Status

Current state:  Accepted

Discussion thread: here

Vote thread:  here

JIRA: KAFKA-19784

PR: https://github.com/apache/kafka/pull/20691

Motivation

Apache Kafka supports rack-aware partition assignment to improve fault tolerance and reduce cross-rack network traffic/cost. However, the current Admin API does not expose rack information for all type of consumer group members, despite this information being available at the protocol level.

Current State

The ConsumerGroupDescribeResponse (API Key 69) protocol includes a rackId field for each group member, as defined in the protocol specification:

{ "name": "RackId", "type": "string", "versions": "0+", 
  "nullableVersions": "0+", "default": "null",
  "about": "The member rack ID." }

However, when users call AdminClient.describeConsumerGroups(), the returned MemberDescription objects do not include this rack information. The rack ID is available in the wire protocol but is discarded during response processing in DescribeConsumerGroupsHandler.

The Similar case is for AdminClient.describeShareGroups(). The current status can be summarized as follows:

Problem Statement

This limitation creates several issues:

1. Monitoring and Observability: Operators cannot determine the rack distribution of consumer group members through the Admin API, making it difficult to verify that rack-aware assignment is working correctly.

2. Inconsistency: Other group types expose rack information:

   • StreamsGroupMemberDescription includes rackId() method
   • The underlying protocol supports it for consumer groups
   • Only the public Admin API omits this information
3. Diagnostics: When troubleshooting rack-aware assignment issues or network problems, operators need to use lower-level tools or custom code to access rack information that should be readily available.
4. Third-party Tools: Monitoring and management tools built on the Admin API cannot display rack information, limiting their usefulness for rack-aware deployments.

Take one example:  currently we have to implemen our AZ/Rack analysis using a workaround — passing the rack information into the clientId field and parsing it afterward.

kafkaConsumerConfig.customConfig(ConsumerConfig.CLIENT_ID_CONFIG, generateClientIdWithRack(ip, rack));

Public Interfaces

We need to do tiny change for the existed public class:
1. Add rack ID support to org.apache.kafka.clients.admin.MemberDescription:

public class MemberDescription {
    private final String memberId;
    private final Optional<String> groupInstanceId;
    private final Optional<String> rackId;  // new field
    private final String clientId;
    //omit other codes        
    public Optional<String> rackId() {  // new method
       return rackId;
    }
    //omit other codes}

2.  Add rack ID support to org.apache.kafka.clients.admin.ShareMemberDescription:

public class ShareMemberDescription{     
    private final String memberId;
    private final Optional<String> rackId; // new field
    private final String clientId;
    //omit other codes        
    public Optional<String> rackId() {  // new method
       return rackId;
    }
    //omit other codes
}

Proposed Changes

 You can refer to https://github.com/apache/kafka/pull/20691:

•  add rackId into MemberDescription and ShareMemberDescription
•  pass through rack ID from protocol response

Compatibility, Deprecation, and Migration Plan 

Backward Compatibility

This change is fully backward compatible:

1. Binary Compatibility:

   • The old constructor remains available (marked as @Deprecated)
   • Existing code will continue to compile and run
   • The new field is an Optional, defaulting to empty()

2. Source Compatibility:

   • Existing code using the old constructor continues to work
   • No changes required to existing applications

3. Behavioral Compatibility:

   • Existing behavior is unchanged
   • Only adds new information when available

Migration Path

For users upgrading:

• No action required
• Rack information automatically available when using Admin API
• Access via memberDescription.rackId() when needed

For developers using MemberDescription:

• Old constructor still works
• Recommended to migrate to new constructor over time
• Old constructor may be removed in a future major version (e.g., Kafka 5.0)

Deprecation Plan

• Mark old constructor as @Deprecated immediately
• Keep it available for at least 2 major releases and then remove them.

Test Plan

We can use follow test to cover the change:

Unit Tests

• Test equality with and without rack ID
• Test rack ID is correctly extracted from DescribeResponse

Integration/System Tests

• Behavior with mixed rack configurations
• Behavior with classic and new protocol groups
  
  

Rejected Alternatives

Alternative 1: Create a New API
Proposal: Create describeConsumerGroupsWithRack() method
Rejection Reason:

• Unnecessary API proliferation
• The information already exists in the protocol
• Extending existing API is more intuitive
• Similar precedent: KIP-345 added groupInstanceId to existing MemberDescription