Status

Current state: Adopted

Discussion thread: here

JIRA: KAFKA-5526 - Getting issue details... STATUS

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Motivation

As far as describing a consumer group goes, the ConsumerGroupCommand currently lists all members of the group along with associated topic partitions and their committed offsets all in one view. For example:

$ bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --describe --group my-group
Note: This will only show information about consumers that use the Java consumer API (non-ZooKeeper-based consumers).
TOPIC                          PARTITION  CURRENT-OFFSET  LOG-END-OFFSET  LAG        CONSUMER-ID                                       HOST                           CLIENT-ID
my-topic                       0          16577           33154           16577      consumer1-81fe8a2a-0b29-4964-a1d4-4741582132bb    /127.0.0.1                     consumer1
my-topic                       1          16578           33155           16577      consumer1-81fe8a2a-0b29-4964-a1d4-4741582132bb    /127.0.0.1                     consumer1
my-topic                       2          16577           33155           16578      consumer2-db6f12f2-e1de-4bb7-93ec-6170fefa8830    /127.0.0.1                     consumer2
my-second-topic                0          33023           44543           11520      consumer7-c1d056fa-eabc-42ed-b557-c848f747ec49    /127.0.0.1                     consumer7
-                              -          -               -               -          consumer8-6078de6a-af63-4c7a-8bad-f03cb3606075    /127.0.0.1                     consumer8
my-other-topic                 1          7901            8652            751        -                                                 -                              -
my-other-topic                 0          7902            8654            752        -                                                 -                              -

The information included in the command output includes:

  • all topic partitions the group is consuming or has consumed from (including those without an active consumer)
  • current offset, log end offset, and the lag for each topic partition in the group
  • all active members of the group (including those without an assigned topic partition)

This KIP suggests breaking down the command to cover some of the information pieces above separately using additional switches. There are a couple of reasons behind this improvement:

  • The above output, with all the missing column values, looks weird and could be difficult to interpret. For a big group, where there are many consumers without assigned partition, the command will output a long list, while the user may only be interested in, for example, active members only.
  • A user is typically interested in one of the information pieces above when issuing the command: either offset information, or members, or topic partitions associated with the group.

As part of this KIP, additional information about the group will also be generated that is currently missing. For example, generation id of the group, or the assignment policy used by each member. 

Public Interfaces

The change required for this includes updating the ConsumerGroupCommand tool:

  1. The output of --describe will change to return one row per topic partition in the group. This means that group members with no assigned partition will not be present in the default output.
  2. A new switch --members will be introduced, that can only be provided along with --describe, to return a list of all active members of the group. For each member the number of assigned topic partitions (without any offset information) and the assignment strategy used by the member are also returned. Additional information (such as member subscription and assignment) can also be provided with a --members --verbose option.
  3. A new switch --offsets will be introduced, that can only be provided along with --describe, that returns the same default output of --describe. I.e., this will be the default switch that can be additionally used with --describe.
  4. A new switch --state will be introduced, that can only be provided along with --describe, to return group-level status information that is returned from the DescribeGroups API call.

Proposed Changes

To describe the proposed changes let us revisit the example above. This KIP proposes to change the --describe option of the ConsumerGroupCommand according to the following:

--describe

This option of the command returns offset and consumer information for each topic partition in the group. This is how the output of the example above would look like:

$ bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --describe --group my-group
Note: This will only show information about consumers that use the Java consumer API (non-ZooKeeper-based consumers).
TOPIC                          PARTITION  CURRENT-OFFSET  LOG-END-OFFSET  LAG        CONSUMER-ID                                       HOST                           CLIENT-ID
my-topic                       0          16577           33154           16577      consumer1-81fe8a2a-0b29-4964-a1d4-4741582132bb    /127.0.0.1                     consumer1
my-topic                       1          16578           33155           16577      consumer1-81fe8a2a-0b29-4964-a1d4-4741582132bb    /127.0.0.1                     consumer1
my-topic                       2          16577           33155           16578      consumer2-db6f12f2-e1de-4bb7-93ec-6170fefa8830    /127.0.0.1                     consumer2
my-second-topic                0          33023           44543           11520      consumer7-c1d056fa-eabc-42ed-b557-c848f747ec49    /127.0.0.1                     consumer7
my-other-topic                 1          7901            8652            751        -                                                 -                              -
my-other-topic                 0          7902            8654            752        -                                                 -                              -

Note that consumers without any assigned partitions will no longer appear in the listing.

