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Status

Current state: [One of "Under Discussion", "Accepted", "Rejected"]"Draft"

Discussion thread: here [Change the link from the KIP proposal email archive to your own email thread]

JIRA: here [Change the link from KAFKA-56391 to your own ticket]

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

Motivation

Describe the problems you are trying to solve.

Public Interfaces

Briefly list any new interfaces that will be introduced as part of this proposal or any existing interfaces that will be removed or changed. The purpose of this section is to concisely call out the public contract that will come along with this feature.

A public interface is any change to the following:

• Binary log format

• The network protocol and api behavior

• Any class in the public packages under clientsConfiguration, especially client configuration

  • org/apache/kafka/common/serialization

  • org/apache/kafka/common

  • org/apache/kafka/common/errors

  • org/apache/kafka/clients/producer

  • org/apache/kafka/clients/consumer (eventually, once stable)

• Monitoring

• Command line tools and arguments

• Anything else that will likely break existing users in some way when they upgrade

Proposed Changes

...

The DescribeGroups protocol v1 currently returns this information for each consumer group:

• error_code
• group_id
• state
• protocol_type
• protocol
• members (only a member summary is returned per member that misses some useful member info too)

Additional info exist or can be derived in a GroupMetadata object on the server side, some of which could be useful if exposed via the DescribeGroups protocol. Here are some examples:

• generationId
• leaderId
• supportedProtocols per member
• subscribed topics per member

Enhancing the protocol with some of this additional info means improving the existing tools that make use of it. For example, using this additional info, the consumer group command's --describe output will provide more information about each consumer group to help with understanding the group behavior, its monitoring, troubleshooting, etc. 

Public Interfaces

This is the latest version (v1) of DescribeGroups protocol:

DescribeGroups Request (Version: 1) => [group_ids] 
  group_ids => STRING
 
DescribeGroups Response (Version: 1) => throttle_time_ms [groups] 
  throttle_time_ms => INT32
  groups => error_code group_id state protocol_type protocol [members] 
    error_code => INT16
    group_id => STRING
    state => STRING
    protocol_type => STRING
    protocol => STRING
    members => member_id client_id client_host member_metadata member_assignment 
      member_id => STRING
      client_id => STRING
      client_host => STRING
      member_metadata => BYTES
      member_assignment => BYTES

The suggestion is to enhance the protocol to version 2 and add some of these currently missing information (* indicates suggested fields to be added to the protocol):

DescribeGroups Request (Version: 2) => [group_ids] 
  group_ids => STRING


DescribeGroups Response (Version: 2) => throttle_time_ms [groups] 
  throttle_time_ms => INT32
  groups => error_code group_id state protocol_type protocol generation_id leader_id new_member_added received_consumer_offset_commits received_transactional_offset_commits [members] 
    error_code => INT16
    group_id => STRING
    state => STRING
    protocol_type => STRING
    protocol => STRING
  * generation_id => INT32
  * leader_id => STRING
    members => member_id client_id client_host member_metadata member_assignment member_subscription [member_protocols]
      member_id => STRING
      client_id => STRING
      client_host => STRING
      member_metadata => BYTES
      member_assignment => BYTES
  *   member_subscription => BYTE
  *   member_protocols => STRING

Proposed Changes

The proposal is to

1. Create version 2 of DescribeGroups protocol to include additional information about the consumer group and also each group member for the API clients.
2. Update the consumer group command to provide the added information in the command output (where applicable). Note that this KIP will improve upon KIP-175 where group state and detailed member information will also be returned by the consumer group command using additional command line options. Therefore, the output of some of the sub-options introduced in KIP-175 will improve to include the newly available information by this KIP. Suggested changes are:
   1. Updating the --state option to also include a GENERATION-ID and LEADER-ID column.
   2. Providing the missing member ASSIGNMENT-STRATEGY value for --members option.
   3. Providing the missing member SUBSCRIPTION value for --members --verbose option.
3. On a related subject but perhaps unrelated to the protocol enhancement, also update the consumer group command to provide filtering by topic as well (similar to how the old consumer offset checker tool supported this filtering).

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Rejected Alternatives

...

Since this KIP follows KIP-175, the output for sub-options --members and --state that are introduced in that KIP will be modified to also include the newly added fields. Therefore, clients who rely on the output for those sub-options may need to be adjusted. It also helps the consumer group --describe command to fill some of the newly added columns introduced by KIP-175 (i.e. ASSIGNMENT-STRATEGY for --members options, and SUBSCRIPTION for --members --verbose option).

Rejected Alternatives

• Providing this additional information in the server logs: It is much more convenient to look up this new pieces of information via the consumer group command tool. Plus, these fields and their underlying concepts are not likely to disappear in the future. So we would not have to worry about making a protocol change that will likely become obsolete in the future.