Stefan Böttcher

Building Semantic Webs for e-government with Wiki technology

E-government webs are among the largest webs in existence, based on the size, number of users and number of information providers. Thus, creating a Semantic Web infrastructure to meaningfully organise e-government webs is highly desirable. At the same time, the complexity of the existing e-government implementations also challenges the feasibility of Semantic Web creation. We therefore propose the design of a two-layer semantic Wiki web, which consists of a content Wiki, largely identical to the traditional web and a semantic layer, also maintained within the Wiki, that describes semantic relationships. This architectural design promises several advantages that enable incremental growth, collaborative development by a large community of non-technical users and the ability to continually grow the content layer without the immediate overhead of parallel maintenance of the semantic layer. This paper explains current challenges to the development of a Semantic Web, identifies Wiki advantages, illustrates a potential solution and summarises major directions for further research.

Reformulating XPath queries and XSLT queries on XSLT views

Applications using XML for data representation very often use different XML formats and thus require the transformation of XML data. The common approach transforms entire XML documents from one format into another, e.g. by using an XSLT stylesheet. Different from this approach, we use an XSLT stylesheet in order to transform a given XPath query or a given XSLT query so that we retrieve and transform only that part of the XML document, which is sufficient to answer the given query. Among other things, our approach avoids problems of replication, saves processing time, and in distributed scenarios, transportation costs.

An integrated commit protocol for mobile network databases

While traditional fixed-wired network protocols like 2-phase-commit guarantee atomicity, we cannot use them in mobile low bandwidth networks where network partitioning, node failure, and message loss may result in blocking. To deploy traditional database applications easily into a mobile environment, there is a demand for a protocol which guarantees an atomic commit of transactions. This paper introduces a protocol which can guarantee such atomic commitment in mobile environments using a combination of commit and consensus protocols. In addition, it takes advantage of mobile network sub-structures like single-hop environments to reduce message transfer costs.

Evaluating xpath queries on XML data streams

Whenever queries have to be evaluated on XML data streams - or when the memory that is available to evaluate the XML data is relatively small compared to the document - DOM based approaches that have to load and store large parts of the document in main memory will fail. In comparison, we present an approach to evaluate XPath queries on SAX streams that supports all axes of core XPath, including the sibling axes. Starting from the XPath query, our approach generates a stack of automata that uses the SAX stream as input and generates the result of the query as an output SAX stream. An evaluation of our implementation shows that in general our approach needs less main memory, but at the same time is faster than both, Saxon and YFilter.

XPath Query Simplification with regard to the Elimination of Intersect and Except Operators

XPath is widely used as an XML query language and is embedded in XQuery expressions and in XSLT stylesheets. In this paper, we propose a rule set which logically simplifies XPath queries by using a heuristic method in order to improve the processing time. Furthermore, we show how to substitute the XPath 2.0 intersect and except operators in a given XPath query with computed filter expressions. A performance evaluation comparing the execution times of the original XPath queries, which contain the intersect and except operators, and of the queries that are the result of our simplification approach shows that, depending on the used query evaluator and on the original query, performance improvements of a factor of up to 350 are possible.

Secure Set Union and Bag Union Computation for Guaranteeing Anonymity of Distrustful Participants

The computation of the union operator for different distributed datasets involves two challenges when participants are curious and can even act malicious: guaranteeing anonymity and guaranteeing security. Anonymity means that the owner of a certain data item cannot be identified provided that more than two participants act. Security means that no participant can underhandedly prevent data items of other participants from being included in the union. We present a protocol for computing both, the set union and the bag union of data sets of different participants that guarantees both properties: anonymity and security even if participants act malicious, i.e.modify messages or change or stop the protocol. We prove the correctness of the protocol and give experimental results that show the applicability of our protocol in a common environment.

XPath query transformation based on XSLT stylesheets

Whenever XML data must be shared by heterogeneous applications, transformations between different application-specific XML formats are necessary. The state-of-the-art method transforms entire XML documents from one application format into another e.g. by using an XSLT stylesheet, so that each application can work locally on its preferred format. In our approach, we use an XSLT stylesheet in order to transform a given XPath query such that we retrieve and transform only that part of the XML document which is sufficient to answer the given query. Among other things, our approach avoids problems of replication, saves processing time and in distributed scenarios, transportation costs.

Reducing sub-transaction aborts and blocking time within atomic commit protocols

Composed Web service transactions executed in distributed networks often require an atomic execution. Guaranteeing atomicity in mobile networks involves a lot more challenges than in fixed-wired networks. These challenges mostly concern network failures, e.g. network partitioning and node disconnection, each of which involves the risk of infinite blocking and can lead to a high number of aborts. In this paper, we introduce an extension to existing atomic commit protocols, which decreases the time during which a resource manager that is involved in a web-service is blocked. In addition, our proposal reduces the number of sub-transaction aborts that arise due to message loss or due to conflicting concurrent transactions by distinguishing re-usable and repeatable sub-transactions from aborting sub-transactions.

A DTD Graph Based XPath Query Subsumption Test

XPath expressions play a central role in querying for XML fragments. We present a containment test of two XPath queries which determines whether a new XPath query XP1 can reuse a previous query result XP2. The key idea is to transform XP1 into a graph which is used to search for sequences of elements which are used in the XPath query XP2.

Detecting privacy violations in sensitive XML databases

Privacy violations and the exposition of sensitive data to a third party may seriously damage the business of a company. Therefore, it is crucial for the company to identify that set of users that may have exposed the sensitive data. To identify that set of users is a problem, when multiple users must have ac cess rights that allow them to access the exposed sensitive data. Our solution to the problem is based on an analysis of the users’ XPath queries. Within a two-step approach, we compare submitted queries with the exposed data to iden tify suspicious queries.