Information Technology Reference
In-Depth Information
for explaining a system's architecture to potential users or external assessors
or conformance testers. It also allows the expression of platform-independent
designs in a commonly understood way without reference to specific techno-
logical choices, making it easier for teams using a model-driven engineering
approach to exchange ideas on requirements and solutions.
1.5
Service Orientation
In recent years, much play has been made of the use of service orientation
as a design principle. From an architectural point of view, however, there is
no significant difference between service-oriented architectures (SOA) and the
architectural framework defined in ODP; current service-oriented schemes can
be seen as a subset of the more general ODP approach.
The main tenets of SOA are that functions should be packaged into loosely
coupled units that provide clearly defined services, and that applications
should be constructed by composition of services that can be discovered dy-
namically based on some form of publication or brokerage mechanism. More
recent SOA activities add to this a distinction between services offered from
different design perspectives, yielding different flavours of service, such as
business services, technical services and so on.
ODP defines service as a fundamental concept, representing the added
value offered as a result of interaction at some interface. Since the ODP ob-
ject model is based on strong encapsulation, this means that there is a close
alignment with the SOA view of service. The discovery and dynamic use as-
pects are covered by the ODP binding model and the definition of common
functions such as the trader. The different types of services are captured by
the ODP viewpoints, with business services being expressed in the enterprise
viewpoint and the technical services being expressed primarily in the compu-
tational viewpoint. However, in the ODP enterprise viewpoint, as we shall
see in the next chapter, there is a greater emphasis on declarative expression
and flexible structures, so the definition of a business service is only one of the
available design tools. Correspondences defined between the different aspects
of a service in different viewpoints are also needed to provide a consistent
specification of the service as a whole.
So what differentiates a service-oriented architecture? The main differ-
ences are not architectural, but are more concerned with raising the engineer-
ing expectations about openness and resilience, largely as a result of years
of implementation experience with web-based systems and of striving for the
widespread adoption of open resource identifiers like Uniform Resource Iden-
tifiers (URIs).
