essence, the fault can be masked by monitoring the system. The probe effect is the difference in behavior between a monitored object and its un-instrumented 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. Event log analysis Event log analysis is known as event composition in active databases, chronicle recognition in artificial intelligence and as real-time logic evaluation in real-time systems. Essentially, event log analysis is used for pattern matching, filtering of event occurrences, and aggregation of event occurrences into composite event occurrences. Commonly, dynamic programming strategies from algorithms are employed to save results of previous analyses for future use, since, for example, the same pattern may be match with the same event occurrences in several consecutive analysis processing. In contrast to general rule processing (employed to assert new facts from other facts, cf. inference engine) that is usually based on backtracking techniques, event log analysis algorithms are commonly greedy; for example, when a composite is said to have occurred, this fact is never revoked as may be done in a backtracking based algorithm. Several mechanisms have been proposed for event log analysis: finite state automata, Petri nets, procedural (either based on an imperative programming language or an object-oriented programming languages), a modification of Boyer–Moore string-search algorithm, and simple temporal networks. See also Event stream processing (ESP) Complex event processing (CEP) Network monitoring Runtime verification (RV) References Operating system technology Network management System monitors Systems management