Semantic Web Service Usage Tasks in WSMO - Enabling Semantic Web Services: The Web Service Modeling Ontology - page 126

o ine composition, it is possible that the exact combination of services that
lead to the overall behavior may be left open to discovery during execution.
In WSMO terms, then, a basic orchestration may describe the composition of
goals, services or some combination thereof.
For example in the DIP project 3 we propose a three-level WSMO compati-
ble approach to orchestration [14]. Orchestrations may be represented either in
abstract state machines (ASMs) [58], in the Cashew workflow language [101],
or in UML activity diagrams [52]. The activity diagram language concentrates
on orchestration as composition of services, since it is intended to support an
o ine composition process which involves discovery, based on UML activity
diagrams. The Cashew language concentrates on the hierarchical composition
of goals via Workflow Patterns [130]. Abstract state machines in general and
in particular the WSMO choreography language are intended to be a neutral,
formal representations for orchestrations. They can compose behavior from
services and goals; and both activity diagrams and Cashew workflows can be
translated into the WSMO language.
In the SUPER project 4 , higher level representations for behavior are con-
sidered from the perspective of business processes. In particular, a semantic
BPEL language is being constructed, preserving the features of BPEL [4]
but being compatible with WSMO and taking advantage of its features, such
as goal-driven execution. In BPEL, behavior from a number of partners is
composed, with each partner able to play several roles within a workflow, de-
scribed as WSDL port types. In order to construct a semantic equivalent, the
participants in semantic BPEL processes are partners that may have several
behaviors described as WSMO goals. A service discovered to fulfil the role of
a partner must support all of these goals and thus the notion of participant
when viewing such a process as defining an orchestration is extended.
9.4.2 Control Flow
In order to allow control-flow to be represented in a state-based manner, and
to allow data-flow to be represented in a semantic manner, WSMO uses the
notion of a “state signature” for orchestration. A state signature is a set of
ontological concepts and relations over which state is expressed - the state at
any point in the execution is understood as the set of instances of all of these
concepts and relations.
Abstract state machines are defined over a state signature by allowing tests
over the current instances as transition guards, and updates as transition
effects. Those updates are specified as logical expressions and can be adds ,
updates and deletes of instances of concepts and relations. Control flow must
therefore be represented as tests over a state. To help to make the control flow
more explicit, two particular forms of ASMs are proposed: control state ASMs,
3 http://dip.semanticweb.org .
4 http://www.ip-super.org .