What is the difference between EvaluateJsonPath and FlattenJson processors?

EvaluateJsonPath and FlattenJson both work with JSON content but serve fundamentally different purposes.

EvaluateJsonPath extracts specific values from a JSON payload using JSONPath expressions and writes those values either to FlowFile attributes or to the FlowFile content. It is a targeted extraction tool — you specify exactly which fields you want and where they go. Configuration requires one or more User-Defined Properties, each mapping a JSONPath expression to a destination attribute name. The JSON content itself is typically not modified when writing to attributes.

FlattenJson takes a nested JSON object and flattens its entire structure into a single-level key-value object, using configurable separator characters to compose the flattened key names. For example:

Input:  {"user": {"address": {"city": "Austin"}}}
Output: {"user.address.city": "Austin"}

FlattenJson is used to normalize deeply nested JSON into flat structures suitable for systems that expect flat schemas (relational databases, Elasticsearch, CSV). It operates on the entire document, producing a new content FlowFile with the flattened JSON.

The choice: use EvaluateJsonPath when you need specific field values as attributes for routing or enrichment; use FlattenJson when you need to restructure the entire document hierarchy into a flat representation for downstream storage.