Published online by Cambridge University Press: 05 June 2012
Stable and unstable predicates
Specifying predicates on the system state provides an important handle to specify, observe, and detect the behavior of a system. This is useful in formally reasoning about the system behavior. By being able to detect a specified predicate in the execution, we gain the ability to monitor the execution. Predicate specification and detection has uses in distributed debugging, sensor networks used for sensing in various applications, and industrial process control. As an example in the manufacturing process, a system may be monitoring the pressure of Reagent A and the temperature of Reagent B. Only when ψ1 = (PressureA > 240 KPa) ∧ (TemperatureB > 300 °C) should the two reagents be mixed. As another example, consider a distributed execution where variables x, y, and z are local to processes Pi, Pj, and Pk, respectively. An application might be interested in detecting the predicate ψ2 = xi + yj + zk < −125. In a nuclear power plant, sensors at various locations would monitor the relevant parameters such as the radioactivity level and temperature at multiple locations within the reactor.
Observe that the “predicate detection” problem is inherently different from the global snapshot problem. A global snapshot gives one of the possible states that could have existed during the period of the snapshot execution. Thus, a snapshot algorithm can observe only one of the predicate values that could have existed during the algorithm execution.
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