Temporal modules

In a federated database environment, different constituents of the federation may use different temporal models or physical representations for temporal information. This paper introduces a new concept, called a temporal module, to resolve these differences, or 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: The first function associates each time point with a set of tuples and the second function links each tuple to 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 to record temporal information. Our solution relies on “information conversions” provided by each constituent. Specifically, each temporal module is extended to provide several “windows” to its information, each in terms of a different time unit. The first step to process a query addressed to the federation is to select suitable windows to the underlying temporal modules. In order to facilitate such a process, 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 above calculus-style language.