Anomaly detection in Azure Stream Analytics
Available in both the cloud and Azure IoT Edge, Azure Stream Analytics offers built-in machine learning based anomaly detection capabilities that can be used to monitor the two most commonly occurring anomalies: temporary and persistent. With the AnomalyDetection_SpikeAndDip and AnomalyDetection_ChangePoint functions, you can perform anomaly detection directly in your Stream Analytics job.
The machine learning models assume a uniformly sampled time series. If the time series is not uniform, you may insert an aggregation step with a tumbling window prior to calling anomaly detection.
The machine learning operations do not support seasonality trends or multi-variate correlations.
Model accuracy and performance
Generally, the model's accuracy improves with more data in the sliding window. The data in the specified sliding window is treated as part of its normal range of values for that time frame. The model only considers event history over the sliding window to check if the current event is anomalous. As the sliding window moves, old values are evicted from the model’s training.
The functions operate by establishing a certain normal based on what they have seen so far. Outliers are identified by comparing against the established normal, within the confidence level. The window size should be based on the minimum events required to train the model for normal behavior so that when an anomaly occurs, it would be able to recognize it.
Keep in mind that the model's response time increases with history size because it needs to compare against a higher number of past events. It is recommended to only include the necessary number of events for better performance.
Gaps in the time series can be a result of the model not receiving events at certain points in time. This situation is handled by Stream Analytics using imputation. The history size, as well as a time duration, for the same sliding window is used to calculate the average rate at which events are expected to arrive.
Spike and dip
Temporary anomalies in a time series event stream are known as spikes and dips. Spikes and dips can be monitored using the Machine Learning based operator, AnomalyDetection_SpikeAndDip.
In the same sliding window, if a second spike is smaller than the first one, the computed score for the smaller spike is probably not significant enough compared to the score for the first spike within the confidence level specified. You can try decreasing the model's confidence level setting to catch such anomalies. However, if you start to get too many alerts, you can use a higher confidence interval.
The following example query assumes a uniform input rate of one event per second in a 2-minute sliding window with a history of 120 events. The final SELECT statement extracts and outputs the score and anomaly status with a confidence level of 95%.
WITH AnomalyDetectionStep AS ( SELECT EVENTENQUEUEDUTCTIME AS time, CAST(temperature AS float) AS temp, AnomalyDetection_SpikeAndDip(CAST(temperature AS float), 95, 120, 'spikesanddips') OVER(LIMIT DURATION(second, 120)) AS SpikeAndDipScores FROM input ) SELECT time, temp, CAST(GetRecordPropertyValue(SpikeAndDipScores, 'Score') AS float) AS SpikeAndDipScore, CAST(GetRecordPropertyValue(SpikeAndDipScores, 'IsAnomaly') AS bigint) AS IsSpikeAndDipAnomaly INTO output FROM AnomalyDetectionStep
Persistent anomalies in a time series event stream are changes in the distribution of values in the event stream, like level changes and trends. In Stream Analytics, such anomalies are detected using the Machine Learning based AnomalyDetection_ChangePoint operator.
Persistent changes last much longer than spikes and dips and could indicate catastrophic event(s). Persistent changes are not usually visible to the naked eye, but can be detected with the AnomalyDetection_ChangePoint operator.
The following image is an example of a level change:
The following image is an example of a trend change:
The following example query assumes a uniform input rate of one event per second in a 20-minute sliding window with a history size of 1200 events. The final SELECT statement extracts and outputs the score and anomaly status with a confidence level of 80%.
WITH AnomalyDetectionStep AS ( SELECT EVENTENQUEUEDUTCTIME AS time, CAST(temperature AS float) AS temp, AnomalyDetection_ChangePoint(CAST(temperature AS float), 80, 1200) OVER(LIMIT DURATION(minute, 20)) AS ChangePointScores FROM input ) SELECT time, temp, CAST(GetRecordPropertyValue(ChangePointScores, 'Score') AS float) AS ChangePointScore, CAST(GetRecordPropertyValue(ChangePointScores, 'IsAnomaly') AS bigint) AS IsChangePointAnomaly INTO output FROM AnomalyDetectionStep
Anomaly detection using machine learning in Azure Stream Analytics
The following video demonstrates how to detect an anomaly in real time using machine learning functions in Azure Stream Analytics.
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