11, 2007, at 1:35 PM, Scott Came wrote:
Ken:
Regarding your #1...
I think this kind of specific guidance is best
reflected in a set of
principles. I don't believe there will be a "one
size fits all" set of
rules that tell you, in every situation, how to
design a service
properly. The best you can do is identify the
principal forces at
play-those factors that, at a minimum, the designer
should at least
consider when setting service boundaries. Those
forces are represented
in principles that say what should characterize a
proper service.
Proper application of these principles (selecting
among the sometimes
competing forces) requires the skill of a designer,
but well-stated
principles accelerate the design process and bound
the designer's
discretion somewhat, in pursuit of an overall design
objective.
And not just any old set of principles will do. SOA
has an overall
purpose-to increase agility, adaptiveness,
responsiveness to change-so
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Ken Laskey
MITRE Corporation, M/S H305 phone: 703-983-7934
7151 Colshire Drive fax: 703-983-1379
McLean VA 22102-7508
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Ken Laskey
MITRE Corporation, M/S H305 phone: 703-983-7934
7151 Colshire Drive fax: 703-983-1379
McLean VA 22102-7508
-----------------------------------------------------------------------------
Ken Laskey
MITRE Corporation, M/S H305 phone: 703-983-7934
7151 Colshire Drive fax: 703-983-1379
McLean VA 22102-7508
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