Architectural Scope of Reference Model

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.    

 

Thomas