Motivation and Goals

Web services are nowadays a popular technology used in practice when diverse software system integration is in demand. The ability to make some functionality available through the Web has inspiring consequences. The Web services provide features which are very interesting also from a research point of view. They are studied from several aspects to propose solutions allowing their more sophisticated exploitation.

One of the intensively researched areas is the study how Web services can be used to dynamically create a functionality, based on the actual requirements. The basic idea is that multiple Web services can be combined together, to form a composite service, supplying more complex needs. The particular composition is realized automatically, on the fly, based on the actual goal. To facilitate this kind of Web service utilization, additional metadata depicting the functionality of single services is required. These metadata are provided in a form of semantic annotations.

The aim of this thesis is to deal with the problem of automatic dynamic semantic Web service composition. The main objective is to deal with problems related to the following issues:

• Functional aspects of Web services. A proper representation of the functional aspects of Web services is crucial for automatic Web service composition. The existing approaches exploit additional meta-data depicting the semantics of the I/O parameters to describe the service behavior. This approach shows to be insufficient. The proposed solutions are oriented to expressing the pre-/post-conditions of Web services. Our aim is to clearly define the pre-/post-conditions and show that it is a feasible way to compose services while considering their pre-/post-conditions.

• QoS optimization. Beside the functional requirements, the user is usually interested also in non-functional properties of Web services. Hence, the QoS optimization during service composition is important. We deal with a service composition aware of the QoS and capable of finding the best solution considering them.

• Changes in the service environment. The Web service environment is frequently changing in time. New services are deployed, some of them are removed. The changes relate also to the QoS attributes, the values of which might evolve in time. Our goal is to develop a composition approach capable of reacting to these changes and thus providing a solution reflecting the actual situation in the service environment.

• Effectiveness. As the Web in general grows, also the set of Web services which are available in repositories is rising. We deal with the problem of performance and scalability of the composition process considering large number of services to be searched and composed.

• Composition system. Our aim is not only to develop a composition method, but also to design a composition system realizing it.

• Continuing user query arrival. In real scenarios, the composition system must dynamically compose services based on the actual user goal. The composition queries may arrive from multiple users. We study the behavior of the composition system due to continuing user query arrival, while reacting also to possible frequent changes in the service environment.

Results

The main contributions of our work, according to the defined objectives of the thesis are summarized in the following points:

• Precise definition of the pre-/post-conditions of Web services and condition compatibility evaluation. We propose a formalization clearly defining the conditions and the way how they are considered during the composition. The part of the formalization is presented in the paper NWeSP09 at Int. Conf. on Next Generation Web Services Practices 2009 (published by IEEE Computer Society press), in the paper WSCA09 at IEEE Congress on Services, Int. Workshop on Web Service Composition and Adaptation 2009 (published by IEEE Computer Society press), and in the paper ICWS10 at IEEE Int. Conf. on Web Services 2010 (published by IEEE Computer Society press).

• Feasible pre-/post-condition aware service composition. We present two methods to condition compatibility evaluation. The first, based on a relational database is presented in WSCA09. The second, introducing condition encoding, is proposed in ICWS10. Our work also contributes by a definition of the rules deciding if the possible data adaptation operations can be used to adjust the outputs of the Web services to fit the required inputs. This is presented in NWeSP09.

• Fast and scalable QoS-driven control-/data-flow design of the composite service. We present an approach based on preprocessing, effective data structures and algorithms. A preliminary version of the approach is presented in CEC09 at Int. Conf. on E-Commerce Technology 2009 (published by IEEE Computer Society press). This paper was a partial requirement to participate Web Services Challenge 2009. An enhanced version of the approach proposes service space restriction, showing dramatic improvement concerning effectiveness. The idea is presented in ICWS10 and the paper ISD10 at Int. Conf. on Information Systems Development 2010 (published by Springer).

• Prompt updates of the internal data structures due to changes in the service environment. We show how our data structures are quickly updated in the case of adding a new service, removing an existing service, or change of the QoS. These issues are depicted in the paper ISD10.

• Software design of the composition system. We present the software architecture of a composition system realizing our composition method, which cares about coordination of different task performed by it. The architecture of a preliminary version of the system is presented in CEC09.

• Examination of the overhead in the composition time due to continuing composition query arrival. We deal with the handling of multiple composition, or service update requests and show that our approach is able to manage them, without a significant effect on the processing time of the requests. This is a subject of the paper ISD10.

• Evaluation of the effect of frequent changes in the service environment. We provide an evaluation of the effect of changes in the service environment, which must be managed with higher priority, to the sojourn time of the composition requests. The results, showing that the processing time of the composition requests is not significantly affected, are presented in ISD10.

Conclusion

The aim of this thesis was to contribute to the field of automatic dynamic semantic Web service composition. It focuses on service description able to capture its functionality more precisely. This is done by defining the pre-/post-conditions of services in terms of predicate logic formulae. The approach achieves much better results in the way of more meaningful compositions. Without consideration of the conditions, the composition may result in an inappropriate solution, not satisfying the user goal from the functionality point of view. Beside the functional aspect of the service composition, we also dealt with the non-functional properties. To achieve user satisfaction, these are important, too. In this thesis we have shown, that it is possible to realize a pre-/post-condition aware, QoS-driven composition, even if large-scale service repositories are considered. This is a crucial issue to make the overall service composition idea a reality. Moreover, in practical scenarios, the service composition must deal with the changes in the service environment and must be able to process multiple queries from different users. We have shown that these issues can be handled effectively, too.

Selected publications

Bartalos, P., Bielikova, M.

QoS Aware Semantic Web Service Composition Approach Considering Pre/Postconditions. In: ICWS ’10: Int. Conf. on Web Services 2010, IEEE Computer Society Press, (2010), pp. 345–352.

Bartalos, P., Bielikova, M.

Effective QoS aware Web service composition in dynamic environment. In: ISD ’10: Int. Conf. on Information Systems Development 2010, Springer, (2010).

Bartalos, P., Bielikova, M.

Composition and undesired web service execution effects. In: WoSS 2010: Proc. of 12th International Symposium on Symbolic and Numeric Algorithms for Scientific Computing, Workshops on Software Services, Timisoara, Romania, (2010).

Bartalos, P., Bielikova, M.

Semantic Web Service Composition Framework Based on Parallel Processing. In: CEC ’09: Int. Conf. on E-Commerce Technology 2009, IEEE Computer Society Press, (2009), pp. 495–498.

Bartalos, P., Bielikova, M.

Fast and Scalable Semantic Web Service Composition Approach Considering Complex Pre/Postconditions. In: Services/WSCA ’09: IEEE Congress on Services, Int. Workshop on Web Service Composition and Adaptation 2009 , IEEE Computer Society Press, (2009), pp. 414–421.

Bartalos, P., Bielikova, M.

Adapting I/O Parameters of Web Services to Enhance Composition. In: NWeSP ’09: Fifth International Conference on Next Generation Web Services Practices 2009 , IEEE Computer Society Press, (2009), pp. 17–22.

Habala, O., Paralic, M., Rozinajova, V., Bartalos, P.

Semantically-Aided Data-Aware Service Workflow Composition. In SOFSEM ’09: Theory and Practice of Computer Science 2009, Springer, LNCS, (2009), pp. 317–328.

Bartalos, P., Bielikova, M.

Enhancing semantic Web services composition with user interaction. In SCC ’08: IEEE Int. Conf. on Services Computing 2008, IEEE Computer Society Press, (2008), pp. 503–506.

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