A Flexible Meta-Wrapper Interface for Autonomous Distributed Information Sources.

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We support flexible query processing with autonomous networked information sources. Flexibility allows a query to be accepted in a dynamic environment with unavailable sources. Flexibility provides the ability to identify equivalent sources, based on their contents; these equivalences are used to eliminate redundancy and provide alternate query plans, when some source is unavailable. We determine the best plan, i.e., the least-cost non-redundant plan, based on a cost-model for autonomous sources. These features are supported by a meta-wrapper component within the mediator. The meta-wrapper interface is defined by a structure and supported operations. WHOQL is a query language for queries and plans; it can represent sequential execution to obtain safe plans, and plans with redundancy (alternatives). A language WHODL defines the mapping from the meta-wrapper interface to
each source. WHODL also describes the contents of a source. This content definition is used to determine equivalences of autonomous sources. We obtain a least-cost non-redundant plan in a dynamic environment. A meta-wrapper cost model uses three underlying sources of information: a selectivity model; a cost model for operators in the meta-wrapper; and a cost estimator for the query response time. The estimator uses a parameterized feedback technique to learn from query feedback, and to determine the relevance of various factors that affect response time. The cost model also provides feedback to the plan generator on low-cost plans. (Also cross-referenced as UMIACS-TR-97-07)