Status

Current state: Under Discussion

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

Motivation

This KIP aims to enforce offsets.topic.replication.factor upon __consumer_offsets topic  auto topic creation.

Kafka brokers have a config called offsets.topic.replication.factor that specify the replication factor for the __consumer_offsets topic. The problem is that this config isn't being enforced upon auto topic creation. If an attempt to auto create the internal topic is made when there are fewer brokers than offsets.topic.replication.factor, the topic ends up getting created anyway with the current number of live brokers. The current behavior is pretty surprising when you have clients or tooling running as the cluster is getting setup. Even if your cluster ends up being huge, you'll find out much later that __consumer_offsets was setup with no replication.

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Rationale for the prior behavior can be found in 

Public Interfaces

Set the offsets.topic.replication.factor to 1 in config/server.properties to maintain existing single-broker quickstart behavior. Note that this does not change the default offsets.topic.replication.factor value of 3 in KafkaConfig.

Proposed Changes

Internal topic creation can happen in five paths:

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1. will now fail topic creation of the internal topic with GROUP_COORDINATOR_NOT_AVAILABLE until the offsets.topic.replication.factor requirement is met.
2. will now fail topic creation of the internal topic and retain existing behavior of failing fail topic creation of the internal topic with INVALID_REPLICATION_FACTOR  until until the offsets.topic.replication.factor  requirement requirement is met.
3. will retain existing behavior. AdminUtils compares cluster size vs. replication factor comparison and throws an InvalidReplicationFactorException if the manually specified replication factor isn't met. If the replication factor is met, it creates the topic, ignoring the broker's offsets.topic.replication.factor config.
4. Same as 3.
5. Same as 3. CreateTopicsResponse including an internal topic will return INVALID_REPLICATION_FACTOR if the manually specified replication factor isn't met.
6. is unrelated, as the zookeeper write will not receive any error from kafka.

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This is a bug fix KIP impacting the setup of new clusters. Users setting up a cluster should keep in mind that the __consumer_offsets topic will not be created until their cluster satisfies the offsets.topic.replication.factor.

Rejected Alternatives

One rejected alternative was to push __consumer_offsets topic creation logic out of the brokers and into the KafkaController. Since the KafkaController can detect broker membership changes through zookeeper, it can create the __consumer_offets topic as soon as the offsets.topic.replication.factor is met. While doable, it is more complicated as it would add even more logic to keep track of in the already complicated KafkaController and would additionally require KafkaApis to instead lookup cluster readiness based on its MetadataCache when responding to GroupCoordinatorRequests.