Integration / Apache NiFi Interview Questions

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.

When would you choose EvaluateJsonPath over FlattenJson for working with JSON FlowFile content?When you need to extract specific field values into FlowFile attributes for routing or conditional logic downstream

✓ Well done — EvaluateJsonPath is a targeted extraction tool; FlattenJson restructures the entire document.

When you need to load deeply nested JSON into a flat relational database table

✗ Try again — flattening a nested document to a relational structure is exactly what FlattenJson is designed for.

When the JSON contains arrays that must be expanded into separate rows

✗ Try again — array expansion is better handled by SplitJSON or the record model, not EvaluateJsonPath or FlattenJson.

Given nested JSON {"order": {"id": 42, "total": 99.50}}, what does FlattenJson produce using a dot separator?{"order.id": 42, "order.total": 99.50}

✓ Well done — FlattenJson joins parent and child key names with the separator character to produce flat keys.

[{"key": "order.id", "value": 42}, {"key": "order.total", "value": 99.50}]

✗ Try again — FlattenJson outputs a flat JSON object, not an array of key-value pairs.

{"id": 42, "total": 99.50}

✗ Try again — that drops the "order" parent key; FlattenJson preserves it by concatenating with the separator.

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Integration / Apache NiFi Interview Questions

What is the difference between EvaluateJsonPath and FlattenJson processors?

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