Temporal modules: An approach toward federated temporal databases

In a federated database environment, different constituents of the federation may use different temporal models or physical representations for the temporal information. This paper introduces a new concept, called a temporal module, to resolve these mismatches among the constituents. Intuitively, a temporal module hides the implementation details of a temporal relation by exposing its information only through two windowing functions: one function associates each time point to a set of tuples and the other function links each tuple with a set of time points. A calculus-style language is given to form queries on temporal modules. Temporal modules are then extended to resolve another type of mismatch among the constituents of a federation, namely, the mismatch involving different time units (e.g., month, week, and day) used in recording temporal information. Our solution to this mismatch relies on "information conversions" provided by each constituent. Specifically, a temporal module is extended to provide several conversion functions; each converts its information to a different time unit. The first step to process a query addressed to the federation is to select suitable conversion functions from the extended temporal modules. In order to do so, time units are formally defined and studied. A federated temporal database model and its query language are proposed. The query language is an extension of the calculus-style language above.