Managing the evolution of XML-based mediation queries

AUTOR(ES)
DATA DE PUBLICAÇÃO

2003

RESUMO

In recent years, the problem of integrating data from heterogeneous and autonomous data sources has received a great deal of attention from the database community research. This problem consists in providing a uniform view of these data sources (called mediation schema or global schema) and defining a set of queries which compute each object in the mediation schema (called mediation queries, mediation mappings or operational mappings). Previous work in data integration can be classified according to the approach used to define the mediation mappings between the data sources and the global schema. The first approach is called global-as-view (GAV) and requires that each object of the global schema be expressed as a view (i.e. a query) on the data sources. In the other approach, called local-as-view (LAV), mediation mappings are defined in an opposite way; each object in a given source is defined as a view on the global schema. In this work, we propose a mediator-based data integration system, which adopts the GAV approach. A distinguishing feature of this system is that besides integrating data it also deals with the problems concerning generation and maintenance of mediation queries. The proposed system uses XML as a common data model for data exchange and integration, and XML Schema language to represent the mediation schema and the source schemas. To provide a highlevel abstraction for information described in an XML schema we propose a conceptual data model, called X-Entity model. We also present the process of converting an XML Schema to an X-Entity schema. This process is based on a set of rules that considers element declarations and type definitions of an XML Schema and generates the corresponding conceptual elements. One of the main problems in the context of the GAV approach is the maintenance of the mappings between the mediation schema and the source schemas. In a dynamic environment, the mediation queries must be flexible enough in order to accommodate new data sources and new usersâ requirements. In this context, we address a novel and complex problem that consists in propagating a change event occurring at the source level or at the user level into the mediation level. To manage the evolution of the mediation level we have defined: i) a set of XEntity schema change operations, ii) a set of propagation primitives reflecting the changes in the mediation level and iii) a set of propagation rules. We also propose a back-end process to execute the propagation of schema changes to the mediation queries. We propose an incremental approach to develop the mediation schema and the mediation queries based on the evolution of the data source schemas and the evolution of the usersâ requirements. More precisely, if a new data source is added, for example, the mediation queries do not need to be recomputed from scratch. Instead, we can add the new data source to the existing queries. When an existing data source is removed, we check the queries, and the ones in which the deleted source appears may be removed or rewritten. In the same way, changes in the usersâ requirements can be reflected in the mediation schema and in the mediation queries for the cases where it is possible to do so. The proposed approach allows the mediation level to evolve incrementally and modifications can be handled easier increasing the system flexibility and scalability

ASSUNTO(S)

management mediation queries evolution of xml-based ciencia da computacao

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