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https://issues.apache.org/jira/browse/KAFKA-10793?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17241976#comment-17241976
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A. Sophie Blee-Goldman commented on KAFKA-10793:
------------------------------------------------
At this point we can only guess, but all signs point to a race condition
between the main consumer thread and the heartbeat thread. One possibility is
that when the future failed it just didn't trigger the `onFailure` callback,
but [~guozhang] & I have both looked through the source code and don't see any
way for this to occur. Another possibility is that the `onFailure` callback was
triggered, but it was invoked too soon_. If the future was completed before we
ever assigned it to the findCoordinatorFuture field, then we would never
actually clear the latest future (we would just set an already-null field to
null again)._
Is this possible? Here's how the AbstractCoordinator builds the request and
assigns the future:
{code:java}
protected synchronized RequestFuture<Void> lookupCoordinator() {
...
findCoordinatorFuture = sendFindCoordinatorRequest(node);
}
{code}
{code:java}
private RequestFuture<Void> sendFindCoordinatorRequest(Node node) {
...
return client.send(node, requestBuilder).compose(new
FindCoordinatorResponseHandler());
}{code}
Inside #compose we call #addListener, which contains this snippet:
{code:java}
if (failed())
fireFailure();
{code}
If the request has already failed by the time we reach this, then we'll trigger
the `onFailure` callback before #compose ever returns – ie before we've
assigned the future to _findCoordinatorFuture_.
The obvious question now is whether it's possible for the request to be failed
in another thread while one thread is in the middle of the synchronized
lookupCoordinator(). The request can be failed by the ConsumerNetworkClient
when polled, during checkDisconnects(). The heartbeat thread actually
synchronizes the entire run loop, so it doesn't seem possible for the hb thread
to fail this request in the background of the main thread during a
lookupCoordinator().
But the inverse is not true: it's possible for the main consumer thread to fail
the request while the hb thread is inside of lookupCoordinator(). The
AbstractCoordinator will poll the network client inside of joinGroupIfNeeded(),
which in not itself synchronized and may be invoked without any locking through
a Consumer#poll.
> Race condition in FindCoordinatorFuture permanently severs connection to
> group coordinator
> ------------------------------------------------------------------------------------------
>
> Key: KAFKA-10793
> URL: https://issues.apache.org/jira/browse/KAFKA-10793
> Project: Kafka
> Issue Type: Bug
> Components: consumer, streams
> Affects Versions: 2.5.0
> Reporter: A. Sophie Blee-Goldman
> Priority: Critical
>
> Pretty much as soon as we started actively monitoring the
> _last-rebalance-seconds-ago_ metric in our Kafka Streams test environment, we
> started seeing something weird. Every so often one of the StreamThreads (ie a
> single Consumer instance) would appear to permanently fall out of the group,
> as evidenced by a monotonically increasing _last-rebalance-seconds-ago._ We
> inject artificial network failures every few hours at most, so the group
> rebalances quite often. But the one consumer never rejoins, with no other
> symptoms (besides a slight drop in throughput since the remaining threads had
> to take over this member's work). We're confident that the problem exists in
> the client layer, since the logs confirmed that the unhealthy consumer was
> still calling poll. It was also calling Consumer#committed in its main poll
> loop, which was consistently failing with a TimeoutException.
> When I attached a remote debugger to an instance experiencing this issue, the
> network client's connection to the group coordinator (the one that uses
> MAX_VALUE - node.id as the coordinator id) was in the DISCONNECTED state. But
> for some reason it never tried to re-establish this connection, although it
> did successfully connect to that same broker through the "normal" connection
> (ie the one that juts uses node.id).
> The tl;dr is that the AbstractCoordinator's FindCoordinatorRequest has failed
> (presumably due to a disconnect), but the _findCoordinatorFuture_ is non-null
> so a new request is never sent. This shouldn't be possible since the
> FindCoordinatorResponseHandler is supposed to clear the
> _findCoordinatorFuture_ when the future is completed. But somehow that didn't
> happen, so the consumer continues to assume there's still a FindCoordinator
> request in flight and never even notices that it's dropped out of the group.
> These are the only confirmed findings so far, however we have some guesses
> which I'll leave in the comments. Note that we only noticed this due to the
> newly added _last-rebalance-seconds-ago_ __metric, and there's no reason to
> believe this bug hasn't been flying under the radar since the Consumer's
> inception
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