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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.