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• What impact (if any) will there be on existing users?

Users who use of Kafka clusters with Zookeeper clients older than 3.5.x won't be able to communicate with a Zookeeper cluster using 3.8.1 Zookeeper cluster. As mentioned in the accompanying JIRA ticket Apache Kafka has been using Zookeeper clients 3.5.x since version 2.4 so versions above and including it should be safe for this upgrade. It is acceptable to break compatibility with Apache Kafka versions prior to 2.4 as they are considered beyond their end of life and are not maintained. (source: Time Based Release Plan#WhatIsOurEOLPolicy).

A Zookeeper cluster is separate from a Kafka cluster.

For example, a user wanting to migrate from a Kafka cluster with version 2.3 and a Zookeeper cluster with version 3.4 to a Kafka cluster with version 3.X (where X is the Kafka version containing this KIP) and Zookeeper cluster with version 3.8.1 would first have to upgrade their Kafka cluster to 3.4.0 (latest version of the time of writing), then update their Zookeeper cluster to 3.8.1 and lastly update their Kafka cluster again from 3.4.0 to 3.X.

As another example, a user wanting to migrate from a Kafka cluster with version 2.4 and a Zookeeper cluster with version 3.5 to a Kafka cluster with version 3.X and Zookeeper cluster with version 3.8.1 could upgrade their Kafka cluster to 3.X and then their Zookeeper cluster to 3.8.1 or vice versa.

Below is an analysis of changes introduced in 3.7.0 and 3.8.0 and whether they will introduce breaking changes in Kafka or not.

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