Status

Current state: Under DiscussionAdopted

Discussion thread: here

JIRA:

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

Motivation

Traditionally, committed offsets in Kafka were expired based on a configurable retention time. In KIP-211, retention semantics were changed to take into account group status. Basically as long as a group is still active, no offsets would be expired. This addressed the problem of losing committed offsets for low-volume partitions which rarely have new data, but it introduced two new problems:

...

The impact of these problems is that it is difficult to rely on lag monitoring using committed offsets. If a consumer is not subscribed to a topic, the lag will just grow. Also, it prevents the broker from cleaning up unused state.

Public Interfaces

Request/Response

We will add a new API to delete committed offsets.

...

{
  "apiKey": X,
  "type": "response",
  "name": "OffsetDeleteResponse",
  "validVersions": "0",
  "fields": [    { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "about": "The top-level error code, or 0 if there was no error." },
    { "name": "ThrottleTimeMs", "type": "int32", "versions": "0+", "ignorable": true,
      "about": "The duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "Topics", "type": "[]OffsetDeleteResponseTopic", "versions": "0+",
      "about": "The responses for each topic.", "fields": [
      { "name": "Name", "type": "string", "versions": "0+",
        "about": "The topic name." },
      { "name": "Partitions", "type": "[]OffsetDeleteResponsePartition", "versions": "0+",
        "about": "The responses for each partition in the topic.",  "fields": [
        { "name": "PartitionIndex", "type": "int32", "versions": "0+",
          "about": "The partition index." },
        { "name": "ErrorCode", "type": "int16", "versions": "0+",
          "about": "The error code, or 0 if there was no error." }
      ]}
    ]}
  ]
}

The error sent back to the client when a group is actively subscribed to the topic is documented bellow:    

public enum Errors {
	...
	GROUP_SUBSCRIBED_TO_TOPIC(86, "The consumer group is actively subscribed to the topic", GroupSubscribedToTopicException::new);
	...
}

public class GroupSubscribedToTopicException extends ApiException {
    public GroupSubscribedToTopicException(String message) {
        super(message);
    }
}

Admin Client

This capability will be exposed through the admin client in the following API:

interface Admin {

  /** 
   * Delete committed offsets for a set of partitions in a consumer group. This will
   * succeed at the partition level only if the group is not actively subscribed
   * to the corresponding topic.
   */
  DeleteConsumerGroupOffsetsResult deleteConsumerGroupOffsets(
     String groupId, 
     Set<TopicPartition> partitions, 
     DeleteConsumerGroupOffsetsOptions options) 
}

@InterfaceStability.Evolving
public class DeleteConsumerGroupOffsetsResult {
  public KafkaFuture<Void> partitionResult(TopicPartition partition);
  public KafkaFuture<Void> all();
}

@InterfaceStability.Evolving
public class DeleteConsumerGroupOffsetsOptions extends AbstractOptions<DeleteConsumerGroupOffsetsOptions> {
}
    

Consumer Group Delete Offset options

Main action

--delete-offsets

This action should be executed independently from other Consumer Group actions.

Required Arguments

Execution

Group Error

$ kafka-consumer-groups.sh --bootstrap-server localhost:9092 --delete-offsets --group unknown --topic test:0,1 --topic foo

Error: Deletion of offsets failed due to: <error message>

Topic/Partition

$ kafka-consumer-groups.sh --bootstrap-server localhost:9092 --delete-offsets --group group1 --topic test:0,1 --topic foo

TOPIC                          PARTITION       STATUS
foo                            Not Provided    Error: <error message>
test                           0               Error: <error message>
test                           1               Error: <error message>

Metrics

We are also taking this opportunity to address a gap in metrics reporting. Currently we do not have any visibility about the rate of writes to the internal __consumer_offsets topic. With this KIP, there are four cases to distinguish, so we will add one meter for each.

• `kafka.server:type=group-coordinator-metrics,name=offset-commits`: Marked for every committed offset. (Note this is different than the offset commit request rate which only gives visibility at the request level)
• `kafka.server:type=group-coordinator-metrics,name=offset-expirations`: Marked for every expired offset
• `kafka.server:type=group-coordinator-metrics,name=offset-deletions`: Marked for every administrative deletion
• `kafka.server:type=group-coordinator-metrics,name=group-completed-rebalances`: Marked every time a rebalance completes (which causes a group metadata entry to be appended to the log).

Proposed Changes

To fix the problem described in the motivation, we need to make the group coordinator aware of consumer subscription semantics. The rebalance protocol was designed to be generic so that it could handle use cases beyond the consumer. For example, it is also used by Kafka Connect. Different group implementations are distinguished by a "ProtocolType" field in the JoinGroup request. For the consumer, the protocol type is "consumer." The group coordinator only allows clients of one protocol type to exist in a group.

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The risk of letting the coordinator parse this schema is that it will be harder to make changes to it in the future. For example, if we want to add a new field, we will have to be careful to upgrade the broker before the clients which which depend on the new field. To address this problem, we propose to let the coordinator parse this data as version 0 only. Effectively it will ignore the value of the version field and just look for the array of subscribed topics. Any trailing bytes will be ignored. Future protocol changes will have to preserve the topic list.

...

Deletion Semantics: Any offset which is eligible for expiration may be deleted even if the group is still active. Usually we do not allow committed offset changes while a group is active because we do not have a mechanism to notify the group of the change. However, offsets which are awaiting expiration do not have this problem because they are not being actively consumed. Hence the deletion can be immediate even when the group is activedone immediately.

Compatibility, Deprecation, and Migration Plan

This is a backwards compatible change. The major impact is how it affects the future evolution of the consumer group protocol. This is documented above.

Rejected Alternatives

None yetWe have considered expiring offsets immediately when the group stops subscribing. The risk is for dynamic groups which have subscriptions changing. If the only member subscribing to a specific topic falls out of the group, then offsets would be lost. Additionally, we wanted to make the behavior consistent with empty group expiration. From an offset expiration perspective, we can treat an empty group as a case where of the subscription is empty, which makes all offsets subject to expiration.