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

Motivation

KAFKA-3853 asks for an improvement to the new-consumer option of the consumer group command. This command, when passed a consumer group that has no consumer (i.e., when the group state is Empty), currently reports an error indicating that there is no active member:

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The requested improvement is returning offsets within the group (and leaving the owner consumer column empty) instead of returning the error message above. The error message can still be printed to to stderr as a warning.

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As shown above, and as explained in the previous section, corresponding topic partitions can be extracted only through the individual (and active) members of the group. The list members is empty if the group is in Empty state, so with this protocol there is no way to extract offsets of an Empty group (problem 1).

Also, offset information for a Stable group is not stored in the above Response schema. Offsets associated with a consumer Stable consumer group are currently extracted using the committed method of KafkaConsumer and by creating a new and dummy member in the group for the sole purpose of making committed calls passing  calls on every topic partition of each of the other consumers in the group to extract the corresponding offsetoffsets. A cleaner approach would be extracting these offsets without having to create this dummy member (problem 2).

Proposed below is version 1 of the DescribeGroups schema to solve that solves both problems above by including the group offset information directly into the Response schema (lines 17-21 below). Note that topic partitions reported under the offsets field are going to be a superset of topic partitions reported under members:member_assignment.

DescribeGroups Request (Version: 1) => [group_ids] 
  group_ids => STRING
 
DescribeGroups Response (Version: 1) => [groups] 
  groups => error_code group_id state protocol_type protocol [members] [offsets]
    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
    offsets => topic partition offset metadata
      topic => STRING
      partition => INT32
      offset => INT64
      metadata => STRING

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There should be no impact to existing users of this protocol. The Request schema remains unchanged, and the Response schema is only expanded. There is no change in the existing Response structure, and, therefore, users who use version 0 of the schema should not be affected. The code behind kafka-consumer-groups.sh will also be updated (as requested in the JIRA) to return more helpful informationadditional information (rows of topic partitions with no assigned consumer). In case it is necessary to provide the old output as well, a new parameter can be defined for the tool to report the new information; while the old way of running the tool still prints out the old output.

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