Summary While the amount of information steadily increases, the requirements on the response time to query these information become more strict. Under those conditions, conventional database systems reach their limits and cannot meet these performance requirements anymore. In recent years, systems with many processing cores are considered to satisfy these demands. Furthermore, these systems include more and more heterogeneous cores tailor‐made to solve one specific task in an efficient manner. However, dedicated hardware accelerators are inflexible and cannot be adapted to the requirements of a dedicated query. Thus, the challenge is orchestrating the diversity of the functionality of all the cores to be optimized for performance/energy efficiency. In this paper, a concept is introduced on how to develop a flexible Field‐Programmable Gate Arrays (FPGA)‐based hardware accelerator to improve the performance of query evaluation in a Semantic Web database. As a first step to the hardware/software system, several joint algorithms are implemented on an FPGA and evaluated against a well‐developed software solution (implemented in C). The comparison shows a significant speedup of up to 10 times. Because of the complexity of the join operator, it is promising that the overall performance of query evaluation can be further enhanced by processing whole queries on an FPGA. Copyright © 2015 John Wiley & Sons, Ltd.

While the paradigm of service oriented architectures and also the concept of service migration has been adapted to wireless sensor networks recently, the consistency demands of stateful service migration have not yet been considered in depth. Many services, for example for the aggregation of data or alarm services, require strict consistency when being migrated from one sensor node to another. In previous papers, we described how atomic commit protocols can be used to guarantee strict consistency of migrated services. Additional communication is required to achieve strict consistency. Therefore we varied the network conditions (i.e. loss rate, density) to determine the best solution in different situations. Finally we designed an adaptive approach which uses the ideal combination of transaction protocol and routing protocol to reduce the communication overhead and save energy to increase the lifetime of wireless sensor network deployments.

Transfering the paradigm of service-oriented architecture (SOA) to sensor networks became an important research area in the last years. Amongst others, a couple of approaches deal with the flexible and robust service discovery and usage of services which takes the unsteady nature of WSNs into account. However, the approaches do not consider the case that services may get unavailable during their activity phase because of node failures. In this case, already collected or computed data will get lost and the service request of the service consumer remains unanswered. In this paper, we propose a redundancy infrastructure for service-oriented WSNs which deals with this special problem. The framework consists of an adaptive data replication technique and a service recovery solution. Whereas the replication technique ensures the survival beyond node failures, the recovery protocol restarts a failed service using the replicated data and enables the service to continue its functionality.

Managing and querying Semantic Web are important issues for Semantic Web applications. Therefore, we have developed a Semantic Web database system with logically and physically optimized SPARQL engines to manage and query RDF data, named LuposDate. In order to present the functionalities of the LUPOSDATE system and engines, we have developed an online demonstration, which is available at http://www.ifis.uni-luebeck.de/index.php?id=luposdate-demo.

XML is the defacto standard for data exchange applications like those in the WWW. However, due to the limited hardware resources, wireless sensor networks abstain from using verbose data formats like XML. Nevertheless, XML as a standardized data exchange format in wireless sensor networks is a means to support more complex data management and heterogeneous networks. Moreover, XML is a key feature towards service-oriented sensor networks that exchange structured information by using SOAP. Recent work has shown that XML can be compressed to meet the general hardware restrictions of sensor nodes while still supporting updates. In this work we outline the vision and benefits of XML usage in wireless sensor networks, show how to evaluate XML queries in wireless sensor networks and how query results can be compressed to lower the comunication overhead. We therefore present an XPath engine on updateable compressed XML data for sensor nodes and an experimental evaluation showing that the performance of our XPath engine fulfills the requirements of today's applications even on sensor nodes.

XPath is a common language for selecting nodes in an XML document. XPath uses so called path expressions which describe a navigation path through semistructured data. In the last years some of the characteristics of XPath have been discussed. Examples include the containment of two XPath expressions p and p' (p /spl sube/ p'). To the best of our knowledge the intersection of two XPath expressions (p /spl cap/ p') has not been treated yet. The intersection of p and p' is the set that contains all XML nodes that are selected both by p and p'. In the context of indexes in XML databases the emptiness of the intersection of p and p' is a major issue when updating the index. In order to keep the index consistent to the indexed data, it has to be detected if an index that is defined upon p is affected by a modifying database operation with the path expression p'. In this paper, we introduce the intersection problem for XPath and give a motivation for its relevance. We present an efficient intersection algorithm for XPath expressions without the NOT operator that is based on finite automata. For expressions that contain the NOT operator the intersection problem becomes NP-complete leading to exponential computations in general. With an average case simulation we show that the NP-completeness is no significant limitation for most real-world database operations.

The work presented in this paper is an attempt to combine two different but closely related research areas in the data base field, namely the quite heavily investigated question of recursive queries in relational data bases on the one hand and the introduction of non first normal form relations on the other. This paper proposes the integration of recursive queries and non first normal form relations by providing a recursion mechanism which can be nested according to the structure of a non first normal form relation. It is shown by examples that this method can be used to formulate queries which are rather cumbersome if only flat relations are used. Especially, in some cases the problem of performing a selection on a recursively defined relation disappears. This question has been the topic of several research papers recently. Moreover, many queries can be formulated in a more natural way than in the world of flat tuples. Therefore, nested recursive queries in combination with non first normal form relations provide a contribution to the problem of modelling complex data.

Current database systems provide only rather limited tools for advanced applications, for example for the database support of knowledge based systems. Especially, it is not possible to represent recursive rules which are denned on the basis of stored data (facts). Using the terminology of PROLOG, this means that database systems provide means for managing facts, but rules are not supported. The semantic gap, therefore, between conventional database systems on the one hand and knowledge base management systems on the other is too large. This paper presents new tools, based on a database programming language, which allow the support of rules on the basis of existing relations. These rules may be recursive, thus providing similar mechanisms as PROLOG does. The tools are set oriented, they allow, therefore, an efficient implementation. Moreover, some disadvantages of PROLOG are avoided, for example infinite recursion. In addition, data types are defined for the representation of updatable rule bases thus providing a first step towards integrated fact and rule management using relational technology.