In an upcoming release you will be able to keep stateful variables using UpdateAttribute (NIFI-1582), the author envisions it being able to support running averages. Also I started an AggregateValues processor under NIFI-2735, but haven't been able to finish it yet, and it works on micro-batches (such as files created from a split processor like SplitJson) rather than rolling windows. In the meantime if you are familiar with a scripting language such as Groovy, Jython, Javascript, or JRuby you could use ExecuteScript or InvokeScriptedProcessor, they have access (via the ProcessContext) to the StateManager, where you could keep state, averages, etc, across flow files over time.
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