A signal processing approach to global predicate monitoring

Global predicate evaluation is a fundamental problem in distributed systems. This paper views it from a different perspective, namely that of the signals and systems area of electrical engineering. It adapts a signal processing approach to address this problem in the context of monitoring of ´health´ of a software system. The global state of the system is viewed as a ´state´ signal which evolves over time. The distributed processes are assumed to possess roughly synchronized clocks. The states of individual processes are periodically sampled and reported to a global monitor. The observed system state constructed by the global monitor is viewed as being composed of two components - the consistent global states and an error signal due to the messages in transit and differences in the local clocks. The global monitor removes the error signal by processing the observed global signal through a low-pass filter. It evaluates the predicates on the filtered signal. The approach presented is applicable to distributed systems which are semi-stationary, i.e. whose internal states of interest remain stable over comparatively long intervals of time. The paper presents the relevant signal processing concepts (p-spectrum and p-filtering), outlines an architecture for global predicate monitoring and describes the signal processing done in the global monitor. The paper then summarizes an evaluation of the approach presented on a small computer aided vehicle dispatch system. The evaluation experiments are described and the results are presented and analyzed.