The Concepts of WSMO - Enabling Semantic Web Services: The Web Service Modeling Ontology - page 66

of the operational competence of the service: (1) choreography decomposes
a capability in terms of interaction with the service, and (2) orchestration
decomposes a capability in terms of the functionality required from other
services.
This distinction reflects the difference between communication and coop-
eration. The choreography defines how to communicate with the service in
order to consume its functionality. The orchestration defines how the over-
all functionality is achieved by the cooperation of more elementary service
providers.
The Web service interface is meant primarily for the description of the
behavior of Web Services and is presented in a way that is suitable for software
agents to determine the behavior of the service and reason about it. It might
be also useful for discovery and selection purposes, and in this description, a
connection to existing Web service specifications, for example WSDL, could
also be specified.
The definition of an interface is given below:
Class interface
hasNonFunctionalProperty type nonFunctionalProperty
importsOntology type ontology
usesMediator type ooMediator
hasChoreography type choreography
hasOrchestration type orchestration

Choreography

The choreography interface describes the behavior of the service from the
client's point of view; this definition is in accordance to the one given in
the W3C Glossary: 1 Web service choreography concerns the interactions of
services with their users. Any user of a Web service, automated or otherwise,
is a client of that service. These users may, in turn, be other Web services,
applications, or human beings.
The state-based mechanism for describing WSMO choreography and or-
chestration interfaces is based on the abstract state machine (ASM) [58]
methodology. An ASM is used to abstractly describe the behavior of a service
with respect to an invocation instance of that service. We have chosen an
ASM model for the description of such interfaces since such a service invoca-
tion (e.g. the purchase of a book from Amazon.com) may consist of a number
of interaction steps. These interactions can be described by a stateful abstract
machine.
ASMs have been chosen as the underlying model for the following three
reasons:

• Minimality . ASMs provide a minimal set of modeling primitives, i.e. they

enforce minimal Ontological commitments. Therefore, they do not intro-

1 http://www.w3.org/2003/glossary .