Service selection and transactional management for web service composition

In addition to functional aspects web services also expose non-functional properties, which describe how the offered functionalities are delivered. In service-oriented systems with real business settings, the non-functional properties play an important role in the service life cycle, including discovery, selection and operation of services. This PhD thesis focuses on two research challenges related to the non-functional aspects of web service composition. The first part of the thesis addresses the problem of selecting web services for a composite application such that the end-to-end Quality of Service (QoS) values perceived by the user meet his/her requirements. This problem is often modeled as a Multiple-Choice MultiDimensional Knapsack problem, which is known to be NP-hard. Therefore, it is expected that any exact solution to this problem will have an exponential computation time. In order to solve this problem, new selection algorithms are presented in this thesis, which are able to satisfy all user’s constraints much faster than exact solutions, while achieving close-tooptimal results. More specifically, a novel hybrid approach is presented, which combines global optimization with local selection in order to benefit form both worlds. In addition, skyline-based algorithms are presented to prune non-interesting services from the search space and thus reducing the computation time dramatically. Several experiments have been conducted using both real and synthetic datasets to evaluate the proposed selection methods. The results of these experiments indicate a significant improvement in performance compared to the state-of-the-art solutions. Another important non-functional aspect that needs to be taken into account when dealing with service compositions is the transactional characteristics of the involved services. A transactional coordination of the composed services is needed in order to ensure a consistent outcome in case of service failures. Due to the inherent autonomy and heterogeneity of web services, current standards for web service transactions relax the ACID properties and rely on compensation models for failure recovery. However, due to relaxing the isolation property transactional dependencies start to emerge between concurrent long-running web service transactions. The second part of this thesis addresses this problem and proposes an extension to the standard framework for web service transactions to enable detecting and handling such transactional dependencies. Moreover, an optimistic protocol for concurrency control and decentralized solutions for handling global dependency cycles are presented. The proposed methods have the advantage that they can be deployed in a fully distributed fashion within the proposed architecture. Experimental evaluation of the concurrency control protocol using extensive simulation of long running web service transactions are also presented.