--describe --members and --describe --members --verbose

A new option --members is introduced that is allowed with --describe only and returns info about active members of the group along with their assigned topic partitions:

$ bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --describe --group my-group --members
Note: This will only show information about consumers that use the Java consumer API (non-ZooKeeper-based consumers).
CONSUMER-ID                                       HOST                           CLIENT-ID              #PARTITIONS
consumer1-81fe8a2a-0b29-4964-a1d4-4741582132bb    /127.0.0.1                     consumer1              2
consumer2-db6f12f2-e1de-4bb7-93ec-6170fefa8830    /127.0.0.1                     consumer2              1
consumer7-c1d056fa-eabc-42ed-b557-c848f747ec49    /127.0.0.1                     consumer7              1

There is an additional --verbose switch that can be used along with this option to include additional details about each group member:

$ bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --describe --group my-group --members --verbose
Note: This will only show information about consumers that use the Java consumer API (non-ZooKeeper-based consumers).
CONSUMER-ID                                       HOST                           CLIENT-ID              #PARTITIONS     ASSIGNMENT
consumer1-81fe8a2a-0b29-4964-a1d4-4741582132bb    /127.0.0.1                     consumer1              2               my-topic(0,1)
consumer2-db6f12f2-e1de-4bb7-93ec-6170fefa8830    /127.0.0.1                     consumer2              1               my-topic(2)
consumer7-c1d056fa-eabc-42ed-b557-c848f747ec49    /127.0.0.1                     consumer7              1               my-second-topic(0)

–describe --offsets

The new --offsets option, that is allowed with --describe only, produces the same result as the --describe option itself. In other words, --offsets is the default sub-option of --describe:

$ bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --describe --group my-group --offsets
Note: This will only show information about consumers that use the Java consumer API (non-ZooKeeper-based consumers).
TOPIC                          PARTITION  CURRENT-OFFSET  LOG-END-OFFSET  LAG        CONSUMER-ID                                       HOST                           CLIENT-ID
my-topic                       0          16577           33154           16577      consumer1-81fe8a2a-0b29-4964-a1d4-4741582132bb    /127.0.0.1                     consumer1
my-topic                       1          16578           33155           16577      consumer1-81fe8a2a-0b29-4964-a1d4-4741582132bb    /127.0.0.1                     consumer1
my-topic                       2          16577           33155           16578      consumer2-db6f12f2-e1de-4bb7-93ec-6170fefa8830    /127.0.0.1                     consumer2
my-second-topic                0          33023           44543           11520      consumer7-c1d056fa-eabc-42ed-b557-c848f747ec49    /127.0.0.1                     consumer7
my-other-topic                 1          7901            8652            751        -                                                 -                              -
my-other-topic                 0          7902            8654            752        -                                                 -                              -

–describe --state

The --state option, that can also be used with --describe only, reports the group level information that comes back in the response of DescribeGroups API call along with the coordinator id of the group:

$ bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --describe --group my-group --state
Note: This will only show information about consumers that use the Java consumer API (non-ZooKeeper-based consumers).
COORDINATOR (ID)       ASSIGNMENT-STRATEGY     STATE          #MEMBERS
localhost:9092 (0)     range                   Stable         3 

 

In the above outputs, the width of the following columns will be dynamically determined (as opposed to the fixed widths that have been applied so far):

  • TOPIC
  • CONSUMER-ID
  • HOST
  • CLIENT-ID
  • COORDINATOR (ID)

Compatibility, Deprecation, and Migration Plan

The proposed changes apply to the new Java-based consumer only. Therefore, the consumer groups based on the old consumer will be unaffected.

Users who use the new-consumer based consumer groups and somehow rely on the output of the --describe option of the ConsumerGroupCommand may have to adjust their clients to use one of the new options provided; for example, in case they rely on the fact the command currently returns members with no assignment. Hopefully, how they use the command output will be simplified with these new options. 

Rejected Alternatives

 

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