Correct View Update Translations via Containment

Given an intensional database (IDB) and an extension database (EDB), the view update problem translates updates on the IDB into updates on the EDB. One approach to the view update problem uses a translation langauge to specify the meaning of a view update. In this paper we prove properties of a translation language. This approach to the view update problem studies the expressive power of the translation language and the computational cost of demonstrating properties of a translation. We use an active rule based database language for specifying translations of view updates. This paper uses the containment of one datalog program (or conjunctive query) by another to demonstrate that a translation is semantically correct. We show that the complexity of correctness is lower for insertion than deletion. Finally, we discuss extensions to the translation language.</p

Partial Answers for Unavailable Data Sources

Many heterogeneous database system products and prototypes exist today; they will soon be deployed in a wide variety of environments. Most existing systems suffer from an Achilles' heel: they ungracefully fail in presence of unavailable data sources. If some data sources are unavailable when accessed, these systems either silently ignore them or generate an error. This behavior is improper in environments where there is a non-negligible probability that data sources cannot be accessed (e.g., Internet). In case some data sources cannot be accessed when processing a query, the complete answer to this query cannot be computed; some work can however be done with the data sources that are available. In this paper, we propose a novel approach where, in presence of unavailable data sources, the answer to a query is a partial answer. A partial answer is a representation of the work that has been done in case the complete answer to a query cannot be computed, and of the work that remains to be done in order to obtain this complete answer. The use of a partial answer is twofold. First, it contains an incremental query that allows to obtain the complete answer without redoing the work that has already been done. Second, the application program can extract information from a partial answer through the use of a secondary query, which we call a parachute query. In this paper, we present a framework for partial answers and we propose three algorithms for the evaluation of queries in presence of unavailable sources, the construction of incremental queries and the evaluation of parachute queries.</p

Unavailable Data Sources in Mediator Based Applications

We discuss the problem of unavailable data sources in the context of two mediator based applications. We discuss the limitations of existing system with respect to this problem and describe a novel evaluation model that overcomes these shortcomings. Mediator systems are being deployed in various environments to provide query access to heterogeneous data sources. When processing a query, the mediator may have difficulty accessing a data source (due to network or server problems). In such cases the mediator is faced with the problem of unavailable data sources. In this paper, we discuss the problem of unavailable data sources in mediator based applications. We first introduce two applications that we are currently developing. The first application concerns a hospital information system; a mediator accesses data sources located in the different services to provide doctors with information on patients. The second application concerns the access to documentary repositories within a network of public and private institutions; a mediator accesses the data sources located in each institution to answer queries asked through a World Wide Web application. We detail the characteristics of these applications in Section 2. We show that these applications are representative of large classes of applications. We then discuss, in Section 3, the impact of unavailable data sources on the design of both applications. We illustrate the limitations of classical mediator systems. We give in Section 4 an overview of a novel sequential model of interaction which fits the needs of both applications and overcomes some of the above mentioned shortcomings. We review related work in Section 5. We conclude and give directions for future work in Section 6.</p

Parachute Queries in the Presence of Unavailable Data Sources

Mediator systems are used today in a wide variety of unreliable environments. When processing a query, a mediator may try to access a data source which is unavailable. In this situation, existing systems either silently ignore unavailable data sources or generate an error. This behavior is inefficient in environments with a non-negligible probability that a data source is unavailable (e.g., the Internet). In the case that some data sources are unavailable, the complete answer to a query cannot be obtained; however useful work can be done with the available data sources. In this paper, we describe a novel approach to mediator query processing where, in the presence of unavailable data sources, the answer to a query is computed incrementally. It is possible to access data obtained at intermediate steps of the computation. We define two new evaluation models and analytically model for these evaluation models the probability of obtaining the answer to a query in the presence of unavailable data sources. The analysis shows that complete answers are more likely in our two evaluation models than in a classical system. We measure the performance of our evaluation models via simulations and show that, in the case that all data sources are available, the performance penalty for our approach is negligible.</p

Query Processing and Inverted Indices in Distributed Text Document Retrieval Systems

The performance of distributed text document retrieval systems is strongly influenced by the organization of the inverted index. This paper compares the performance impact on query processing of various physical organizations for inverted lists. We present a new probabilistic model of the database and queries. Simulation experiments determine those variables that most strongly influence response time and throughput. This leads to a set of design trade-offs over a wide range of hardware configurations and new parallel query processing strategies.</p

Performance Issues in Distributed Shared-Nothing Information Retrieval Systems

Many information retrieval systems provides access to abstracts. For example Stanford University, through its FOLIO system, provides access to the INSPEC database of abstracts of the literature on physics, computer science, electrical engineering, etc. In this article this database is studied by using a trace-driven simulation. We focus on a physical index design which accommodates truncations, inverted index caching, and database scaling in a distributed shared-nothing system. All three issues are shown to have a strong effect on response time and throughput. Database scaling is explored in two ways. One way assumes an ``optimal'' configuration for a single host and then linearly scales the database by duplicating the host architecture as needed. The second way determines the optimal number of hosts given a fixed database size.</p

A Framework for Classifying Scientific Metadata

The scientific community, public organizations and administrations have generated a large amount of data concerning the environment. There is a need to allow sharing and exchange of this type of information by various kinds of users including scientists, decision-makers and public authorities. Metadata arises as the solution to support these requirements. We present a formal framework for classification of metadata that will give a uniform definition of what metadata is, how it can be used and where it must be used. This framework also provides a procedure for classifying elements of existing metadata standards.</p

Scaling Heterogeneous Distributed Databases and the Design of DISCO

Access to large numbers of data sources introduces new problems for users of heterogeneous distributed databases. End users and application programmers must deal with unavailable data sources. Database administrators must deal with incorporating new sources into the model. Database implementors must deal with the translation of queries between query languages and schemas. The Distributed Information Search COmponent (Disco) addresses these problems. Query processing semantics are developed to process queries over data sources which do not return answers. Data modeling techniques manage connections to data sources. The component interface to data sources flexibly handles different query languages and translates queries. This paper describes (a) the distributed mediator architecture of Disco, (b) its query processing semantics, (c) the data model and its modeling of data source connections, and (d) the interface to underlying data sources.</p

An Introduction to the e-XML Data Integration Suite

This paper describes the e-XML component suite, a modular product for integrating heterogeneous data sources under an XML schema and querying in real-time the integrated information using XQuery, the emerging W3C standard for XML query. We describe the two main components of the suite, i.e., the repository for warehousing XML and the mediator for distributed query processing. We also discuss some typical applications.</p

Validating Mediator Cost Models with Disco

Disco is a mediator system developed at INRIA for accessing heterogeneous data sources over the Internet. In Disco, mediators accept queries from users, process them with respect to wrappers, and return answers. Wrapper provide access to underlying sources. To efficiently process queries, the mediator performs cost-based query optimization. In a heterogeneous distributed database, cost-estimate based query optimization is difficult to achieve because the underlying data sources do not export cost information. Disco's approach relies on combining a generic cost model with specific cost information exported by wrappers. In this paper, we propose a validation of Disco's cost model based on experimentation with real Web data sources. This validation shows the efficiency of our generic cost model as well as the efficiency of more specialized cost functions.</p