Information Technology Reference
In-Depth Information
element with the atomic services as parameters. The
list object
is a structure with
First
and
Rest
parts and could be described by an expression like:
First
(
a
0
,
Rest
(
First
(
a
1
),
Rest
(…))).
In
CLO
algorithm, the first parameter of the expression is examined and the
OWLSProcess
algorithm is called for this. The result becomes the head of the
constructing list, because it is the service or the composition of services that will be
executed first. Then, the
CLO
algorithm is called recursively for
C'
, the composite
web service
C
with
a
0
omitted. The result of this call is set as the
Rest
part. Finally,
the constructed list object is returned.
The last step in converting the composite web service functional representation to
OWL-S description is the creation of the profile description. Here, the composite web
service is treated as an atomic service with specific inputs and outputs. The construc-
tion of this description is merely based on the methods provided by the OWL-S API's
functions.
4
Conclusions and Future Work
Web services are playing an important role in the web applications development field,
with which many different systems through the globe can communicate and exchange
data using the World Wide Web. Users that need a specific functionality can retrieve
the desired web services from the UDDI registries and use them to create the output
they are looking for.
SOA architecture has contributed to the rapid and easy web applications develop-
ment, using as units the web services and combining them to create new, complex
services of advanced functionality that can serve even as complete business models.
The composition methods studied in this paper differ on user's involvement level.
Some initial solutions, of limited autonomy, use workflows and leave the details
regarding the location the appropriate services execution and their order to the user. In
some more creative solutions, the user doesn't have to find the exact services that will
be used, but just provides a description of them. The discovering of services that
match with the descriptions and the execution of the resulting workflow are automati-
cally performed without the intervention of the user.
In later studies, the autonomy of the composition procedure is increased. Semantic
information concerning the web services is used to describe in a semantic level their
functionality. Languages such as OWL-S are used for this purpose. In this way, concept
matching becomes possible and so is the check whether two or more services can
cooperate. The semantic information is used also by automatic web services composi-
tion via planning methods, which are examined in this paper. The composition problem
is treated as a planning problem and solved by algorithms of the field.
The result is a plan encoded in planning languages, such as PDDL+ that describes
the services that will be used for the composition and the way in which they will be
combined to create the desired composite web service. But, for this final service to be
available to other users too and to be published in a UDDI registry as an atomic web
service and take part to possible future compositions, semantic description of the
service have to be created.
