Mutual Consistency in Real-Time Databases

A real-time database is composed of real-time objects whose values remain valid only within their validity intervals. Each object in the database models a real world entity. The freshness of these objects is maintained by update transactions that sample the real world entities. The literature proposes various ways to derive a schedule of transactions that preserves the freshness (also known as absolute consistency) of these objects. But these approaches do not take care of the mutual consistency of the objects, i.e., whether together they represent a logical state of the system. We investigate the problem of checking whether, given an update transaction schedule, a periodic query would be able to read mutually consistent values. We propose solutions for both single- and multiple-query cases in the presence of non-preemptable query executions. Specifically, we first investigate formulas that give the maximal value of mutual gaps among a set of data read at a certain point in time. (A mutual gap for two object values read from the database refers to the difference between the times at which the two objects were updated.) We then propose design approaches to (1) decide the period and relative deadline of a query so that it would guarantee mutual consistency; (2) decide if a given set of queries with relative deadlines and periods can guarantee mutual consistency. Finally, we suggest ways of reducing the complexity of our proposed approaches for both harmonic periods and general cases