Getting Started with Apache NiFi
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Routing on Attributes

One of the most powerful features of NiFi is the ability to route FlowFiles based on their Attributes. The primary mechanism for doing this is the RouteOnAttribute Processor. This Processor, like UpdateAttribute, is configured by adding user-defined properties. Any number of properties can be added by clicking the "+" button in the top-right corner of the Properties tab in the Processor's Configure dialog.

Each FlowFile's Attributes will be compared against the configured properties to determine whether or not the FlowFile meets the specified criteria. The value of each property is expected to be an Expression Language expression and return a boolean value. For more on the Expression Language, see the Expression Language Reference section below.

After evaluating the Expression Language expressions provided against the FlowFile's Attributes, the Processor determines how to route the FlowFile based on the Routing Strategy selected. The most common strategy is the "Route to Property name" strategy. With this strategy selected, the Processor will expose a Relationship for each property configured. If the FlowFile's Attributes satisfy the given expression, a copy of the FlowFile will be routed to the corresponding Relationship. For example, if we had a new property with the name "begins-with-r" and the value "${filename:startsWith(\'r')}" then any FlowFile whose filename starts with the letter 'r' will be routed to that Relationship. All other FlowFiles will be routed to 'unmatched'.