THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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More complex streaming pipelines generally need to keep some sort of operator state in order to execute the application logic. Examples include, keeping some aggregation or summary of the received elements, or we can imagine more complex states such as keeping a state-machine for detecting patterns for fraudulent financial transactions or holding a model for some machine learning application. While in all mentioned cases we keep some kind of summary of the input history, the concrete requirements vary greatly from one stateful application to another.
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