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The solution analyzed in [16] also translates the composition problem to PDDL
descriptions and suggests that in this way an appropriate planner could be found each
time according to the problem so as to provide an improved solution. The paper
presents a three step technique for the creation of a composite web service with the
first step being the translation of the OWL-S descriptions and OWL ontologies to
PDDL domain and problem descriptions; the second one is the creation of a plan that
solves the problem with the execution of a planner; the third one is the translation of
the plan to a new OWL-S description of the resulting composite web service. Howev-
er, the paper focuses only on the first step of the procedure. Some assumptions are
made to ease the translation function, such as considering that each atomic process
has either effects or outputs but not both simultaneously. Also, the authors of the
paper do not deal with OWL-S process models that have composite process using
Repeat-While and Repeat-Until or Any-Order and Split-Join constructs. The algo-
rithm proposed, deals separately with the OWL-S process model, the atomic and sim-
ple processes, the sequence, if-then-else, choice and split processes and with the
OWL-S target service description to create the domain and problem descriptions. The
process of choosing the appropriate planner for each problem and the translation of
the plan to OWL-S description of the new service are not elaborated in the paper.
The aforementioned methods tackle the problem of web services composition
using a variety of fully or partially automated techniques. However, they don't deal
with the task of expressing the resulting composite service in OWL-S, taking advan-
tage of the supported control constructs.
3
Translating PDDL to OWL-S
This section analyzes the method for translating a composite web service expressed in
the PDDL language to the corresponding OWL-S description. The translation
completes in two phases. The first one concerns the extraction of all the required
information from the plan for the creation of a composite web service's functional
representation. The second is about the conversion of this representation to an OWL-S
description of the resulting composite web service.
3.1
Constructing the Composite WS
The first step in the creation of an OWL-S description based on data derived from a
PDDL plan is the manipulation of these data and their conversion to a composite web
service functional representation. This representation refers to the available simple or
atomic web services and the order in which they should be executed and is structured
using the OWL-S control constructs
sequence
,
split
and
split-join
.
In the following algorithm the functional representation of a composite web service
C
is represented as a predicate
f
(
a
0
,
a
1
,...,
a
n
), where
f
is the control construct used to
describe the composition structure and
a
0
,
a
1
,…,
a
n
stand for the simple web services
that participate in the composition. Each
a
i
could be another composite service or, in a
simpler case, an atomic process, which is represented as
atomic
(
a
i
).
The developed algorithm consists of three general steps, as shown in
Fig. 1
. The
first step concerns the parsing of the files associated with the composition planning
problem and the extraction of all the information needed in the next steps. In the
