[jira] [Commented] (KAFKA-5896) Kafka Connect task threads never interrupted

["Randall Hauch (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KAFKA-5896?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16358670#comment-16358670 ] 

Randall Hauch commented on KAFKA-5896:
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[~ewencp], do you have any thoughts on this? I know in the past you've talked about not wanting to do this since some developers won't properly implement the interruption. I agree that not everyone implements it correctly, but we could at least _try_ to cancel the tasks. 

> Kafka Connect task threads never interrupted
> --------------------------------------------
>
>                 Key: KAFKA-5896
>                 URL: https://issues.apache.org/jira/browse/KAFKA-5896
>             Project: Kafka
>          Issue Type: Bug
>          Components: KafkaConnect
>            Reporter: Nick Pillitteri
>            Assignee: Nick Pillitteri
>            Priority: Minor
>
> h2. Problem
> Kafka Connect tasks associated with connectors are run in their own threads. When tasks are stopped or restarted, a flag is set - {{stopping}} - to indicate the task should stop processing records. However, if the thread the task is running in is blocked (waiting for a lock or performing I/O) it's possible the task will never stop.
> I've created a connector specifically to demonstrate this issue (along with some more detailed instructions for reproducing the issue): https://github.com/smarter-travel-media/hang-connector
> I believe this is an issue because it means that a single badly behaved connector (any connector that does I/O without timeouts) can cause the Kafka Connect worker to get into a state where the only solution is to restart the JVM.
> I think, but couldn't reproduce, that this is the cause of this problem on Stack Overflow: https://stackoverflow.com/questions/43802156/inconsistent-connector-state-connectexception-task-already-exists-in-this-work
> h2. Expected Result
> I would expect the Worker to eventually interrupt the thread that the task is running in. In the past across various other libraries, this is what I've seen done when a thread needs to be forcibly stopped.
> h2. Actual Result
> In actuality, the Worker sets a {{stopping}} flag and lets the thread run indefinitely. It uses a timeout while waiting for the task to stop but after this timeout has expired it simply sets a {{cancelled}} flag. This means that every time a task is restarted, a new thread running the task will be created. Thus a task may end up with multiple instances all running in their own threads when there's only supposed to be a single thread.
> h2. Steps to Reproduce
> The problem can be replicated by using the connector available here: https://github.com/smarter-travel-media/hang-connector
> Apologies for how involved the steps are.
> I've created a patch that forcibly interrupts threads after they fail to gracefully shutdown here: https://github.com/smarter-travel-media/kafka/commit/295c747a9fd82ee8b30556c89c31e0bfcce5a2c5
> I've confirmed that this fixes the issue. I can add some unit tests and submit a PR if people agree that this is a bug and interrupting threads is the right fix.
> Thanks!



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