Rita Steinmetz

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.

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.

Testing Containment of XPath Expressions in Order to Reduce the Data Transfer to Mobile Clients

Within mobile client-server applications which access a server-side XML database, XPath expressions play a central role in querying for XML fragments. Whenever the mobile client can use a locally stored previous query result in order to answer a new query instead of accessing the server-side database, this can significantly reduce the data transfer from the server to the client. In order to check whether or not a previous query result can be reused for a new XPath query, we present a containment test of two XPath queries which combines two steps. At first, we use the DTD in order to check whether all paths selected by one XPath expression are also selected by the other XPath expression. Then we right-shuffle predicate filters of both expressions and start a subsumption test on them.

Finding the Leak: A Privacy Audit System for Sensitive XML Databases

Whenever private information that is legally used by multiple employees of a company has been illegally exposed to a third party, it is of significant importance to the concerned company to find the information leak in its staff for a variety of reasons, e.g., to keep confidence of its customers. In this paper, we present a privacy audit system for XML databases and the XPath query language which uses the concept of an audit query to describe the secret information. For a given audit query, our system returns a set of suspicious user queries that may have used the secret information. Suspicious user queries are identified in a sequence of four steps: first, a static analysis based on the time constraints; second, a comparison of the nodename tests of the audit query and the user queries; third, an analysis of the associations of the node-name tests found in the audit query and in the user queries; and finally, a test on ’historic data’. Furthermore, we discuss privacy violation detection in case of an attacker who submits multiple queries and externally compares the results.

Data management for mobile Ajax web 2.0 applications

Whenever Ajax applications on mobile devices have to retrieve large XML data fragments from a remote server, a reduction of the exchanged data volume may be crucial to manage limited bandwidth and limited energy of the mobile device. We propose to use an XML compression technique that compresses an XML document to a binary directed acyclic graph (DAG) and to use DAG-based DOM evaluation on the client side. Our experiments show that the data transfer for applications like amazon or eBay can be reduced to 70% of the original data transfer needed.

Embedding XML schema constraints in search-based intersection tests for XPath query optimization

Standards like XML schema and XPath play a central role in distributed XML data processing. XPath query optimization can benefit from a tester that checks whether the intersection of data fragments described by two XPath expressions is empty for all valid database states. In this paper, we contribute a fast but incomplete intersection test for XPath expressions that reflects type constraints like sub-types and extensions defined in XML schema. The key idea is to transform the XML schema types into a hierarchy of constraints, and to translate these constraints into an automaton representing all the XML document paths that are valid according to this XML schema. Finally, the intersection test can be reduced to a search in these automata.

DTD-driven structure preserving XML compression

Whenever XML is used as the data format to exchange large amounts of data or even for data streams, the verbose behavior of XML is one of the bottle necks. While compression of XML data seems to be a way out, it is essential for a variety of applications that the compression result still can be parsed, searched, transformed or modified efficiently. In order to support efficient search in compressed XML data, we have devel oped a compression technique that links two components: the first component uses the DTD to perform a structure-preserving compression of XML markup data, while the second uses a trie for the compression of text constants and attribute values.