Method Engineering: A Service-Oriented Approach - Intentional Perspectives on Information Systems Engineering - page 342

6 The Composition Process

This section describes the method service composition process. Method services
are considered as process fragments, so they can be composed to build complex
processes. The objective of service composition is to create new processes by com-
bining processes of existing services. Service composition is generated on the fly
based on a developer's request. This approach is in contrast to the solutions provided
by classical workflow approaches where activities in a process are pre-planned and
pre-specified [ 21, 23] .
In SO2M, service composition is seen as an iterative process (cf. Fig. 10) . The
entry of this process is a request formulated by a developer. The treatment of the
request consists in searching, selecting and organizing all the services necessary for
satisfying the request. We call “composition graph” (Gc), the graph that specifies
all the services and their relationships participating to request satisfaction [ 6] . The
composition process result is a whole development method (or a fragment) defined
from the execution graphs of the services appearing in the composition graph. The
result is represented by a process graph (Gp). The process graph can be executed in
a certain context to elaborate a particular IS [6] .
Every iteration in the composition process contains three activities: discovery,
composition and refinement. These activities are repeated until the initial request
is satisfied with a set of services. The number of iterations varies according to the
granularity of the goal in the request. Indeed, the more abstract will be the goal,
the more important will be the number of services in the composition graph and the
number of iterations in the composition process. It is important to note that simple
processes, as constituent of decisional processes or composite processes, initiate
new iterations in the composition process. Simple processes lead, at composition
time, to explore the service base to find the service the more suitable to realize the
process. In this way, at any time, composition takes into account the current state of
the services base.
At each iteration the composition graph (Gc) is extended and the process graph
(Gp) is refined. Each iteration is driven by a goal. During the first iteration, the goal
is the request one. In the following iterations, the goal results from one (or some)
simple process(es) defined in a composite or decisional process of a method service .

Goal

Composition

Discovery

Refinement

Gc and Gp
Graphs

Fig. 10 An iteration in the
composition process