[ravisharm]

Summary

KafkaInputReader: Kafka header/key/topic values now win over payload columns of the same name.

 buildBlendedEventMap.get(key)
-  val = payloadRow.get(key)
-  if val == null -> headerMap.get(key)
+  val = headerMap.get(key)
+  if val == null -> payloadRow.get(key)

Applies to read() and sample(). Needed so enrichment headers (T0, Metrics Monetization) can't be shadowed by a payload column.

Evidence

• Before: header foo=fromHeader + payload foo=x, empty header prefix
  testHeaderWinsOnNameCollision -> ComparisonFailure: expected:<fromHeader> but was:<x>
  After: KafkaInputFormatTest Tests run: 12, Failures: 0, Errors: 0

Note: the Kafka module has no junit-vintage-engine, so its JUnit 4 tests run 0 tests under Surefire by default. I added the engine temporarily to run them and did not commit that.

Merge Danger

Door: two-way

Revert the 4-line change.

Blast Radius: Kafka ingestion

Behavior changes only for specs using KafkaInputFormat with a header, key or topic column name that equals a payload column. With the default kafka.header. prefix, collisions are rare. Anyone relying on payload-wins would now see header values.

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

The focused change matches the intended precedence, with no blocking issues identified in static review; tests were not executed.

No open issues 🎉

What changed in this PR

Updates Kafka ingestion so non-null header, key, and topic values take precedence over same-named payload columns.

Changes:

• Reverses shared lookup precedence for read() and sample().
• Adds a header-collision regression test that also checks unaffected payload fields and timestamps.
• Updates the lookup documentation.

File	Description
extensions-core/​kafka-indexing-service/​src/​test/​java/​org/​apache/​druid/​data/​input/​kafkainput/​KafkaInputFormatTest.java	Tests header precedence on name collisions.
extensions-core/​kafka-indexing-service/​src/​main/​java/​org/​apache/​druid/​data/​input/​kafkainput/​KafkaInputReader.java	Prioritizes Kafka metadata over payload values and documents the behavior.

_{Review effort: Balanced}