Partial Answers for Unavailable Data Sources,
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)
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