Status

Current state: Under DiscussionAccepted

Discussion thread: here

JIRA:

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The old consumer based group management allows for explicitly deleting a consumer group. This functionality is missing in the new consumer based group management where offsets are stored in a Kafka internal topic. The only way a consumer group can be removed is when its offsets are expired. The default retention time for committed offsets is currently 1 day (KIP-186 aims to increase that to 7 days). The override for this retention time is through the CommitOffsetOffsetCommit API. Therefore, the offset cache can easily expand for consumer groups that are just waiting for their offsets to expire. With KIP-211 that proposes removing the retention_time field from the CommitOffset OffsetCommit API, the need for an alternate group deletion mechanism is more eminent than ever.

To mimic the tooling behavior of the old consumer group tool, this KIP proposes supporting the --delete switch for Kafka stored group offsets (new consumer) too. The behavior of --delete for the new consumer will be similar to that of the old consumer. For example, if group has active consumers the deletion will fail. In other words, a group can be deleted only when its state is Empty.

Public Interfaces

While existing APIs remain unchanged a new API DeleteGroups is introduced to remove offsets of the given group(s) from the internal offset cache.

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DeleteGroups Request (Version: 0) => [group_id] 
  group_id => STRING
 
DeleteGroups Response (Version: 0) => throttle_time_ms [group_error_codes]
  throttle_time_ms => INT32
  group_error_codes => group_id error_code 
    group_id => STRING
    error_code => INT16

In addition to this new API, we define the necessary operation/resource permission to use the API as Delete/Group. In other words, when SASL authentication is enabled a user needs to have Delete operation access to a group in order to delete it. With this new Delete operation, the Group resource type will have four associated operations: Describe, Read, Delete, All. The All operation implies the other three, and the Read operation implies Describe. There is no imply relationship between Delete and Read or Describe.

Potential error codes returned by the API (at the top level or per-group level or both) include:

• 0: None
• 15: COORDINATOR_NOT_AVAILABLE
• 24: INVALID_GROUP_ID
• 30: GROUP_AUTHORIZATION_FAILED
• ??: NON_EMPTY_GROUP
• ??:GROUP_ID_NOT_FOUND

Note that this KIP proposes two new error codes:

• NON_EMPTY_GROUP: addresses the case where the group is not empty (i.e. group state is not Empty) and cannot be deleted.
• GROUP_ID_NOT_FOUND: addresses the case where the requested group id does not exist.

A --delete switch for Similar to how the consumer group command line tool supports describing a single group at a time, the deletion functionality will be implemented the same way (deleting one group at a time). The protocol, however, supports deleting multiple groups in one requesttool (kafka-consumer-groups.sh) will by supported for Kafka stored group offsets (new consumer) too, which behaves similar to its old consumer counterpart. For example, if group has active consumers the deletion will fail. In other words, a group can be deleted only when its state is Empty.

Compatibility, Deprecation, and Migration Plan

The changes proposed by this KIP apply to the new Java-based consumer only, and are backward compatible because the suggested API does not already exist. New clients that work with older brokers will not be able to make use of this new API.

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1. Using the newly introduced offset reset functionality of the consumer group command tool by injecting a retention_time of 0. : This alternative was rejected in favor of KIP-211, that aims at removing the retention_time field from the OffsetCommit API.
2. Exposing the group deletion functionality in org.apache.kafka.clients.admin.AdminClient: This was left as a future work as no group-related functionality has been implemented in AdminClient yet.
3. Providing the functionality to delete individual offsets: This was also left as a potential future work to keep the initial implementation simple. If, in the future, it turns out that per-partition offset reset is not sufficient and there is a need to delete individual offsets, it can be proposed and implemented in its own KIP.
4. Having a high level error code in the response protocol: This was rejected because one error code cannot normally be generalized to all groups that are requested be deleted. It may very well be the case that a Errors.COORDINATOR_NOT_AVAILABLE error would apply to some groups (groups to be deleted may belong to different coordinators), Errors.GROUP_AUTHORIZATION_FAILED would apply to some other, etc.