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services composition is the use of AI planning technology. Each web service is
represented as a planning operator and the desired composite service's inputs and
outputs form the initial state and the goals respectively. The plans that arise are
encoded in languages such as PDDL [5] that describe the actions, that is the web ser-
vices, that must be executed and the order of their execution.
The contribution of this paper focuses on the automatic translation of the plans,
expressed in PDDL, to OWL-S descriptions [10] that take advantage of the OWL-S
control constructs and facilitate the automatic invocation of the composite service.
Specifically, information from the PDDL descriptions of the domain, the composition
problem, and the plan is used to create a functional representation of the composition.
This representation describes with a specific syntax the way each atomic web service
is connected to each other in order to produce the final output. In a second phase, this
functional representation is utilized to generate the OWL-S descriptions of the new
composite web service.
In terms of functionality, the method described in this paper is merely based on the
PDDL descriptions of the planning operators and does not explicitly deal with semantic
information of the initial atomic services. Therefore, it can be applied to compositions
arising from both syntactic and semantic matching of inputs and outputs of the atomic
services. However, since the final expression will be encoded in OWL-S language, we
will use the notion of semantic web service throughout the rest of the paper.
In the sections to follow, the relative research field is explored. The suggested
technique is analyzed in detail and some conclusions along with future directions are
given. Specifically, the rest of the paper is organized as follows:
In section 2, the field of automated web services composition using AI Planning
techniques is presented and some studies on the field are exposed. In section 3, the de-
veloped method for translating the PDDL plans to OWL-S descriptions is analyzed.
This section is divided into two sub-sections, reflecting the two phases of the method; in
the first sub-section, the algorithm that creates the functional representation describing
the composition is presented, whereas in the second sub-section, the method for con-
verting this representation to OWL-S description is described. Finally, in the last
section, conclusions of the research so far are given along with some ideas on how the
developed algorithms and the web services composition procedure could be enhanced.
2
Related Work
The process of automated web services composition by the point of view of planning
has been studied extensively. The most important advantage of this approach is the
dynamic character that is offered to the composition process, which reduces a lot the
interference of the user.
One of the most known systems in the field of web services composition via plan-
ning is SHOP2 (Simple Hierarchical Ordered Planner), [15]. It is based on HTN plan-
ning (Hierarchical Task Network) methods [14]. One basic difference between
SHOP2 and the other HTN systems is that it locates all the actions of the plan in the
same order that they will be later executed. In this way, the current state of the system
in every step of the planning procedure is known and inference mechanisms or heuris-
tic techniques can be used to augment the effectiveness and the efficiency of the
whole process.
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