Method Engineering: A Service-Oriented Approach - Intentional Perspectives on Information Systems Engineering - page 332

of services via service requests, service discovery and service invocation. In con-
trast, components are application integrated artifacts. In the first case, an application
results from services request while, in the second case, the application consists of
components integration.
CBSE and SOE also differ about organizational aspect. Usually, components are
statically organized with links predefined at design-time. On the contrary, ser-vices
can dynamically be organized with “computed” links. Services possess the ability
of engaging other services in order to complete complex transactions. So, service
delivery often makes use of composition mechanisms to coordinate several ser-
vices that together fulfill a consumer's request. Resulting composite services may
be used as basic services in further compositions of services. Static and dynamic
compositions are one of the major challenges for adoption of the service-oriented
approach. Indeed, development of electronic distributed and flexible business appli-
cations requires automated composition and integration of services in the current
dynamic context of the web infrastructure.

2.2 Principles of a Service-Oriented Paradigm

In this section, we define fundamental principles for a service paradigm defini-
tion. Each principle is introduced and argued. Impact of each principle on service
modeling is presented.

Principle No 1: Goal-oriented specification. We adopt an approach based on
usage. In service modeling, we consider that a service exists for delivering a
solution to achieve a goal. For instance, in the pedagogic domain, a service can
satisfy a learner's goal while in a business domain a service can satisfy a client's
problem.
Whatever the domain, we consider that a service is intended to meet consumers'
needs, that is what the consumers intend. Then, interface of services should empha-
size what a service can do for its consumers. We propose to model services with a
goal-oriented specification [ 3, 24, 31] . This presents two main advantages:

• Goal modeling emphasizes problem-related knowledge rather than solution one.

It then induces a separation problem/solution.

• Goal orientation supports service usage specification at a high level of descrip-

tion. Goal orientation emphasizes the “why to use” a service, so goal orientation
allows reducing the semantic distance between available services and consumer's
requests.

Moreover, goal orientation supports alignment of a service with the domain
strategy and processes.
From the provider perspective, each service must have an interface which
ex-presses a goal. From the consumer perspective, discovering and invoking a
service means expressing a goal.