Architectural Scope of Reference Model

[Thomas Erl <terl@serviceorientation.org>]

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