...

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

There are additional info 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. 

...

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

...

Compatibility, Deprecation, and Migration Plan

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

There are other information available for a consumer group that could be included in the KIP. For example:

...

Instead of adding this information to the protocol (and having to explain to users what it means and also having to continue exposing it in the future even if implementation details change) we could report them as log messages them (when necessary) on the broker side.