Workflow Model for Semantic Web Services
Studing real-life Grid applications, we came to the conclusion that
current approaches for Semantic Web service (SWS) description are
not particularly suited to describe the kinds of workflows that
are relevant for Grid applications. The requirements in these
applications include
-
The ability to describe {\em multi-party interactions} with respect to a
single service, with the different parties distinguished by the
{\em roles} they occupy within the service operation. This is a
result of the the need to be able to describe, with respect to a
single service, either strict requirements or simply the
possibility it has to cooperate with other services and {\em what
form that cooperation can take}, during operation.
-
The ability to describe both interactions initiated by the
described service as well as interactions that are initiated by
other parties. This provides a more flexible model than what is
currently available and enables the support of more sophisticated
services, which can, for example, notify clients about the state
of progress or call upon other services to perform specific
actions.
-
The ability to describe the state changes a service goes through that
are relevant to the interacting parties, caused either by events
internal to the service or as a result of interactions with
outside parties. This enables a service user to understand which
actions it must initiate with regards to a service model and which
are not depended on the service itself but are relevant to how it
can interact with the service. Furthermore, it enables other
parties, such as security devices~\cite{ashri_aaaisymp:04}, to
monitor the progress of interaction with a service, so as to
ensure that the service is not misused.
Realizations of parts of these requirements can be found in
existing approaches. For example, WS-Choreography
defines roles and request and response protocols, and
Interaction Protocols in DAML define state-oriented
and message-centered protocols. Nevertheless, there exist
no approach that captures all of the above listed
required features.
As a first step towards a Grid application workflow model we therefore
propose an interaction protocols model that describes, in a
service-centric way, a service's interactions with clients and other
services. We combine ideas from other approaches to meet
Grid application requirements, extending existing models
with additional concepts such
as events to reflect internal state changes of services,
and different types of messages to express direction of message flow
(incoming vs outgoing).
| Ontological Model for Service Interaction Protocol |