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Kafka, mail # user - Offset committing on rebalance


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Re: Offset committing on rebalance
Jun Rao 2013-08-15, 15:52
Yes, during rebalances, messages could be re-delievered since the new owner
of a partition starts fetching from the last checkpointed offset in ZK.

For reasons on why rebalances happen a lot, see
https://cwiki.apache.org/confluence/display/KAFKA/FAQ#FAQ-Whyaretheremanyrebalancesinmyconsumerlog%3F

Thanks,

Jun
On Thu, Aug 15, 2013 at 8:36 AM, Ian Friedman <[EMAIL PROTECTED]> wrote:

> It's a simple enough patch, but wouldn't this mean that messages still in
> process when a rebalance happens could get delivered to another consumer if
> we end up losing the partition? Rebalances seem to happen very frequently
> with a lot of consumers for some reason… And it doesn't seem like a
> consumer is guaranteed or likely to retain ownership of a partition it's in
> the middle of consuming after a rebalance.
>
> --
> Ian Friedman
>
>
> On Thursday, August 15, 2013 at 10:53 AM, Jun Rao wrote:
>
> > We are only patching blocker issues in 0.7. 0.8 beta1 has been released
> and
> > most dev effort will be on 0.8 and beyond. That said. This particular
> case
> > is easy to fix. If you can port the patch in
> > https://issues.apache.org/jira/browse/KAFKA-919 o the 0.7 branch , we
> can
> > commit that to the 0.7 branch.
> >
> > Thanks,
> >
> > Jun
> >
> >
> > On Wed, Aug 14, 2013 at 9:30 PM, Ian Friedman <[EMAIL PROTECTED] (mailto:
> [EMAIL PROTECTED])> wrote:
> >
> > > Ugh.
> > >
> > > Is there any way to make this work in 0.7, or is transitioning to 0.8
> the
> > > only way? My operations engineers spent a lot of effort in configuring
> and
> > > hardening our 0.7 production install, and 0.8 isn't released yet. Not
> to
> > > mention having to integrate the new client side code.
> > >
> > > Either way, thanks for all your help Jun.
> > >
> > > --
> > > Ian Friedman
> > >
> > >
> > > On Thursday, August 15, 2013 at 12:21 AM, Jun Rao wrote:
> > >
> > > > Yes, this is an issue and has been fixed in 0.8.
> > > >
> > > > Thanks,
> > > >
> > > > Jun
> > > >
> > > >
> > > > On Wed, Aug 14, 2013 at 5:21 PM, Ian Friedman <[EMAIL PROTECTED](mailto:
> [EMAIL PROTECTED]) (mailto:
> > > [EMAIL PROTECTED] (mailto:[EMAIL PROTECTED]))> wrote:
> > > >
> > > > > Hey guys,
> > > > >
> > > > > I designed my consumer app (running on 0.7) to run with autocommit
> off
> > > and
> > > > > commit manually once it was done processing a record. The intent
> was so
> > > > > that if a consumer died while processing a message, the offset
> would
> > > > >
> > > >
> > >
> > > not be
> > > > > committed, and another box would pick up the partition and
> reprocess
> > > >
> > >
> > > the
> > > > > message. This seemed to work fine with small numbers of consumers
> > > >
> > >
> > > (~10).
> > > > > But now that I'm scaling it out, I'm running into a problem where
> it
> > > >
> > >
> > > looks
> > > > > like messages that consumers picked up and then errored on are not
> > > >
> > >
> > > getting
> > > > > processed on another machine.
> > > > >
> > > > > After investigating the logs and the partition offsets in
> zookeeper, I
> > > > > found that in ZookeeperConsumerConnector.scala
> closeFetchersForQueues,
> > > > > called during the rebalance process, will commit the offset
> regardless
> > > > >
> > > >
> > >
> > > of
> > > > > the autocommit status. So it looks like even if my consumer is in
> the
> > > > > middle of processing a message, the offset will be committed, and
> even
> > > > >
> > > >
> > >
> > > if
> > > > > the processing fails, it will never be picked up again. Now that I
> > > >
> > >
> > > have a
> > > > > lot of consumer nodes, the rebalancer is going off a lot more often
> > > >
> > >
> > > and I'm
> > > > > running into this constantly.
> > > > >
> > > > > Were my assumptions faulty? Did I design this wrong? After reading
> the
> > > > > comment in the code I understand that if it didn't commit the
> offset
> > > > >
> > > >
> > >
> > > there,
> > > > > the message would just get immediately consumed by whoever ended up
> > > >
> > >
> > > owning
> > > > > the partition, even if we were in the middle of consuming it