Thorsten Möller

A platform to support decentralized and dynamically distributed P2P composite OWL-S service execution

In a large variety of applications, it is increasingly important to provide application functionality in a modular way by means of (Web) services. At the same time, pre-defined applications are no longer suitable to cope with the high functional dynamics that can be found in novel e-business, e-health, and e-science applications. In contrast, dynamic application creation, i.e., applications that are assembled ad hoc by service composition and usually instantiated very few times, are more and more becoming prevalent. Form a systems point of view, large scale application environments like the Internet create scalability requirements towards distributed execution of composite (Web) services which go beyond the traditional non-distributed approach to manage composite services. The contribution of this paper is threefold. First, we present a novel approach that combines those aspects by using different technologies in a distributed environment to dynamically distribute composite service execution in situations where it is beneficial or required. Second, the approach considers semantic annotation of services to facilitate new possibilities for data and service co-ordination. Third, the approach also incorporates the interfaces needed to integrate service execution with semantic service composition planners to allow for dynamic forward failure recovery by contingency service re-planning. These concepts are currently developed on the basis of the peer-to-peer platform OSIRIS NEXT which supports dynamically distributed and decentralized execution of composite semantic services that are described based on OWL-S.

Next-generation applications in healthcare digital libraries using semantic service composition and coordination

Healthcare digital libraries (DLs) increasingly make use of dedicated services to access functionality and/or data. Semantic (web) services enhance single services and facilitate compound services, thereby supporting advanced applications on top of a DL. The traditional process management approach tends to focus on process definition at build time rather than on actual service events in run time, and to anticipate failures in order to define appropriate strategies. This paper presents a novel approach where service coordination is distributed among a set of agents. A dedicated component plans compound semantic services on demand for a particular application. In failure, the planner is reinvoked to define contin- gency strategies. Finally, matchmaking is effected at runtime by choosing the appropriate service provider. These combined technologies will provide key support for highly flexible next-generation DL applications. Such technologies are under development within CASCOM.

Semantic Web Service Execution

Service execution comprises all the activities that need to be carried out at runtime to invoke one or several (Web) services in a coordinated manner. These activities include initiation, control and validation of service invocations. Since each service is supposed to create side effects as manifested by the functionality that it implements, both the service user and service provider are interested that certain properties for execution are guaranteed. The two most prominent properties are guaranteed termination and reliability, that is, sustaining a consistent state before and after execution even in the presence of failures. Those aspects become of particular interest when it comes to (i) execution in distributed environments where more than one software entity might be involved, and (ii) execution of composite services, i.e., processes.

Optimized data access for efficient execution of Semantic Services

Executing Semantic Services requires, in contrast to traditional SOAP-based Web Services, frequent read and write accesses to graph-based semantic data stores - for instance, for the evaluation of preconditions or the materialization of service effects. Therefore, the overall performance of semantic service execution, in particular for composite services, is strongly affected by the efficiency of these reads and writes. In this paper we present two data access optimization techniques for semantic data stores: Prepared Queries and Frame Caching. The former reduces the costs for repeated query evaluation, e.g., in loops. The latter provides rapid access to frequently read triples or subgraphs based on materialized views using a Frame-based data structure. The described techniques have been implemented and evaluated on the basis of OSIRIS Next, our open infrastructure for Semantic Service support.