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https://issues.apache.org/jira/browse/KAFKA-19148?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
 ]

Julian Bergner updated KAFKA-19148:
-----------------------------------
    Description: 
*Issue Summary:*

We're observing unclean leader election even though 
{{{}unclean.leader.election.enable=false{}}}.

*Scenario:*

During a partition reassignment, if we promote a non-ISR broker to leader and 
simultaneously remove the current leader from the ISR, Kafka still elects a new 
leader from outside the ISR. This behavior contradicts the expected behavior 
when unclean leader election is explicitly disabled.

*Details:*

*Original ISR:* [1, 2]
 * *New ISR after reassignment:* [3, 2]

 * *Kafka Versions Tested:*

 * 
 ** Kafka 4.0.0 (KRaft mode)
 ** Kafka 3.9.0 (Kraft mode)
 ** Kafka 3.9.0 (Zookeeper mode)

 * *Observation:*

 * 
 ** The behavior differs between the two modes.

 * 
 ** In Kraft, unclean leader election occurred, which should not happen with 
the config set to {{{}false{}}}.

 * 
 ** In Zookeeper no unclean leader election occurred.

*Attachments:*
Docker Compose files and reproduction steps for both:
 * Kafka 4.0.0 (KRaft)

 * Kafka 3.9.0 (Zookeeper)

  was:
*Issue Summary:*

We're observing unclean leader election even though 
{{{}unclean.leader.election.enable=false{}}}.

*Scenario:*

During a partition reassignment, if we promote a non-ISR broker to leader and 
simultaneously remove the current leader from the ISR, Kafka still elects a new 
leader from outside the ISR. This behavior contradicts the expected behavior 
when unclean leader election is explicitly disabled.

*Details:*

*Original ISR:* [1, 2]
 * *New ISR after reassignment:* [3, 2]

 * *Kafka Versions Tested:*

 * 
 ** Kafka 4.0.0 (KRaft mode)

 * 
 ** Kafka 3.9.0 (Kraft mode)
 ** Kafka 3.9.0 (Zookeeper mode)

 * *Observation:*

 * 
 ** The behavior differs between the two modes.

 * 
 ** In Kraft, unclean leader election occurred, which should not happen with 
the config set to {{{}false{}}}.

 * 
 ** In Zookeeper no unclean leader election occurred.

*Attachments:*
Docker Compose files and reproduction steps for both:
 * Kafka 4.0.0 (KRaft)

 * Kafka 3.9.0 (Zookeeper)


> Potential Unclean Leader Election in KRaft Despite 
> unclean.leader.election.enable=false
> ---------------------------------------------------------------------------------------
>
>                 Key: KAFKA-19148
>                 URL: https://issues.apache.org/jira/browse/KAFKA-19148
>             Project: Kafka
>          Issue Type: Bug
>          Components: controller
>    Affects Versions: 4.0.0, 3.9.0
>            Reporter: Julian Bergner
>            Priority: Critical
>         Attachments: Readme_Kraft.md, Readme_Zookeeper.md, 
> docker-compose_kraft.yml, docker-compose_zookeeper.yml
>
>
> *Issue Summary:*
> We're observing unclean leader election even though 
> {{{}unclean.leader.election.enable=false{}}}.
> *Scenario:*
> During a partition reassignment, if we promote a non-ISR broker to leader and 
> simultaneously remove the current leader from the ISR, Kafka still elects a 
> new leader from outside the ISR. This behavior contradicts the expected 
> behavior when unclean leader election is explicitly disabled.
> *Details:*
> *Original ISR:* [1, 2]
>  * *New ISR after reassignment:* [3, 2]
>  * *Kafka Versions Tested:*
>  * 
>  ** Kafka 4.0.0 (KRaft mode)
>  ** Kafka 3.9.0 (Kraft mode)
>  ** Kafka 3.9.0 (Zookeeper mode)
>  * *Observation:*
>  * 
>  ** The behavior differs between the two modes.
>  * 
>  ** In Kraft, unclean leader election occurred, which should not happen with 
> the config set to {{{}false{}}}.
>  * 
>  ** In Zookeeper no unclean leader election occurred.
> *Attachments:*
> Docker Compose files and reproduction steps for both:
>  * Kafka 4.0.0 (KRaft)
>  * Kafka 3.9.0 (Zookeeper)



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