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Subject: Re: [soa-rm] Architectural Scope of Reference Model
I suspect when we get to specifying what gives a service unique identity,
Will we ever reach that point? That is, is the question of whether or not a service has a unique identity within the scope of a RM? My take is no - it's a design issue, and potentially an architecture issue depending on how such a unique identity needs to be generated/maintained (i.e. may have an "identity engine" component in an architecture - not to be confused with identity management for security).
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From: Scott Came [mailto:firstname.lastname@example.org]
Sent: Wednesday, March 30, 2005 12:44 PM
Subject: Re: [soa-rm] Architectural Scope of Reference Model
Hello everyone, I've been lurking so far, but let me jump in on this one...
Thomas' key question, as I read his post, was not whether messages should be part of the RM, but whether service requestors should be.
In my view, having services without service requestors doesn't make a lot of sense. So I would like to suggest that service requestors are a proper element in the RM.
If you grant that, then the relationship between a service requestor and a service implies the exchange of some information (in the general case). We might not call that a "message", but it does exist. Is there another, more appropriately abstract, name for the elements of the service's interface that specify the structure of information exchange that occurs between requestor and service?
Looking at it another way (and forgive me if this is getting ahead of where we should be)...I suspect when we get to specifying what gives a service unique identity, it may well be a (perhaps qualified) name plus a set of interface elements (operations/behaviors, if you will, and the "signature" of those operations). If so, then we'll need a name for the elements that represent the input and output parameters (again, forgive the "concrete" terminology) of each operation.
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> Thank you for this very elegant summary!
> I think the answer may be in the definition of a "reference model" vs.
> "architecture". I think case studies will help clear up this
> confusion. A reference model will normally not contain "messages" as a
> 1. Please look at the OSI Reference model. This is a communication
> stack yet it does not contain messages:
> This does not contain any "message" although messages will occur in
> implementations using the reference model
> 2. The ITA Reference Model likewise does not have "messages":
> 3. RCS Reference Model
> Again - no messages even though there is an element marked
> "Communications" in figure one.
> The reference Model should not contain "messages" as a component. That
> belongs in architecture or implementations based on the reference
> model. I have never encountered one reference model with concrete terms
> in it. If it had such, it would not be abstract.
> We must think abstract, not concrete.
> Duane Nickull
> Thomas Erl wrote:
>> Some thoughts regarding the on-going discussion of whether a message
>> element should be part of our reference model:
>> As per our chosen definition of architecture, in order to describe
>> service-oriented architecture we need to:
>> 1. Define elements that comprise the structure of a system.
>> 2. Define external properties of these elements.
>> 3. Define relationships between these elements.
>> 4. Define the overall structure of the system.
>> (not necessarily in this order)
>> Starting with the first point, different element collections have been
>> proposed in the two position papers submitted so far. As has been
>> discussed, the MacKenzie/Nickull paper does not identify a message
>> element, whereas Kohring's does.
>> A related difference I noticed when reviewing these papers is that
>> Kohring's establishes a broader range of SOA elements. Specifically,
>> both service provider and requestor (consumer) roles are separately
>> identified and described. As mentioned in item #3 above, we are
>> required to define the relationship between the elements we define.
>> Therefore, it makes sense that this paper includes a separate element
>> (message) that can be used to help describe the relationship between a
>> service and its requestor.
>> The elements identified in the MacKenzie/Nickull paper are:
>> - Service
>> - Service Description
>> - A form of advertisement to facilitate discoverability.
>> - Service Contract
>> - Data Model
>> These elements form a narrower architectural scope, leading to a
>> proposed architecture that revolves primarily around the service (or a
>> service assuming the provider role). Because a service requestor is
>> not explicitly identified as a separate element, it makes sense that
>> an element representing some unit of communication (message or
>> otherwise) is also not identified. Within this model's scope, the
>> definition of a relationship between a service and its requestor
>> (beyond details implied by description, contract, data model, and
>> advertisement elements) is not a requirement.
>> I believe that in order to address the issue of whether a message is a
>> legitimate element within the reference model, we should begin
>> by clearly defining the scope of our abstract architecture. Given that
>> we are establishing core elements that are expected to be present in
>> all forms of SOA, this raises the question: Does an architecture
>> require the presence of both a service provider and a service
>> requestor (the coffee shop and the patron) in order to be classified
>> "service-oriented"? If yes, we must define this relationship. To
>> properly do so, we very well may need to further identify and define a
>> separate element to represent an abstract unit of communication passed
>> between them.
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