Denis Washington created KAFKA-8635:
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Summary: Unnecessary wait when looking up coordinator before
transactional request
Key: KAFKA-8635
URL: https://issues.apache.org/jira/browse/KAFKA-8635
Project: Kafka
Issue Type: Bug
Components: clients
Affects Versions: 2.2.1, 2.3.0
Reporter: Denis Washington
In our Kafka Streams applications (with EOS enabled), we were seeing mysterious
long delays between records being produced by a stream task and the same
records being consumed by the next task. These delays turned out to always be
around {{retry.backoff.ms}} long; reducing that value reduced the delays by
about the same amount.
After digging further, I pinned down the problem to the following lines in
{{org.apache.kafka.clients.producer.internals.Sender#runOnce}}:
{{} else if (transactionManager.hasInFlightTransactionalRequest() ||
maybeSendTransactionalRequest()) {}}
{{ // as long as there are outstanding transactional requests, we simply
wait for them to return}}
{{ client.poll(retryBackoffMs, time.milliseconds());}}}}}}
{{ return;}}
{{ }}}
This code seems to assume that, if {{maybeSendTransactionalRequest}} returns
true, a transactional request has been sent out that should be waited for.
However, this is not true if the request requires a coordinator lookup:
{{if (nextRequestHandler.needsCoordinator()) {}}
{{ targetNode =
transactionManager.coordinator(nextRequestHandler.coordinatorType());}}
{{ if (targetNode == null) {}}
{{ transactionManager.lookupCoordinator(nextRequestHandler); }}}}
{{ break;}}
{{ }}}
{{ ...}}
{{lookupCoordinator()}} does not actually send anything, but just enqueues a
coordinator lookup request for the {{Sender}}'s next run loop iteration.
{{maybeSendTransactionalRequest}} still returns true, though (the {{break}}
jumps to a {{return true}} at the end of the method), leading the {{Sender}} to
needlessly wait via {{client.poll()}} although there is actually no request
in-flight.
I _think_ the fix is to let {{maybeSendTransactionalRequest}} return false if
it merely enqueues the coordinator lookup instead of actually sending anything.
But I'm not sure, hence the bug report instead of a pull request.
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