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Status

Current state: [One of "Under Discussion", "Accepted", "Rejected"]

Discussion thread: here [Change the link from the KIP proposal email archive to your own email thread]

Vote thread: here

JIRA: here [Change the link from KAFKA-1 to your own ticket]

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

Motivation

Describe the problems you are trying to solve.

Public Interfaces

Briefly list any new interfaces that will be introduced as part of this proposal or any existing interfaces that will be removed or changed. The purpose of this section is to concisely call out the public contract that will come along with this feature.

A public interface is any change to the following:

• Binary log format

• The network protocol and api behavior

• Any class in the public packages under clientsConfiguration, especially client configuration

  • org/apache/kafka/common/serialization

  • org/apache/kafka/common

  • org/apache/kafka/common/errors

  • org/apache/kafka/clients/producer

  • org/apache/kafka/clients/consumer (eventually, once stable)

• Monitoring

• Command line tools and arguments

• Anything else that will likely break existing users in some way when they upgrade

Proposed Changes

Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Test Plan

Describe in few sentences how the KIP will be tested. We are mostly interested in system tests (since unit-tests are specific to implementation details). How will we know that the implementation works as expected? How will we know nothing broke?

Rejected Alternatives

KIP-932: Queues for Kafka introduced share groups to Apache Kafka. This KIP introduces additional group configurations to give users more control over share groups. It also formalizes the manner in which group configurations with limits specified by broker configurations are validated and enforced.

Proposed Changes

Some of the configuration for share groups in KIP-932 is specified at the broker level without an equivalent group configuration.

First, it enables group configuration for the maximum number of delivery attempts, so error tolerance can be configured for each group. For records which are currently in-flight when the limit is reduced, a final delivery attempt which exceeds the reduced limit may be made. The delivery count limit is not bounds checked at the start of the delivery attempt because fetching is not concerned with archiving records. It's a boundary case that only occurs when the limit decreases and messages are already in-flight. When a delivery attempt completes, we check whether the number of deliveries has reached or exceeded the limit, and archive the records accordingly. If the limit was decreased, it's possible that the limit has been exceeded in which case a final delivery attempt is permitted. If the limit was increased, it's possible that a record was nearing its final attempt but now it has more delivery attempts available. The throttling that occurs as the final attempts are made might have been applied previously, but can be relaxed to match the new limit.

Second, it enables group configuration for the record lock limit per share-partition, so the balance between resource consumption and concurrency can be configured for each group. The defaults for the equivalent broker configurations were chosen to be suitable for most situations, but group overrides give administrators more control if they desire. In a similar way, if the limit is decreased, the number of record locks will gradually decrease to fit within the new limit as record delivery completes.

Finally, KIP-1222: Acquisition lock timeout renewal in share consumer explicit mode introduced renewal of acquisition locks. This enables processing of records to be extended indefinitely by repeatedly renewing the acquisition locks. A new group configuration is added to control whether this behavior is permitted for the group. If the value of the configuration is set to false, a renew acknowledgement will be rejected with error code INVALID_RECORD_STATE  and an error message which explains that renewing acquisition locks is not enabled for the group.

Validation and enforcement of group configurations

Group configurations have been introduced in several KIPs (848, 932, 1071) in which broker configurations specify the permitted limits of the group configurations. For example, the group configuration consumer.session.timeout.ms  is not allowed to be less than the broker configuration group.consumer.min.session.timeout.ms  or greater than group.consumer.max.session.timeout.ms . When the group configuration is updated, it is simple to validate it against the broker configuration limits. However, if a broker is restarted and the broker configuration has been changed, the group configurations might now fall outside the permitted limits. Prior to this KIP, in some cases, the broker will not start because the group configuration validations fail. In others, the group configurations are only checked when they are being updated. This KIP also resolves these inconsistencies.

• When a group configuration is updated, the value is validated against the broker configuration limits. If the validation fails, the request is rejected. (No change)
• When a broker configuration is updated, the existing group configurations are not validated. This ensures that the administrator is able to tighten or relax limits easily.
• When a broker starts, if a group configuration lies outside the broker configuration limits, the broker is permitted to start.
• When a group configuration is evaluated, the effective value of a group configuration is capped by the broker limits.

Public Interfaces

Configuration

Broker configuration

New configurations are added to specify the upper and lower bounds of the associated group configurations.

Group configuration

Compatibility, Deprecation, and Migration Plan

The changes in this KIP are limited to adding a few new configurations to control existing behavior. There should be no compatibility concerns.

Test Plan

The feature will be thoroughly tested with unit and integration tests.

Rejected Alternatives

None so farIf there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.