Status

Current state: Under Discussion

Discussion thread: here

JIRA: Unable to render Jira issues macro, execution error.

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

Motivation

As explained in the corresponding JIRA, the consumer behavior is not consistent when it comes to using an empty ("") group id (which is the default):

• If the consumer subscribes to a topic using this group id it will get an invalid group id error from the broker during a poll (JoinGroupRequest fails group validation if group id is ""):
  Exception in thread "main" org.apache.kafka.common.errors.InvalidGroupIdException: The configured groupId is invalid
• For a stand-alone consumer, that assigns partitions to itself, using the default group id ("") does not cause an error. It can fetch last committed offset, seek to a partition offset, fetch messages, and commit offsets without having to worry about the empty string group id.

This has been summarized in table below.

This can be confusing to users as they would likely expect the consumer treat the default group id the same way in these two cases. Using the default group id of "" is troublesome, and consumers are not normally expected to do that in a production setting. While in the long term the usage of "" as group id should be disallowed, this KIP suggests some improvements to how this group id works.

Public Interfaces

• KafkaConsumer semantics will be slightly modified for the stand-alone consumer to disallow fetching or committing offsets for the empty group id (""). If there is no current offset, consumer will use the auto.offset.reset configuration to determine the next offset to fetch from.
• The default group id will also change to null so that consumers that want to use an empty ("") group id for coordinator-free consumption would have to explicitly specify that group id.

Proposed Changes

This is a detailed list of improvements proposed by this KIP:

1. Changing the default group id to null 
2. A new OffsetCommit API version in which, committing offsets for group id "" leads to an error. 

Change Default group.id to null

The default value of consumer config group.id will be changed from "" to null. This means that if standalone consumers want to keep using the group id "" they would have to explicitly provide the config (e.g. props.put(ConsumerConfig.GROUP_ID_CONFIG, "");).

This is how these two group ids will work

OffsetCommit API Enhancement

The current OffsetCommit protocol looks like this.

OffsetCommit Request (Version: 3) => group_id generation_id member_id retention_time [topics] 
  group_id => STRING
  generation_id => INT32
  member_id => STRING
  retention_time => INT64
  topics => topic [partitions] 
    topic => STRING
    partitions => partition offset metadata 
      partition => INT32
      offset => INT64
      metadata => NULLABLE_STRING


OffsetCommit Response (Version: 3) => throttle_time_ms [responses] 
  throttle_time_ms => INT32
  responses => topic [partition_responses] 
    topic => STRING
    partition_responses => partition error_code 
      partition => INT32
      error_code => INT16

The new version of the protocol is the same, with an additional constraint that committing offsets when group_id is "", results in a new type of error: ILLEGAL_OFFSET_COMMIT.

OffsetCommit Request (Version: 4) => group_id generation_id member_id retention_time [topics] 
  group_id => STRING
  generation_id => INT32
  member_id => STRING
  retention_time => INT64
  topics => topic [partitions] 
    topic => STRING
    partitions => partition offset metadata 
      partition => INT32
      offset => INT64
      metadata => NULLABLE_STRING


OffsetCommit Response (Version: 4) => throttle_time_ms [responses] 
  throttle_time_ms => INT32
  responses => topic [partition_responses] 
    topic => STRING
    partition_responses => partition error_code 
      partition => INT32
      error_code => INT16

    ...
    ILLEGAL_OFFSET_COMMIT(73, "The standalone consumer belonging to an empty (\"\") or null group id is not allowed to commit offsets",
        new ApiExceptionBuilder() {
            @Override
            public ApiException build(String message) {
                return new IllegalOffsetCommitException(message);
            }
    }
    ...

Consumers with the default group id ("") that use assign(...) to consume from hand-picked partitions will not be able to commit offsets. If they attempt an offset commit they will receive an error. A new error will be introduced in org.apache.kafka.common.protocol.Errors for this purpose.

Stand-alone consumers using the default group id will have to seek to an offset before start fetching; or the auto.offset.commit value will be used to determine the starting point of fetch. They will no longer be able to start from a previously committed fetch position.

Compatibility, Deprecation, and Migration Plan

The empty group id will be deprecated and its support will be removed in the next major release. For now, a server-side warning will be logged to indicate this deprecation.

Stand-alone consumers that used the default group id for fetching messages and committing offsets would no longer be able to do that. This should be acceptable because, in general, consumers are not expected to use the default group id for offset commit, which is a dangerous and troublesome practice. This KIP aims at closing the existing gap in using the default group id. Therefore, it is advised that these consumers start using valid group names for committing offsets.

Rejected Alternatives

• Keeping "" as the default group id: The reason for not choosing this option is that using the empty group id could be unknowingly done at the moment (because of it being the default), and since it is not a recommended practice and could lead to issues, it will no longer be the default. Anyone who wants to keep using it should explicitly specify it and be aware if the restrictions set around it by this KIP.