Status

Current state: "Under Discussion" 

Discussion thread: here 

JIRA: here 

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

Motivation

Currently Mirrormaker uses deprecated AlterConfigs API when syncing topic configurations for broker compatibility to 0.11.0. In addition, Mirromaker has a configurable ConfigPropertyFilter  class to select the topic configuration properties to replicate and uses org.apache.kafka.connect.mirror.DefaultConfigPropertyFilter by default. The default class excludes certain topic configurations from replication such as follower.replication.throttled.replicas and leader.replication.throttled.replicas. 

However, the AlterConfigs API replaces any existing configuration with the new configuration. Due to this, additional configurations set on a remote topic get cleared up when MirrorMaker syncs topic configurations whether they are excluded from replication or not. For example this prevents running Cruise Control on the target cluster as it may set follower.replication.throttled.replicas and leader.replication.throttled.replicas.

MirrorMaker should not clear topic configurations that are excluded from replication.

In addition, source and target topic configurations could have different default values. Currently, Mirrormaker overwrites configurations on target topic even if the configurations on the source topic have not been changed but happened to have different default values. However, user may only want to sync configurations that they have changed but leave the ones with default values. 

Public Interfaces

This KIP proposes to migrate the IncrementalAlterConfigs API for syncing topic configurations in MirrorMaker. The IncrementalAlterConfigs API has been around for several years since it was introduced in Kafka 2.3.0 and addresses the shortcoming of the AlterConfigs API. In order not to break the compatibility, we will introduce a new setting to MirrorMaker to give the option to enable and disable IncrementalAlterConfigs API for syncing topic configurations. This new setting is expected to serve as a temporary measure until the next major release when the API is always used.

It also proposes adding the ability to use the target cluster's default for target topic configurations, rather than using the source cluster's default. For instance, users would be able to configure ConfigPropertyFilter to check if the configuration on the source topic is the default and decide whether or not to replicate it to the target topic. 

Proposed Changes

1) Extend ConfigPropertyFilter  class to have a new method called shouldReplicateSourceDefault which would always return true by default. User would be able to configure this method to return false for a configuration that is DEFAULT_CONFIG.

2) Add a new configuration setting to MirrorMaker:

• Name: use.incremental.alter.configs

• Description: Deprecated. Whether to automatically use the IncrementalAlterConfigs API for syncing topic configurations. This configuration will be removed in Kafka 4.0 and the IncrementalAlterConfigs API will be used by default. Users should make sure that the target cluster is running at least Kafka 2.3.0 or later. 

• Type: String

• Default: requested

• When set to "requested", MirrorMaker will use the IncrementalAlterConfigs for syncing topic configurations. If it receives an error from an incompatible broker, will use the deprecated AlterConfigs API instead and log a WARN message e.g. "The target cluster <ALIAS> is not compatible with IncrementalAlterConfigs API. Therefore using deprecated AlterConfigs API for syncing topic configurations."

• When explicitly set to "never", MirrorMaker will use the deprecated AlterConfigs API for syncing topic configurations.
• When explicitly set to "required", MirrorMaker will use the IncrementalAlterConfigs API for syncing topic configurations. If it receives an error from an incompatible broker, the MirrorMaker will report this to the user and fail the connector.

3) From Kafka 4.0, MirrorMaker will use IncrementalAlterConfigs API for syncing topic configuration. 

Compatibility, Deprecation, and Migration Plan

By default, the new setting will be set to "required"  which means we'll use IncrementalAlterConfigs if possible, and fallback to AlterConfigs API if incompatible. This means there will be a behaviour change while syncing topic configurations in MM2 after this KIP. For example, if changing retentions.ms of a topic in the source cluster, filtered configurations such as follower.replication.throttled.replicas of the topic currently get cleared in the target cluster. After this KIP, the current value of follower.replication.throttled.replicas would be kept in the target cluster unless the target broker's version is older than Kafka 2.3.0. 

This setting will be marked as deprecated immediately. 

In the next major version:

• use.incremental.alter.configs will be removed.
• MirrorMaker will use the IncrementalAlterConfigs API to sync topic configurations. 

Users running the latest MirrorMaker connector against a target cluster with brokers older than Kafka 2.3.0, will have to upgrade.

Test Plan

Currently, the integration tests for MirrorMaker do not check if the target topic has the expected configurations after applying configuration filters, hence this bug was not caught. New integration tests that checks target topic configurations once they have been synced and tests different settings of the new configuration will be added. 

Rejected Alternatives

• We could have changed the current updateConfigs() method to call DescribeConfigs API before AlterConfigs so that it had an incremental mode. This would have avoided the migration, however, the topics configurations could change between DescribeConfigs and AlterConfigs calls so we could still end up syncing incorrect topic configurations. We would need to stop using the deprecated API at some point anyway.