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).
h1. Motivation
Since Apache Kafka 4.0, ZooKeeper has been removed and KRaft is the only supported mode ([KIP-500|https://cwiki.apache.org/confluence/display/KAFKA/KIP-500%3A+Replace+ZooKeeper+with+a+Self-Managed+Metadata+Quorum]). In KRaft mode, a single process can serve as a broker, a controller, or both. [KIP-631|https://cwiki.apache.org/confluence/display/KAFKA/KIP-631%3A+The+Quorum-based+Kafka+Controller] 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. The documentation describes {{node.id}} as required in KRaft mode, yet the synonym mechanism allows users to omit {{node.id}} entirely if {{broker.id}} is set, contradicting the documented requirement.
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}}.
h1. 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.
||Config||Type||Default||4.3||5.0||
|{{broker.id}}|INT|\-1|*Deprecated*|*Removed*|
|{{node.id}}|INT|_(required)_|No change|No change|
Note: 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.
No changes to public APIs, network protocols, metrics, or command-line tools.
h1. Proposed Changes
h2. Phase 1: Deprecation (Apache Kafka 4.3)
# *Log deprecation warning*: When {{broker.id}} is explicitly set in the server configuration, log a WARN-level message at startup:
{code}
The 'broker.id' configuration is deprecated and will be removed in Apache Kafka 5.0. Please use 'node.id' instead.
{code}
# *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.
h2. Phase 2: Removal (Apache Kafka 5.0)
# *Remove {{broker.id}} configuration*: The {{BROKER_ID_CONFIG}} will be removed from {{ServerConfigs}}. Setting it will result in an unknown configuration warning.
# *Remove synonym logic*: Remove the {{broker.id}}/{{node.id}} synonym handling from {{AbstractKafkaConfig.populateSynonyms()}}.
# *Retain internal {{brokerId()}} accessor*: The {{brokerId()}} method will continue to exist for internal use, delegating to {{nodeId()}}.
h2. Behavior Matrix
The following table summarizes the expected behavior for each configuration scenario across version ranges:
||Configuration||4.0 ~ 4.2 (Current)||4.3 ~ 4.x (Deprecated)||5.0 (Removed)||
|Only {{node.id}} set|Valid|Valid|Valid|
|Only {{broker.id}} set|Valid; {{node.id}} is auto-populated from {{broker.id}}|Valid + *deprecation warning*; {{node.id}} is auto-populated from {{broker.id}}|{{ConfigException}}: {{node.id}} is required|
|Both set, same value|Valid|Valid + *deprecation warning*|Valid; {{broker.id}} is ignored with *unknown config warning*|
|Both set, different values|*Error*: {{node.id}} must equal {{broker.id}}|*Error*: {{node.id}} must equal {{broker.id}}|Valid; {{broker.id}} is ignored with *unknown config warning*|
|Neither set|{{ConfigException}}: {{node.id}} is required|*Error*: {{node.id}} is required|{{ConfigException}}: {{node.id}} is required|
h1. Compatibility, Deprecation, and Migration Plan
h2. Migration Steps
Users should replace {{broker.id}} with {{node.id}} in their server configuration. If both are configured, simply remove {{broker.id}}.
h1. Test Plan
h2. 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).
h2. 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}}.
h1. Rejected Alternatives
_None._