Status

Current state: Under Discussion

Discussion thread: None

JIRA:

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

Since Apache Kafka 4.0, ZooKeeper has been removed and KRaft is the only supported mode (KIP-500). In KRaft mode, a single process can serve as a broker, a controller, or both. KIP-631 therefore introduced node.id to replace broker.id as the canonical node identifier.

However, the legacy broker.id configuration still exists and is treated as a synonym of node.id. If only one is set, the other is automatically populated with the same value. If both are explicitly set, they must be equal; otherwise, a ConfigException is thrown.

Having two configuration properties that must always hold the same value causes unnecessary confusion and contradicts the documentation, which describes node.id as required in KRaft mode.

Related ZooKeeper-era configurations such as broker.id.generation.enable and reserved.broker.max.id were already removed in Kafka 4.0, making broker.id the only remaining legacy identifier config. We should deprecate it to simplify the configuration surface and guide all users toward the single, canonical configuration: node.id.

Public Interfaces

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

Notes:

• 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 be omitted 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 4.3)

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

   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.

Phase 2: Removal (Apache Kafka 5.0)

1. Remove broker.id configuration: The BROKER_ID_CONFIG will be removed from ServerConfigs. Setting it will result in an unknown configuration warning.
2. Remove synonym logic: Remove the broker.id/node.id synonym handling from AbstractKafkaConfig.populateSynonyms().
3. Retain internal brokerId() accessor: The brokerId() method will continue to exist for internal use, delegating to nodeId().

Behavior Matrix

The following table summarizes the expected behavior for each configuration scenario across version ranges:

Note: In the 5.0 column, "ignored" means broker.id is treated as an unrecognized configuration, which logs a standard unknown config warning.

Compatibility, Deprecation, and Migration Plan

Migration Steps

Users should replace broker.id with node.id in their server configuration. If both are configured, simply remove broker.id.

Downgrade Considerations

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 4.3)

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).

Phase 2: Removal (Apache Kafka 5.0)

Unit tests to verify:

• broker.id is no longer recognized and is ignored with an unknown config warning.
• Setting only broker.id without node.id results in a ConfigException.

Rejected Alternatives

Throw ConfigException when broker.id is set in 5.0

Instead of ignoring broker.id as an unrecognized configuration in 5.0, we could explicitly throw a ConfigException. However, this would force users to update their configuration files before upgrading, making the upgrade process more disruptive. Ignoring unrecognized configurations is consistent with how other removed configurations are handled in Kafka.

References

• KIP-500: Replace ZooKeeper with a Self-Managed Metadata Quorum
• KIP-631: The Quorum-based Kafka Controller