Nick Pillitteri created KAFKA-5896:
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Summary: 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
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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