Plants need both data acquisition without any loss and detection of state changes in restricted times. The paper proposes the active ring model for active real-time database systems which acquire data from industrial plants, provide the data for applications, and detect state changes to notify them to applications. The model incorporates the ECA mechanism into circular storage areas. In the ECA mechanism, conditions whose evaluation time is bounded are separated from others. The model is cognizant of data freshness. Since deadlines of transactions are set according to data freshness, the detection is integrated with the real-time acquisition. The paper includes an experimental result revealing that the active ring model is effective to provide service according to data freshness.
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