Table of Contents

Status

Current state: Under Discussion Accepted

Discussion thread: https://lists.apache.org/thread/6zsr70lpzqp96g4jf6kngyms4kkz1tp1

Vote thread: N/A https://lists.apache.org/thread/dtc48g4zjdj4s4n0hm99hm0xoho5sp1n

JIRA:

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This KIP proposes to deprecate the broker.id server configuration property in Apache Kafka 4.3 4 and remove it in Apache Kafka 5.0.

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• The default value of -1 for broker.id is effectively unused in KRaft mode. If node.id is set, the synonym mechanism overwrites broker.id with the value of node.id. If neither is set, node.id validation fails with a ConfigException before the default is ever used.
• The documentation describes node.id as required in KRaft mode, but it can currently actually be omitted if currently if broker.id is set, because the synonym mechanism automatically populates node.id from broker.id. After the removal in 5.0, node.id must be explicitly set.

The behavior is the same for all process.roles (broker, controller, or combined). No changes to public APIs, network protocols, metrics, or command-line tools.

Proposed Changes

Phase 1: Deprecation (Apache Kafka 4.

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4)

1. Log deprecation warning: When broker.id is explicitly set in the server configuration, log a WARN-level message at startup:

   Code Block
   The 'broker.id' configuration is deprecated and will be removed in Apache Kafka 5.0. Please use 'node.id' instead.

2. Preserve backward compatibility: The existing synonym mechanism will continue to function. Users who only set broker.id will still have it automatically copied to node.id. The validation requiring both to be equal (if both are explicitly set) also remains unchanged.

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Downgrading from 5.0 to 4.x is safe as long as the same configuration file is used — any config that was valid on 4.x before the upgrade remains valid after the rollback. The only edge case is if broker.id and node.id are set to different values on a 5.0 binary (where broker.id is ignored) and then downgraded to 4.x, which would result in a ConfigException. In practice, this cannot happen if the config was originally used on 4.x.

Test Plan

Phase 1: Deprecation (Apache Kafka 4.

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4)

Unit tests to verify:

• A deprecation warning is logged when broker.id is explicitly set (with or without node.id).
• Backward compatibility: setting only broker.id still works (existing tests in KafkaConfigTest already create configs with only broker.id set).

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