Angela Bonifati

Schema Matching and Mapping

Schema Matching and Mapping provides an overview of the ways in which the schema and ontology matching and mapping tools have addressed information systems requirements. Topics include effective methods for matching data, mapping transformation verification, mapping-driven schema evolution and merging.

Comparative analysis of five XML query languages

XML is becoming the most relevant new standard for data representation and exchange on the WWW. Novel languages for extracting and restructuring the XML content have been proposed, some in the tradition of database query languages (i.e. SQL, OQL), others more closely inspired by XML. No standard for XML query language has yet been decided, but the discussion is ongoing within the World Wide Web Consortium and within many academic institutions and Internet-related major companies. We present a comparison of five, representative query languages for XML, highlighting their common features and differences.

Designing data marts for data warehouses

Data warehouses are databases devoted to analytical processing. They are used to support decision-making activities in most modern business settings, when complex data sets have to be studied and analyzed. The technology for analytical processing assumes that data are presented in the form of simple data marts, consisting of a well-identified collection of facts and data analysis dimensions (star schema). Despite the wide diffusion of data warehouse technology and concepts, we still miss methods that help and guide the designer in identifying and extracting such data marts out of an enterprisewide information system, covering the upstream, requirement-driven stages of the design process. Many existing methods and tools support the activities related to the efficient implementation of data marts on top of specialized technology (such as the ROLAP or MOLAP data servers). This paper presents a method to support the identification and design of data marts. The method is based on three basic steps. A first top-down step makes it possible to elicit and consolidate user requirements and expectations. This is accomplished by exploiting a goal-oriented process based on the Goal/Question/Metric paradigm developed at the University of Maryland. Ideal data marts are derived from user requirements. The second bottom-up step extracts candidate data marts

Dynamic XML documents with distribution and replication

The advent of XML as a universal exchange format, and of Web services as a basis for distributed computing, has fostered the apparition of a new class of documents: dynamic XML documents. These are XML documents where some data is given explicitly while other parts are given only intensionally by means of embedded calls to web services that can be called to generate the required information. By the sole presence of Web services, dynamic documents already include inherently some form of distributed computation. A higher level of distribution that also allows (fragments of) dynamic documents to be distributed and/or replicated over several sites is highly desirable in todays Web architecture, and in fact is also relevant for regular (non dynamic) documents.The goal of this paper is to study new issues raised by the distribution and replication of dynamic XML data. Our study has originated in the context of the Active XML system [1, 3, 22] but the results are applicable to many other systems supporting dynamic XML data. Starting from a data model and a query language, we describe a complete framework for distributed and replicated dynamic XML documents. We provide a comprehensive cost model for query evaluation and show how it applies to user queries and service calls. Finally, we describe an algorithm that, for a given peer, chooses data and services that the peer should replicate to improve the efficiency of maintaining and querying its dynamic data.

Active rules for XML: A new paradigm for E-services

Abstract. XML is rapidly becoming one of the most widely adopted technologies for information exchange and representation. As the use of XML becomes more widespread, we foresee the development of active XML rules, i.e., rules explicitly designed for the management of XML information. In particular, we argue that active rules for XML offer a natural paradigm for the rapid development of innovative e-services. In the paper, we show how active rules can be specified in the context of XSLT, a pattern-based language for publishing XML documents (promoted by the W3C) which is receiving strong commercial support, and Lorel, a query language for XML documents that is quite popular in the research world. We demonstrate, through simple examples of active rules for XSLT and Lorel, that active rules can be effective for the implementation of e-commerce services. We also discuss the various issues that need to be considered in adapting the notion of relational triggers to the XML context.

XPath lookup queries in P2P networks

We address the problem of querying XML data over a P2P network. In P2P networks, the allowed kinds of queries are usually exact-match queries over file names. We discuss the extensions needed to deal with XML data and XPath queries. A single peer can hold a whole document or a partial/complete fragment of the latter. Each XML fragment/document is identified by a distinct path expression, which is encoded in a distributed hash table. Our framework differs from content-based routing mechanisms, biased towards finding the most relevant peers holding the data. We perform fragments placement and enable fragments lookup by solely exploiting few path expressions stored on each peer. By taking advantage of quasi-zero replication of global catalogs, our system supports fast full and partial XPath querying. To this purpose, we have extended the Chord simulator and performed an experimental evaluation of our approach.

Schema mapping verification: the spicy way

Schema mapping algorithms rely on value correspondences - i.e., correspondences among semantically related attributes - to produce complex transformations among data sources. These correspondences are either manually specified or suggested by separate modules called schema matchers. The quality of mappings produced by a mapping generation tool strongly depends on the quality of the input correspondences. In this paper, we introduce the Spicy system, a novel approach to the problem of verifying the quality of mappings. Spicy is based on a three-layer architecture, in which a schema matching module is used to provide input to a mapping generation module. Then, a third module, the mapping verification module, is used to check candidate mappings and choose the ones that represent better transformations of the source into the target. At the core of the system stands a new technique for comparing the structure and actual content of trees, called structural analysis. Experimental results show that, by carefully designing the comparison algorithm, it is possible to achieve both good scalability and high precision in mapping selection.

XQueC: A query-conscious compressed XML database

XML compression has gained prominence recently because it counters the disadvantage of the verbose representation XML gives to data. In many applications, such as data exchange and data archiving, entirely compressing and decompressing a document is acceptable. In other applications, where queries must be run over compressed documents, compression may not be beneficial since the performance penalty in running the query processor over compressed data outweighs the data compression benefits. While balancing the interests of compression and query processing has received significant attention in the domain of relational databases, these results do not immediately translate to XML data. In this article, we address the problem of embedding compression into XML databases without degrading query performance. Since the setting is rather different from relational databases, the choice of compression granularity and compression algorithms must be revisited. Query execution in the compressed domain must also be rethought in the framework of XML query processing due to the richer structure of XML data. Indeed, a proper storage design for the compressed data plays a crucial role here. The XQueC system (XQuery Processor and Compressor) covers a wide set of XQuery queries in the compressed domain and relies on a workload-based cost model to perform the choices of the compression granules and of their corresponding compression algorithms. As a consequence, XQueC provides efficient query processing on compressed XML data. An extensive experimental assessment is presented, showing the effectiveness of the cost model, the compression ratios, and the query execution times.

XQueC: pushing queries to compressed XML data

Publisher Summary This chapter reveals that the loader and compressor convert XML documents in a compressed, yet queryable format. The compressed repository stores the compressed documents and provides: access methods to this compressed data, and a set of compression-specific utilities that enable, e.g., the comparison of two compressed values. The query processor optimizes and evaluates XQuery queries over the compressed documents. Its complete set of physical operators allows for efficient evaluation over the compressed repository. The chapter motivates the choice of the storage structures for compressed XML, and of the compression algorithms employed. It describes the XQueC query processor, its set of physical operators, and outlines its optimization algorithm.

Distributed databases and peer-to-peer databases: past and present

The need for large-scale data sharing between autonomous and possibly heterogeneous decentralized systems on the Web gave rise to the concept of P2P database systems. Decentralized databases are, however, not new. Whereas a definition for a P2P database system can be readily provided, a comparison with the more established decentralized models, commonly referred to as distributed, federated and multi-databases, is more likely to provide a better insight to this new P2P data management technology. Thus, in the paper, by distinguishing between db-centric and P2P-centric features, we examine features common to these database systems as well as other ad-hoc features that solely characterize P2P databases. We also provide a non-exhaustive taxonomy of the most prominent research efforts toward the realization of full-fledged P2P databases.