Partial Answers for Unavailable Data Sources

Database Seminar May 9, 1997

Speaker: Anthony Tomasic, INRIA


Many heterogeneous database system products and prototypes exist today; they are deployed in a wide variety of environments. All existing systems (that we are aware of) suffer from an Achilles' heel: if some sources of data are unavailable when accessed by the heterogeneous database system, these systems either silently ignore the unavailable source of data or generate an error, i.e. they ungracefully fail. This behavior is improper in environments where there is a non-negligible probability that data sources cannot be accessed (e.g., Internet). In this talk, we propose a novel approach to this issue where, in presence of unavailable data sources, the answer to a query is a ``partial answer.'' A partial answer is itself a query that results from the partial evaluation of the original query; it is composed of the data that have been obtained and processed during the evaluation and of a representation of the unfinished work to be done. Partial answers can be resubmitted to the system in order to obtain the final answer to the original query, or another partial answer. Additionally, the application program can extract information from a partial answer through the use of a secondary query. This secondary query is called a ``parachute query.'' In this talk we give a taxonomy of types of partial answers and parachute queries. We present algorithms for the evaluation of queries in presence of unavailable data sources, and we describe an implementation.

Joint work with Philippe Bonnet (BULL/Dyade, Grenoble, France)