Monitored Objects and The Probe Effect
As discussed by Gait, when an object is monitored, its behavior is changed. In particular, in any concurrent system in which processes can run in parallel, this poses a particular problem. The reason is that whenever sensors are introduced in the system, processes may execute in a different order. This can cause a problem if, for example, we are trying to localize a fault, and by monitoring the system we change its behavior in such a way that the fault may not result in a failure; in essence, the fault can be masked by monitoring the system. The probe effect is the difference in behavior between a monitored object and its uninstrumented counterpart.
According to Schütz, we can avoid, compensate for, or ignore the probe effect. In critical real-time system, in which timeliness (i.e., the ability of a system to meet time constraints such as deadlines) is significant, avoidance is the only option. If we, for example, instrument a system for testing and then remove the instrumentation before delivery, this invalidates the results of most testing based on the complete system. In less critical real-time system (e.g., media-based systems), compensation can be acceptable for, for example, performance testing. In non-concurrent systems, ignorance is acceptable, since the behavior with respect to the order of execution is left unchanged.
Read more about this topic: Event Monitoring
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