Stratos uses Complex Event Processor (CEP) to do real-time monitoring, based on the health statistics that are published to the CEP from the cartridges and services. The CEP does temporal (i.e., time-based) queries to analyze all the event streams that are being sent to it, and sends summarized information to the Auto-scaler. The event processing engine is an event aggregator/accumulator, which takes lots of events and produces messages that the Auto-scaler uses to make elasticity decisions in a more granular manner. Currently, Stratos uses WSO2 CEP; however, Stratos supports any Complex Event Processor, by provides an event publishing interface, which is in the <STRATOS_SOURCE_HOME>/components/org.apache.stratos.common/src/main/java/org/apache/stratos/common/statistics/
publisher/StatisticsPublisher.java file. If you wish to plug your CEP to Stratos, you need to first implement this interface, and also implement the corresponding logic in the logic method, to publish the statistics. The current CEP implementation that Stratos uses can be found in the <STRATOS_SOURCE_HOME>/components/org.apache.stratos.common/src/main/java/org/apache/stratos/common/statistics/
publisher/wso2/cep/WSO2CEPStatisticsPublisher.java file.

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