Apache NiFi RecordPath Guide
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Function Usage

In addition to retrieving a field from a Record, as outlined above in the Filter section, we sometimes need to refine which fields we want to select. Or we may want to return a modified version of a field. To do this, we rely on functions. The syntax for a function is <function name> <open parenthesis> <args> <close parenthesis>, where <args> represents one or more arguments separated by commas. An argument may be a string literal (such as 'hello') or a number literal (such as 48), or could be a relative or absolute RecordPath (such as ./name or /id). Additionally, we can use functions within a filter. For example, we could use a RecordPath such as /person[ isEmpty('name') ]/id to retrieve the id field of any person whose name is empty. A listing of functions that are available and their corresponding documentation can be found below in the Functions section.