Policy based contracting in semantic web service markets

Web services generalize the idea of the Web beyond the exchange of simple Web pages in order to enable the provision of a broad range of different services. By composing Web services, cross-organizational and collaborative business processes can be realized in a highly dynamic and flexible way, which is particularly important if services have to be automatically procured at runtime. However, achieving a higher degree of automation is obstructed by the informal nature of legal, contractual and organizational regulations, the numerous and complex service descriptions including manifold customization possibilities, and the open and heterogeneous nature of the Web service market. In this thesis, semantic technologies that provide more explicit meaning of information are employed to address these problems. These technologies facilitate the exchange of information in heterogeneous systems and increase the share of machine-understandable data accessible for automated decision-making. We introduce the Core Policy Ontology in order to capture regulations as well as preferences by means of goal and utility function policies, respectively. Furthermore, we introduce the Core Ontology of Bids that facilitates customization of Web services to specific user needs by efficiently representing highly configurable Web service offers and requests. Analogously, we derive the Core Contract Ontology from the Core Policy Ontology to formally represent Web service contracts. Thereby, we provide an open, transparent and interoperable representation of contracts and enable a tight integration of contractual information with the collaborative business interactions they govern. In order to show the applicability of the presented ontologies, we introduce an automated contracting mechanism that includes algorithms for automated matching, allocation, contract formation and contract monitoring. It exploits the semantic descriptions provided by the ontology framework and thereby enables logic-based matching between offers and requests and the specification of policies on hierarchical sets of service characteristics. Since declarative matching and allocation rules are used to define the mechanism, it can be dynamically adapted to new domains or settings. For the efficient allocation of Web services in heterogeneous environments, we present a novel approach that enables the integration of semantic matching and efficient optimization techniques such as linear programming. Moreover, the mechanism can be used to verify whether a Web service invocation adheres to the obligations stated in the contract. The contracting mechanism is prototypically implemented using WS-BPEL and the ontology reasoner KAON2. The evaluation of the prototype indicates that Web service contracting is applicable in practice and that semantic matching of requests and offers is particularly important for settings with highly customizable services.