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Anomaly detection is critical for web-based software systems. Anecdotal evidence suggests that in these systems, the accuracy of a static anomaly detection method that was previously ensured is bound to degrade over time. It is due to the significant change of data distribution, namely concept drift, which is caused by software change or personal preferences evolving. Even though dozens of anomaly detectors have been proposed over the years in the context of software system, they have not tackled the problem of concept drift. In this paper, we present a framework, StepWise, which can detect concept drift without tuning detection threshold or per-KPI (Key Performance Indicator) model parameters in a large scale KPI streams, take external factors into account to distinguish the concept drift which under operators' expectations, and help any kind of anomaly detection algorithm to handle it rapidly. For the prototype …