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https://issues.apache.org/jira/browse/KAFKA-15767?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17911495#comment-17911495
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David Jacot commented on KAFKA-15767:
-------------------------------------
Removed the fix version as this is not planned for 4.0.
> Redesign TransactionManager to avoid use of ThreadLocal
> -------------------------------------------------------
>
> Key: KAFKA-15767
> URL: https://issues.apache.org/jira/browse/KAFKA-15767
> Project: Kafka
> Issue Type: Improvement
> Components: clients, producer
> Affects Versions: 3.6.0
> Reporter: Kirk True
> Assignee: Kirk True
> Priority: Minor
> Labels: transactions
>
> A {{TransactionManager}} instance is created by the {{KafkaProducer}} and
> shared with the {{Sender}} thread. The {{TransactionManager}} has internal
> states through which it transitions as part of its initialization,
> transaction management, shutdown, etc. It contains logic to ensure that those
> state transitions are valid, such that when an invalid transition is
> attempted, it is handled appropriately.
> The issue is, the definition of "handled appropriately" depends on which
> thread is making the API call that is attempting an invalid transition. The
> application thread expects that the invalid transition will generate an
> exception. However, the sender thread expects that the invalid transition
> will "poison" the entire {{TransactionManager}} instance.
> So as part of the implementation of KAFKA-14831, we needed a way to change
> logic in the {{TransactionManager}} on a per-thread basis, so a
> {{ThreadLocal}} instance variable was added to the {{TransactionManager}} to
> keep track of this. However, the use of ThreadLocal instance variables is
> generally discouraged because of their tendency for memory leaks, shared
> state across multiple threads, and not working with virtual threads.
> The initial implementation attempt of KAFKA-14831 used a context object,
> passed in to each method, to affect the logic. However, the number of methods
> that needed to be changed to accept this new context object grew until most
> of the methods in {{TransactionManager}} needed to be updated. Thus all the
> affected call sites needed to be updated, resulting in a much larger change
> than anticipated.
> The task here is to remove the use of the {{ThreadLocal}} instance variable,
> let the application thread and {{Sender}} thread keep their desired behavior,
> but keep the change to a minimum.
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