@Carrick That property does support Expression Language, but you need to understand what this property is doing with the EL you provided. ${trigger.type} <-- This is an expression language statement. It is asking NiFi to check for the existence of an attribute on the incoming FlowFile, assigned to a Process group, found in the NiFi registry file, found in a JVM property and finally found in NiFi user env variable. (check in that order, first match ends search.) If not found nothing is returned. If found the value assigned to that attribute is returned. So in your case what is being returned is 'mouse' or 'keyboard'. So now the merge content processor is using mouse or keyboard as the correlation attribute name. And in you case there is no attribute named mouse or keyboard so all the flowfiles with null values are ending up in the same bin. As soon as you removed the EL and simply defined the attribute to use for correlation it worked. Thanks, Matt
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